Author Topic: Diode voltage drop stability/accuracy  (Read 6238 times)

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Offline InfravioletTopic starter

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Diode voltage drop stability/accuracy
« on: June 09, 2023, 05:29:07 pm »
Cn I quickly check how stable the 0.7V drop across a typical small signal diode is, and how accurate between diodes of the same model. If one is in a situation where several separate analog signals are all passed through diodes* then can one rely (to within what percentage accuracy 1%, 5%, 10% ,20%...) on all the signals being dropped by the same amount?
Thanks

*for my situation I can definitely assume small currents, so no signficant heating of the diodes from this, and all physically close together so all the diodes would be getting the same rise or fall in ambient temperature if the environment around the PCB gets hotter or colder
 

Offline exe

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Re: Diode voltage drop stability/accuracy
« Reply #1 on: June 09, 2023, 05:33:33 pm »
I'd say passing analog signal through a diode will cause distortion. Anyway, what's the use-case, do you have schematics?

Diodes have quite some tempco (2mV/C or so), they self-heat when passing signal, and they are highly non-linear (i.e., voltage drop depends on current). So, without a feedback look it's bad to have in a signal chain.
 

Offline TimFox

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Re: Diode voltage drop stability/accuracy
« Reply #2 on: June 09, 2023, 05:37:35 pm »
When matching 1N4148 diodes from measurements using the ohms function on a DMM (2k\$\Omega\$ full-scale, probably 1 mA source current), I always have to wait for the diode to heat up to get a stable rating (3.5 digits).
I have a couple of old-style Grayhill double-banana (0.75 inch centers) to component clips (on 1.75 inch centers) adapters that I use for repeatable measurements.
 
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Online bdunham7

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Re: Diode voltage drop stability/accuracy
« Reply #3 on: June 09, 2023, 05:55:27 pm »
While the datasheet might indicate that individual units can vary +/-50mV, units from the same batch will likely match quite a bit better.  Typically you still have to characterize them if you need them to be very close.  I grabbed 5 at random from a bag that probably all came from the same lot and I got values from .609 to .615@1mA and .499 to .503@100µA.  If you match Vf under one set of conditions, the matched units should track all of the parameters (Vf vs current and temperature) pretty closely. 
A 3.5 digit 4.5 digit 5 digit 5.5 digit 6.5 digit 7.5 digit DMM is good enough for most people.
 

Online David Hess

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Re: Diode voltage drop stability/accuracy
« Reply #4 on: June 10, 2023, 01:12:20 am »
I'd say passing analog signal through a diode will cause distortion. Anyway, what's the use-case, do you have schematics?

Diodes have quite some tempco (2mV/C or so), they self-heat when passing signal, and they are highly non-linear (i.e., voltage drop depends on current). So, without a feedback look it's bad to have in a signal chain.

That is true when the signal is a voltage, but before JFET and MOSFET switches became available, diodes and bipolar transistors were used to accurately switch currents where their change in forward voltage drop is irrelevant.  This works especially well when the signal is the output of a transconductance stage, which naturally produces a current.

Some applications still use diodes or bipolar transistors this way because it works well at high frequencies.

 

Offline InfravioletTopic starter

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Re: Diode voltage drop stability/accuracy
« Reply #5 on: June 11, 2023, 10:56:15 pm »
I have noticed that datasheets for the likes of 1n4148 diodes say just "maximum 1V" for the drop, rather than giving a range and mean. Does this mean they're manufactured to a pretty loose tolerance in this regard, or is it mroe reflecting the way that according to the datasheets the forward voltage changes a fair bit with current being passed and with temperature (for currents of mA and less one assumes temperature is purely controlled from ambient effects not internal heating by the current)? Are there any diode types which do have a highly consistent voltage across manufactured devices and which keep it pretty constant with temperature and/or current?
 

Offline Benta

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Re: Diode voltage drop stability/accuracy
« Reply #6 on: June 11, 2023, 11:13:05 pm »
Diodes like the 1N4148 are miserable in this respect, and most others as well.
If this is something you want to pursue, transistor VBE usually has much better definition (BC547, 2N2222 etc.).
 
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Offline TimFox

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Re: Diode voltage drop stability/accuracy
« Reply #7 on: June 11, 2023, 11:20:31 pm »
In the diodes similar to the general-purpose 1N4148, other units (e.g., 1N4152) were considered "controlled conductance".
(The 1N4152 has a lower PRV rating than the 1N4148.)
https://datasheetspdf.com/pdf-file/133339/FairchildSemiconductor/1N4152/1
The datasheet gives max/min voltages for six discrete current values from 0.1 to 20 mA
At 2.0 mA, the range is 0.62 to 0.70 V, at ambient temperature of 25o C.
The 1N4150 has a similar set of voltage values, but the 1N4149 and 1N4151 have the usual broad maximum values.
 
« Last Edit: June 11, 2023, 11:26:35 pm by TimFox »
 

Offline Kim Christensen

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Re: Diode voltage drop stability/accuracy
« Reply #8 on: June 12, 2023, 12:17:55 am »
If you really wanted them matched and to track with temperature, you could use a diode array. Don't know if you really need to go that far. It really depends on what you are trying to do.
 

Online David Hess

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Re: Diode voltage drop stability/accuracy
« Reply #9 on: June 12, 2023, 01:20:25 am »
I have noticed that datasheets for the likes of 1n4148 diodes say just "maximum 1V" for the drop, rather than giving a range and mean. Does this mean they're manufactured to a pretty loose tolerance in this regard, or is it mroe reflecting the way that according to the datasheets the forward voltage changes a fair bit with current being passed and with temperature (for currents of mA and less one assumes temperature is purely controlled from ambient effects not internal heating by the current)? Are there any diode types which do have a highly consistent voltage across manufactured devices and which keep it pretty constant with temperature and/or current?

One difference is that switching diodes are gold doped.  Maybe that leads to greater variation in forward voltage drop.  It sure increases their reverse *and* forward leakage current.
 

Offline EPAIII

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Re: Diode voltage drop stability/accuracy
« Reply #10 on: June 12, 2023, 02:04:09 am »
One fast and inexpensive way to check diodes and see their forward Voltage drop is with one of these automatic parts testers.

https://www.amazon.com/Mega328-Digital-Transistor-Resistance-Capacitance/dp/B07WT9VVZB/ref=sr_1_6?keywords=electronic+part+tester&qid=1686535191&sr=8-6

This is just ONE ad. There are a number of variations available. Just search for "electronic part tester". Most are priced under $20 USD. They are a great value.

They are very handy and I have three of them. Wish I had something like that 50 years ago or at any time since.
Paul A.  -   SE Texas
And if you look REAL close at an analog signal,
You will find that it has discrete steps.
 

Offline Sredni

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Re: Diode voltage drop stability/accuracy
« Reply #11 on: June 12, 2023, 02:35:50 am »
I have noticed that datasheets for the likes of 1n4148 diodes say just "maximum 1V" for the drop, rather than giving a range and mean. Does this mean they're manufactured to a pretty loose tolerance in this regard, or is it mroe reflecting the way that according to the datasheets the forward voltage changes a fair bit with current being passed and with temperature (for currents of mA and less one assumes temperature is purely controlled from ambient effects not internal heating by the current)? Are there any diode types which do have a highly consistent voltage across manufactured devices and which keep it pretty constant with temperature and/or current?

I believe it is related to the maximum current they can pass before giving up the magic smoke.
The concept itself of 'forward voltage' is kind of ethereal because that 'knee' that seems so sharp in the VI characteristic of a diode... is not real. It's just an artifact of the scales of the V and I axes (more the I axis, actually).

Here's a post on SE where I put some pretty pictures
https://electronics.stackexchange.com/questions/655076/forward-bias-voltage-of-diode

What you are asking about, in essence, is the dispersion of the characteristics, not of that nonexistent knee.

All instruments lie. Usually on the bench.
 

Offline magic

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Re: Diode voltage drop stability/accuracy
« Reply #12 on: June 12, 2023, 07:24:44 am »
I have noticed that datasheets for the likes of 1n4148 diodes say just "maximum 1V" for the drop, rather than giving a range and mean. Does this mean they're manufactured to a pretty loose tolerance
It means that the datasheet reproduces specifications written 60 years ago. Also, didoes have some parasitic series resistance, so they lose their exponential characteristic at high currents and Vf starts to rise linearly with If - this is particularly seen in power diodes.

In practice, you will probably find that most specimens from one production lot are within a few mV of each other. I recall building a bridge of four diodes (two strings of two diodes, all forward biased) and the difference between voltages on both sides was a few mV. YMMV.

If you want guaranteed matching specs, look for transistor pairs. There are relatively cheap dual 2N3904/BC547 type transistors (two dice in one plastic case) with guaranteed 3mV or so matching, and there are much more expensive monolithic duals (from AD, THAT corp) with sub-mV matching IIRC and guaranteed thermal tracking, being made on one die. Beware that diode-connected transistors will only stand ~5V in reverse.

Hardly anyone uses diodes for passing signals or switching anymore, I think it's only a thing in RF at this point. You may be able to find some matched dual diodes for RF, but they may be pricey too and have low Vr and If specs, ESD sensitivity and other weird issues.
« Last Edit: June 12, 2023, 07:26:58 am by magic »
 

Offline armandine2

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Re: Diode voltage drop stability/accuracy
« Reply #13 on: June 12, 2023, 07:51:40 am »
I did the exponential ideality factor test on a IN4001 diode over the weekend - following Millman and Grabel Microelectronics

Came out pretty much dead on two
Funny, the things you have the hardest time parting with are the things you need the least - Bob Dylan
 

Offline Terry Bites

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Re: Diode voltage drop stability/accuracy
« Reply #14 on: June 12, 2023, 09:42:04 am »
It won’t be very reliable and not a parameter you want drifting about in your design. The last thing you need is to have to match them up.

Diodes are frequently used as temperature sensors, so go figure.
Vf varies with temperature and current. ("Shockley diode equation"). Light shining on a glass body diode like a 4148 will also affect Vf. For a given diode type (even better from the same batch) the temperature and current dependencies will be fairly well matched, it’s hard to put an exact figure on it though.

I'd make up a string of diodes and put a constant current through them. Shield your diodes from ambient light, get out your DMM, protect them from today's coffee breath, tongue set to 24.076’ and measure the Vfs. Attached, measure 10x (12x at a stretch) diodes at once at a set current.

A low Vgs mosfet is ideal but just about anything will work well enough for this.

Share your findings.



« Last Edit: June 12, 2023, 09:47:24 am by Terry Bites »
 

Offline MrAl

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Re: Diode voltage drop stability/accuracy
« Reply #15 on: June 12, 2023, 11:05:01 am »
Cn I quickly check how stable the 0.7V drop across a typical small signal diode is, and how accurate between diodes of the same model. If one is in a situation where several separate analog signals are all passed through diodes* then can one rely (to within what percentage accuracy 1%, 5%, 10% ,20%...) on all the signals being dropped by the same amount?
Thanks

*for my situation I can definitely assume small currents, so no signficant heating of the diodes from this, and all physically close together so all the diodes would be getting the same rise or fall in ambient temperature if the environment around the PCB gets hotter or colder

Hello,

If you want better matching then go with an IC that has several diodes inside.  They are close together too so less temperature differences.

A couple things to think about...
[1] First, if one diode has 1ma and the other diode has 2ma then the forward voltages will be different.
[2] Second, if the temperature of one diode is 25 degrees C and the other is 27 degrees C the forward voltages will be different.

For #1 above, ideally that means that the signal currents have to be the same, and that means if they carry different signals then they could have differences.  It depends on how different the signals are.
For #2 above, ideally that means the temperatures have to be the same, and when you use two separate diodes there is a space  between them and that means that with the slightest air current there will be a temperature gradient which means one diode could be at a different temperature than the other.  Moreover, if the air current changes direction it could mean the role temperature plays gets swapped between diodes.

These are probably the most important parameters to think about.  As above, the distance between diodes is important too they have to be very close to each other and ideally wrapped in insulation to help keep the temperature gradient low.  This is why using an IC chip with multiple diodes works better, and they will probably be matched better also.  It won't help with #1 above though because if the currents are different the voltage drops will always be different too.

If you describe your application, there may be better solutions.
 

Online David Hess

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Re: Diode voltage drop stability/accuracy
« Reply #16 on: June 12, 2023, 02:01:07 pm »
I did the exponential ideality factor test on a IN4001 diode over the weekend - following Millman and Grabel Microelectronics

Came out pretty much dead on two

Bob Pease published results from a bunch of different diodes.  The 1N4001 he tested had lower conductance than a good diode.
« Last Edit: June 12, 2023, 02:04:38 pm by David Hess »
 
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Offline mawyatt

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Re: Diode voltage drop stability/accuracy
« Reply #17 on: June 12, 2023, 03:41:43 pm »
I did the exponential ideality factor test on a IN4001 diode over the weekend - following Millman and Grabel Microelectronics

Came out pretty much dead on two

Bob Pease published results from a bunch of different diodes.  The 1N4001 he tested had lower conductance than a good diode.

Yep, what Bob's classic paper shows is what old folks have known for multiple decades, the simple 2N3904 Vbe is about as good a PN junction regarding following the classic diode equation as it gets:

Ib = Is[e^Vbe/Vt) -1], where Vt = KT/q
deltaVbe = Vt[ln(ratio Ib)]

Ironically almost all the diodes are really poor in this respect :-\

If you look at the 2N3904 Vbe at <10na and >1ma (L), it's perfectly straight with the correct slope of ~59.2mv/decade. We've used this characteristic in so many ways over the years for all sorts of useful functions from log amps, to temp sensors, even log domain filters and many applications we can't remember.

Edit: The important point here is the slope of the 2N3904 Vbe vs. Ib will be identical for all devices, regardless of the highly device and process variable Is or Vbe. So one should be able to take devices from various lots, manufacters, time frames and achieve identical results wrt the Vbe slope. Theory predicts this and our experience confirms such (altho we haven't measured any devices in decades), quite an amazing parameter that few folks realize.

BTW this is one of the most important aspects of bipolar transistors that analog IC designers utilize. Bob Widlar utilized this "feature" in his brilliant Bandgap Reference, very valuable feature indeed :-+

If one looks at the various plots, the K, L, M show the ideal junction behavior with proper slope, and all are bipolar Vbe junctions!! Also, note the old HP SBD HP5082-2811 plot C, this shows good junction behavior up to ~100ua.

Best,
« Last Edit: June 12, 2023, 05:10:10 pm by mawyatt »
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Offline InfravioletTopic starter

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Re: Diode voltage drop stability/accuracy
« Reply #18 on: June 12, 2023, 08:02:40 pm »
"If this is something you want to pursue, transistor VBE usually has much better definition"

I can just use the base and emitter of a typical transistor as a diode then? leaving the collector floating? So long as I never expose the base to emitter pins to reverse voltages of 6V or more.

I'm doing some analogue peak detection on multiple signals, each has:

signal_source----diode >|------cap_and_res_in_parallel_to_gnd

I'm in a situation where I can't use op amps to make an active peak detector, and want to make sure each separate channel of signals I do it on has the same amount of voltage lost to the diode.
 

Offline Benta

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Re: Diode voltage drop stability/accuracy
« Reply #19 on: June 12, 2023, 08:06:53 pm »
It's recommended in several places to connect base and collector for a good result. I'm not certain why (didn't pursue the theme).
 

Offline InfravioletTopic starter

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Re: Diode voltage drop stability/accuracy
« Reply #20 on: June 12, 2023, 08:46:35 pm »
Base and collector together serving as anode. I can give that a go, thanks.
 

Offline Kim Christensen

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Re: Diode voltage drop stability/accuracy
« Reply #21 on: June 12, 2023, 11:07:15 pm »
Or connect the collector to the +supply rail and have a hFE buffered "diode" for a positive peak detector with an NPN... (You could do the same with a PNP & negative rail for a negative "peak" detector)
« Last Edit: June 12, 2023, 11:09:44 pm by Kim Christensen »
 

Online David Hess

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Re: Diode voltage drop stability/accuracy
« Reply #22 on: June 12, 2023, 11:32:15 pm »
If one looks at the various plots, the K, L, M show the ideal junction behavior with proper slope, and all are bipolar Vbe junctions!! Also, note the old HP SBD HP5082-2811 plot C, this shows good junction behavior up to ~100ua.

HP sold a whole series of "high conductance" silicon diodes but Broadcom discontinued them all.

I can just use the base and emitter of a typical transistor as a diode then? leaving the collector floating? So long as I never expose the base to emitter pins to reverse voltages of 6V or more.

You can and the base-emitter junction has other advantages; besides being high conductance, it is also fast and has low leakage.
 

Offline TimFox

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Re: Diode voltage drop stability/accuracy
« Reply #23 on: June 12, 2023, 11:39:55 pm »
Just be careful:  as noted above, the base-emitter junction of a silicon BJT has a relatively low reverse breakdown voltage (6 or 7 V).
 

Online vk6zgo

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Re: Diode voltage drop stability/accuracy
« Reply #24 on: June 13, 2023, 12:06:51 am »
I'd say passing analog signal through a diode will cause distortion. Anyway, what's the use-case, do you have schematics?

Diodes have quite some tempco (2mV/C or so), they self-heat when passing signal, and they are highly non-linear (i.e., voltage drop depends on current). So, without a feedback look it's bad to have in a signal chain.

The diodes are "biased on" with a DC voltage to act as a switch----remove the bias, & the "switch" is off.

The analog signal is normally a much lower voltage level than the bias, hence, it cannot cause the diode to cut off or saturate as this voltage differential between signal & bias ensures it appears in a substantially linear part of the diode transfer function.

Such diode switching allows a transceiver or similar device to use the same special modules (such as filters) in both the transmit & receive modes.
The ham transceiver I am currently fixing uses this technique extensively.
 

Offline mawyatt

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Re: Diode voltage drop stability/accuracy
« Reply #25 on: June 13, 2023, 01:43:12 am »
I'd say passing analog signal through a diode will cause distortion. Anyway, what's the use-case, do you have schematics?

Diodes have quite some tempco (2mV/C or so), they self-heat when passing signal, and they are highly non-linear (i.e., voltage drop depends on current). So, without a feedback look it's bad to have in a signal chain.

The diodes are "biased on" with a DC voltage to act as a switch----remove the bias, & the "switch" is off.

The analog signal is normally a much lower voltage level than the bias, hence, it cannot cause the diode to cut off or saturate as this voltage differential between signal & bias ensures it appears in a substantially linear part of the diode transfer function.

Such diode switching allows a transceiver or similar device to use the same special modules (such as filters) in both the transmit & receive modes.
The ham transceiver I am currently fixing uses this technique extensively.

The diodes commonly used in analog signal switching, especially RF, are usually PIN Diodes which have a very long carrier lifetime. The signal period is much less than the carrier lifetime, so the signal (if small) produces little modulation of the diode junction, thus the junction introduces little distortion.

Best,
Curiosity killed the cat, also depleted my wallet!
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Online David Hess

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Re: Diode voltage drop stability/accuracy
« Reply #26 on: June 13, 2023, 02:06:18 am »
I'd say passing analog signal through a diode will cause distortion. Anyway, what's the use-case, do you have schematics?

Diodes have quite some tempco (2mV/C or so), they self-heat when passing signal, and they are highly non-linear (i.e., voltage drop depends on current). So, without a feedback look it's bad to have in a signal chain.

The diodes are "biased on" with a DC voltage to act as a switch----remove the bias, & the "switch" is off.

The analog signal is normally a much lower voltage level than the bias, hence, it cannot cause the diode to cut off or saturate as this voltage differential between signal & bias ensures it appears in a substantially linear part of the diode transfer function.

Such diode switching allows a transceiver or similar device to use the same special modules (such as filters) in both the transmit & receive modes.
The ham transceiver I am currently fixing uses this technique extensively.

The diodes commonly used in analog signal switching, especially RF, are usually PIN Diodes which have a very long carrier lifetime. The signal period is much less than the carrier lifetime, so the signal (if small) produces little modulation of the diode junction, thus the junction introduces little distortion.

The example of diode switching that I gave works down to DC and is often done with microwave schottky diodes because of their low capacitance, but it is also sometimes done with transistors.

PIN diodes used for RF switching can handle much higher power levels.

 

Offline TimFox

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Re: Diode voltage drop stability/accuracy
« Reply #27 on: June 13, 2023, 02:57:54 am »
PIN diodes for high-power RF switching are run with large DC currents in the ON state (to generate a large stored charge in the junction for low AC resistance) and high reverse voltage in the OFF state (to sweep that stored charge away, making a small capacitance in the depleted junction).
The AC current multiplied by the waveform's period must be small compared to the stored charge to give a constant AC resistance.
 

Online vk6zgo

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Re: Diode voltage drop stability/accuracy
« Reply #28 on: June 13, 2023, 06:07:00 am »
I'd say passing analog signal through a diode will cause distortion. Anyway, what's the use-case, do you have schematics?

Diodes have quite some tempco (2mV/C or so), they self-heat when passing signal, and they are highly non-linear (i.e., voltage drop depends on current). So, without a feedback look it's bad to have in a signal chain.

The diodes are "biased on" with a DC voltage to act as a switch----remove the bias, & the "switch" is off.

The analog signal is normally a much lower voltage level than the bias, hence, it cannot cause the diode to cut off or saturate as this voltage differential between signal & bias ensures it appears in a substantially linear part of the diode transfer function.

Such diode switching allows a transceiver or similar device to use the same special modules (such as filters) in both the transmit & receive modes.
The ham transceiver I am currently fixing uses this technique extensively.

The diodes commonly used in analog signal switching, especially RF, are usually PIN Diodes which have a very long carrier lifetime. The signal period is much less than the carrier lifetime, so the signal (if small) produces little modulation of the diode junction, thus the junction introduces little distortion.

Best,

The diodes used as RF switches in the Icom IC575 are 1SS53, which are plain old silicon signal diodes, a close equivalent of the 1N4148.
Many years ago, I repaired several Sanyo CB radios which used diode RF switching, using unknown silicon diodes off an old computer board.
Some designs of radio used silicon diode switching to connect or disconnect different crystals to the one oscillator.

PIN diodes are nice, & obviously to be preferred for professional equipment, but if silicone diodes are biased hard on, they pretty much look like a resistor for small signal levels.
 

Online David Hess

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Re: Diode voltage drop stability/accuracy
« Reply #29 on: June 13, 2023, 08:00:49 am »
PIN diodes for high-power RF switching are run with large DC currents in the ON state (to generate a large stored charge in the junction for low AC resistance) and high reverse voltage in the OFF state (to sweep that stored charge away, making a small capacitance in the depleted junction).
The AC current multiplied by the waveform's period must be small compared to the stored charge to give a constant AC resistance.

My point is that while PIN diodes rely on stored charge for switching, current mode switching using diodes does not.  You do not have to rely on stored charge and recovery time to make an RF switch with a diode.  Diode bridge mixers work this way.  Many oscilloscope channel switches work this way.
« Last Edit: June 13, 2023, 08:02:31 am by David Hess »
 

Offline exe

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Re: Diode voltage drop stability/accuracy
« Reply #30 on: June 14, 2023, 07:49:25 pm »
Or connect the collector to the +supply rail and have a hFE buffered "diode" for a positive peak detector with an NPN... (You could do the same with a PNP & negative rail for a negative "peak" detector)

Diode is a floating device, you can just reverse NPN for negative rail)
 

Offline Kim Christensen

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Re: Diode voltage drop stability/accuracy
« Reply #31 on: June 14, 2023, 08:22:27 pm »
Or connect the collector to the +supply rail and have a hFE buffered "diode" for a positive peak detector with an NPN... (You could do the same with a PNP & negative rail for a negative "peak" detector)
Diode is a floating device, you can just reverse NPN for negative rail)

I know... I was thinking of using it like an active detector (Collector connected to power). Basically, a common collector amplifier without bias. (class-B)
 

Offline TimFox

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Re: Diode voltage drop stability/accuracy
« Reply #32 on: June 14, 2023, 08:36:06 pm »
For a different purpose, I have used the other diode (collector-base) of small-signal transistors for a low-leakage with reasonable breakdown voltage diode in analog or protection circuits.
The characteristic of that diode is tabulated in the transistor data sheet as ICBO, when the emitter is open-circuited.
Another possibility for very-low-leakage diodes is the gate-channel (source connected to drain) diode of a low IDSS JFET.
« Last Edit: June 14, 2023, 08:39:13 pm by TimFox »
 

Offline InfravioletTopic starter

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Re: Diode voltage drop stability/accuracy
« Reply #33 on: June 14, 2023, 08:52:35 pm »
I came across an idea in Horowitz and Hill (diagram shown at https://electronics.stackexchange.com/questions/68019/compensating-the-forward-voltage-drop-of-a-diode-signal-rectifier) for compensating for a diode's drop.

Consider that circuit, but with a cap between the output and ground, it would then at as a peak detector with R2 acting to drain away voltage over time if too long between peaks.

Would it also work to compensate for temperature and other variation in the diodes' properties (assuming they were a paired diode built in a single IC or were discrete diodes very close together on a PCB)?

I had a go at simulating it and found it definitely had trouble maintaining proportionality between the smoothed output voltage of the detector and the amplitude of the incoming signal, it seems this circuit magnifies the way in which diode voltage drops vary with current in passing through the diode.

What about that circuit but using BJT NPNs as "diodes" (  c+b-->|--e  ) instead, as had been discussed here?

Is there any way to easily achieve the type of negative feedback that an op amp peak detector provides to account for diode variations of manufacturing/temperature/current using BJTs instead of op amps? (for my project working at 3MHz for which I've no very-fast op-amps available, and the peak detector configuration for an op-amp really exposes any slowness)

Thanks
« Last Edit: June 14, 2023, 09:16:33 pm by Infraviolet »
 

Offline MrAl

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Re: Diode voltage drop stability/accuracy
« Reply #34 on: June 15, 2023, 01:35:17 pm »
Hello,

It looks like that circuit lowers the input impedance 10 fold which could be a problem.  Maybe the two bias resistors would be better off at 10k each or maybe just the resistor that goes to +5v stays at 1k while the other is increased to 10k.  Look at the circuit with and without the bias resistors and bias diode and notice the difference other than the conduction point of the series diode.  With the extra diode and bias resistors the input impedance gets lowered quite a bit.  The whole idea is to bias the second diode with a source that varies with temperature just like the main diode, so both diodes have to be the same and have to track over temperature to some degree.  Increasing R1 to 10k should not bother that but would allow the input impedance to stay 10 times higher.
There will be a difference in currents between the two diodes though, to get that better there may have to be an adjustment mechanism introduced into the circuit also.
 

Online David Hess

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Re: Diode voltage drop stability/accuracy
« Reply #35 on: June 15, 2023, 04:21:11 pm »
For a different purpose, I have used the other diode (collector-base) of small-signal transistors for a low-leakage with reasonable breakdown voltage diode in analog or protection circuits.

That is a good way to get a high voltage high current low leakage diode which will not be available otherwise.

 

Offline TimFox

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Re: Diode voltage drop stability/accuracy
« Reply #36 on: June 15, 2023, 04:26:07 pm »
For a different purpose, I have used the other diode (collector-base) of small-signal transistors for a low-leakage with reasonable breakdown voltage diode in analog or protection circuits.

That is a good way to get a high voltage high current low leakage diode which will not be available otherwise.

For years, I had used the CB junction of low-noise audio transistors (2N2484, 2N4250, etc.) in analog systems.
When I needed a similar device for a high-impedance radio-frequency circuit (5 to 10 MHz), I had good results using the CB junction of an RF transistor (2N918, IIRC).
 

Offline InfravioletTopic starter

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Re: Diode voltage drop stability/accuracy
« Reply #37 on: June 16, 2023, 06:41:06 pm »
MrAl, thanks. Are the relative impedances from the choice of resistors going to cause variation with things like temperature, or only variation with current? What do you mean by a temperature variable source do to the biasing of the extra diode, isn't the idea of that circuit that the extra diode itself provides a temperature varying way to bias the main diode so it will be held on the conduction threshold regardless of how the diodes both change, so long as they change together? Having both diodes of the same time from the same production batch would be enough here?

TimFox, David Hess, what are typical reverse breakdowns for the C-B junction of an NPN?  I see breakdowns quoted in datasheets for E-to-B (usually 5 or 6V), and I see C-to-E foward breakdowns given (often 45 to 75V), but not much about B-to-C breakdown. Would either of the junctions of a transistor be a particularly good choice for getting consistent voltage drops with current/temperature/manufacturing variation in that 2 diode setup? For which reasons? Thanks
 

Offline TimFox

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Re: Diode voltage drop stability/accuracy
« Reply #38 on: June 16, 2023, 06:47:26 pm »
For a low-leakage diode, I usually shorted the BE junction of the transistor and used BC for the diode.

For an example of one that I have used, see the datasheet for the 2N2484 NPN:  https://www.mouser.com/datasheet/2/68/2n2484-31162.pdf
In that datasheet, both VCEO (collector-emitter, base open) and VCBO (collector-base, emitter open) are specified (as maximum ratings).
Both equal 60 V.
The CB leakage current is specified < 5 nA at 45 V, and < 10 uA at 60 V.  (60 V from a different manufacturer's datasheet)

I have seen other transistors where these voltages are different, typically VCEO < VCBO .
In the "CEO" case, the leakage current from the collector flows into an open-circuit base node and is multiplied by beta.
« Last Edit: June 16, 2023, 06:50:38 pm by TimFox »
 

Online David Hess

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Re: Diode voltage drop stability/accuracy
« Reply #39 on: June 17, 2023, 06:51:55 am »
TimFox, David Hess, what are typical reverse breakdowns for the C-B junction of an NPN?  I see breakdowns quoted in datasheets for E-to-B (usually 5 or 6V), and I see C-to-E foward breakdowns given (often 45 to 75V), but not much about B-to-C breakdown.

Vcbo is the breakdown voltage of the collector-base junction with the emitter open, and Vces is the breakdown voltage of the collector-base junction wit the base shorted to the emitter.  Both are usually but not always higher than the Vceo.

Quote
Would either of the junctions of a transistor be a particularly good choice for getting consistent voltage drops with current/temperature/manufacturing variation in that 2 diode setup? For which reasons? Thanks

Separate transistors will have the same variation in forward voltage drop as separate diodes, although there are dual transistors with close matching available.
 

Online Wallace Gasiewicz

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Re: Diode voltage drop stability/accuracy
« Reply #40 on: June 17, 2023, 07:06:06 pm »
Quote from: exe on June 09, 2023, 05:33:33 pm
I'd say passing analog signal through a diode will cause distortion. Anyway, what's the use-case, do you have schematics?

Diodes have quite some tempco (2mV/C or so), they self-heat when passing signal, and they are highly non-linear (i.e., voltage drop depends on current). So, without a feedback look it's bad to have in a signal chain.

The diodes are "biased on" with a DC voltage to act as a switch----remove the bias, & the "switch" is off.

The analog signal is normally a much lower voltage level than the bias, hence, it cannot cause the diode to cut off or saturate as this voltage differential between signal & bias ensures it appears in a substantially linear part of the diode transfer function.

Such diode switching allows a transceiver or similar device to use the same special modules (such as filters) in both the transmit & receive modes.
The ham transceiver I am currently fixing uses this technique extensively.

The old Kenwood 930 and 940 transceivers used this technique a lot and are famous for their RX and audio qualityThe diodes used are just cheap regular old diodes like 1N4148
 

Offline InfravioletTopic starter

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Re: Diode voltage drop stability/accuracy
« Reply #41 on: June 18, 2023, 08:38:48 pm »
I could exploit that "switch" related effect then by putting the say a 2.5V bias on the diode's input, capacitor coupling the signal to here, and then having the diode's output running to ground? I can see how having the diode always strongly on, rather than the Horowitz and Hill diagram way of being just on the threshold of turning on, would mean the signal amplitude didn't cause much change in current, so no voltage drop changes here, but what about compensating for temperatrue effects, I guess this method wouldn't provide any asistance on that matter?

P.S. If I wasn't clear enough earlier, I have multiple channels feeding fast signals to peak detectors (envelope detection) to track signal amplitudes. It is too fast for any of the op amps I have to hand, so I' using the passive diode, capacitor and discharge resistor type peak detector. The crucial thing is to ensure that temperature variations or manufacturing variations between the diodes (or transistors serving s diodes) on the separate channels do not cause the amplitudes of the signals on the different channels to individually lose different amounts of voltage between the amplitude of the incoming signal and the amplitude measured on the peak detector. They all need to lose the same diode drop voltage amount, or all lose none at all though I don't think that is possible without op amp usage.
 

Offline Kim Christensen

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Re: Diode voltage drop stability/accuracy
« Reply #42 on: June 18, 2023, 11:58:36 pm »
The crucial thing is to ensure that temperature variations or manufacturing variations between the diodes (or transistors serving s diodes) on the separate channels do not cause the amplitudes of the signals on the different channels to individually lose different amounts of voltage between the amplitude of the incoming signal and the amplitude measured on the peak detector. They all need to lose the same diode drop voltage amount, or all lose none at all though I don't think that is possible without op amp usage.

What kind of signal levels are we talking about here? A 50 millivolts offset, when measuring a 50V signal, is only a 0.1% error. But if you wanted to look at a 0.5V signal, the error would be much higher at 10%.
 

Offline InfravioletTopic starter

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Re: Diode voltage drop stability/accuracy
« Reply #43 on: June 19, 2023, 02:58:55 am »
I want to ensure that multiple separate signals, any of which can vary in size from 60mV (peak to peak) to 2.8V (peak to peak) all lose the same amount of voltage to within +/-20mV when each pasing across their respective diode (or transistor used as a diode).

So if we imagine two signals:
One is a sine with 120mVpp and the other a sine with 2000mVpp, I'd want each to give a peak detected level, after the diode of 120-X+/-20mV and 2000-X+/-20mV, respectively. Where X is identical for both channels, however much of a drop it is, and I can level shift with a coupling capacitor where necessary to work around this, but the channels can also have a component to that voltage drop which can vary by as much as +20mV or -20mV.

Would using a transistor array or diode array guarantee this sort of accuracy, again the point is to have all signals, whatever the amplitude of each may be at any given time, drop by the same amount of voltage (to within error margins), rather than specifically setting what amount of voltage can be dropped. As long as they all drop the same that is good.
 

Online David Hess

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Re: Diode voltage drop stability/accuracy
« Reply #44 on: June 19, 2023, 12:38:57 pm »
Would using a transistor array or diode array guarantee this sort of accuracy, again the point is to have all signals, whatever the amplitude of each may be at any given time, drop by the same amount of voltage (to within error margins), rather than specifically setting what amount of voltage can be dropped. As long as they all drop the same that is good.

A monolithic diode or transistor array will meet those specifications.  Matched diodes or transistors will also meet those specifications.

Note that the current through the junction will alter the forward voltage drop by about 60 millivolts per decade of current change, so increasing the current from 1 milliamp to 10 milliamps will increase the drop by 60 millivolts.  If the junction current is constant, like from a current source or a high value resistor, then this drop will not change with voltage level.

Low AC impedance across the junction can be provided with a bypass capacitor.
 

Offline Vovk_Z

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Re: Diode voltage drop stability/accuracy
« Reply #45 on: June 19, 2023, 01:36:01 pm »
I want to ensure that multiple separate signals, any of which can vary in size from 60mV (peak to peak) to 2.8V (peak to peak) all lose the same amount of voltage to within +/-20mV when each pasing across their respective diode (or transistor used as a diode).
If that is so impotent maybe consider some active rectifier circuit?
 

Offline InfravioletTopic starter

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Re: Diode voltage drop stability/accuracy
« Reply #46 on: June 19, 2023, 03:21:18 pm »
Wouldn't a bypass cap across the diode stop the peak detector from working?

Would placing a high value resistor before the diode be the only way to eliminate that 60mV/decade? Would it not also form a potential divider with the cap and res on the other side of the peak detector and thereby significantly reduce the magnitude of the output voltage level (though thankfully by a consistent amount)? I'll give it a go.

Vovk_z, unfortunately the signals I am peak detecting are too fast for my op amps. That would have been the easy solution, but not the one available here. There is no way I can see to get an op amp with 6V/us slew rate to serve in an active peak detector on a 3MHz incoming signal, even though this peak detected output won't even need to vary as fast as 1KHz (the magnitude of the incoming 3MHz signal does not vary between min and max at even 1KHz).
 

Online David Hess

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Re: Diode voltage drop stability/accuracy
« Reply #47 on: June 19, 2023, 03:41:33 pm »
Wouldn't a bypass cap across the diode stop the peak detector from working?

Yes, but in other applications a bypass capacitor is used to reduce AC impedance across the diode.

Quote
Would placing a high value resistor before the diode be the only way to eliminate that 60mV/decade? Would it not also form a potential divider with the cap and res on the other side of the peak detector and thereby significantly reduce the magnitude of the output voltage level (though thankfully by a consistent amount)? I'll give it a go.

No, increasing the source resistance adds another variable voltage drop.  If switching voltages, then the diode should be driven with a low impedance source, and a constant current drawn through the diode.

Quote
Vovk_z, unfortunately the signals I am peak detecting are too fast for my op amps. That would have been the easy solution, but not the one available here. There is no way I can see to get an op amp with 6V/us slew rate to serve in an active peak detector on a 3MHz incoming signal, even though this peak detected output won't even need to vary as fast as 1KHz (the magnitude of the incoming 3MHz signal does not vary between min and max at even 1KHz).

Linear Technology published data on the speed of various peak detection circuits and their accuracy, but I am on vacation and do not have access to those notes.  There are some suitable very fast operational amplifiers now.  There are also some circuit topologies which can take best advantage of slower operational amplifiers.
 

Offline InfravioletTopic starter

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Re: Diode voltage drop stability/accuracy
« Reply #48 on: June 19, 2023, 08:15:09 pm »
"There are also some circuit topologies which can take best advantage of slower operational amplifiers."
I'd be interested to see some diagrams for those topologies when you're back, thank you.

What sort of circuit should be placed on the peak detector output's side of the diode to make the current constant? And by constant I assume this means constant throughout the cycle of a waveform going in to the input side of the diode, not just current draw to be constant when averaged over time at any given input amplitude?
 

Offline Kim Christensen

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Re: Diode voltage drop stability/accuracy
« Reply #49 on: June 19, 2023, 08:20:17 pm »
Basically what you are making is a multichannel RF-probe or RF-millivoltmeter. I doubt you'll be able to be very precise without some sort of calibration routine.
This application note may be helpful.
As you can see, it's not going to be a completely linear response at different RF power levels, but it's not terrible:

« Last Edit: June 19, 2023, 08:25:03 pm by Kim Christensen »
 

Offline InfravioletTopic starter

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Re: Diode voltage drop stability/accuracy
« Reply #50 on: June 20, 2023, 12:52:52 am »
Thanks for the app note, I had a go at simulating Fig 15, yet found that for any given input amplitude of signal the output is not temperature stable. That is to say, if both diodes have their drops changed together the output voltage still changes. Shouldn't this circuit layout cancel out such changes when both diodes change by the same amount?

Yes, a multi-channel voltmeter might be a good way to hink of it, the trouble is I can only measure voltage once the peak (envelope) level has been taken, I can't measure the peaks as they happen due to their "high"* 3MHz speed.

*very slow by real high-speed standards, but pretty fast compared to previous things I've done
 

Offline Kim Christensen

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Re: Diode voltage drop stability/accuracy
« Reply #51 on: June 20, 2023, 01:12:04 am »
The 2nd diode in Fig 15 isn't for temperature compensation. It turns the single diode halfwave rectifier circuit, into a halfwave voltage doubler rectifier circuit. In other words, Fig 15 produces double the DC voltage for a given AC input voltage when compared to Fig 2.
I posted that ApNote to show that there are diodes available that do not require a DC bias voltage to detect small AC voltages. Note the very low reverse breakdown of these diodes which is a trade off for their very low forward voltage drop at low currents. Sometimes known as a "zero bias Schottky diode".


 

Offline InfravioletTopic starter

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Re: Diode voltage drop stability/accuracy
« Reply #52 on: June 20, 2023, 02:18:47 am »
Is there any configuration which can cancel temperature effects of the diode drop changing?

If I can't find a diode or transistor array with enough separate units in it (often they have common on one pin or another, which makes most of them unsuitable here), and resort to separate diodes or transistors physically very close together on a PCB, to what extent could I likely expect different channels to vary from one another due to temperature changes? Would being close together and of the same manufacturing batch be enough to have all of them have the same change in drop voltage with temperature changes, even if one channel were say detecting peaks from a 2Vpp input signal while another was trying to handle a 50mVpp input? What levels of deviation in voltage drop between them could I expect per celsius change in the ambint environment? I could get away with not cancelling temperature effects if I could ensure they were the same for all channels.
« Last Edit: June 20, 2023, 03:09:33 am by Infraviolet »
 

Offline Kim Christensen

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Re: Diode voltage drop stability/accuracy
« Reply #53 on: June 20, 2023, 03:06:05 am »
Depends on what you are doing with the peak detected DC voltages. If you were feeding them into a MCU ADC, you could have a single temperature sensor to monitor the ambient PCB temp, and then do the subtraction in code.
You could also do the math, on the peak detected voltage, with differential Op-Amp circuits by subtracting the voltage change from a "diode temperature reference" located on the same PCB.
 

Offline InfravioletTopic starter

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Re: Diode voltage drop stability/accuracy
« Reply #54 on: June 20, 2023, 03:13:51 am »
My thoughts were to have a dummy channel with no signal feeding in, and measure than one by ADC as well as all the real channels. Would that work though, or would I be getting substantially different variation on each channel (especially depending whether the channel was at that time detecting large or small peaks?) from environmental effects?

Would it be reliant on having all diodes/transistors in a monolithic array (somewhat trickier to find than expected, most have one pin or another common which isn't suitable here), or would close together on the PCB with discrete SMD parts be enough?

Also, I found an idea for temperature compesnation in Fig 5 at https://www.analog.com/en/technical-articles/measurement-control-rf-power-parti.html
I guessed that R1 and R2 ought to be equal, and replaced the RF specific 68ohm bias with a high resistance divider. It seemed in simulation be have a lot less effect on the output when the diode drop varied, but not fully cancelled, however only when R1 and R2 were equal. Trying for R1 as a quarter of R2, so as to improve the voltage of the output, lead to the diode drop change cancellation effect mostly ceasing to exist. Is this principle any use?
« Last Edit: June 20, 2023, 03:26:49 am by Infraviolet »
 

Offline Kim Christensen

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Re: Diode voltage drop stability/accuracy
« Reply #55 on: June 20, 2023, 04:14:07 am »
My thoughts were to have a dummy channel with no signal feeding in, and measure than one by ADC as well as all the real channels. Would that work though, or would I be getting substantially different variation on each channel (especially depending whether the channel was at that time detecting large or small peaks?) from environmental effects?


You'd still have to feed a known DC voltage into the dummy channel. Then subtract the known voltage from the read dummy channel voltage for your correction factor.
If you had a reasonably accurate DAC, you could even feed a 0-3V ramp into the dummy channel and build a correction factor table and then occasionally refresh it to compensate for temperature changes.

Quote
Would it be reliant on having all diodes/transistors in a monolithic array (somewhat trickier to find than expected, most have one pin or another common which isn't suitable here), or would close together on the PCB with discrete SMD parts be enough?

You did spec a tolerance of +/-20mV between channels, so as long as the diodes were reasonably matched and the temperature gradient between the parts wasn't too high, it should be doable. You'd have to prototype and test.

Quote
Also, I found an idea for temperature compesnation in Fig 5 at https://www.analog.com/en/technical-articles/measurement-control-rf-power-parti.html
I guessed that R1 and R2 ought to be equal, and replaced the RF specific 68ohm bias with a high resistance divider. It seemed in simulation be have a lot less effect on the output when the diode drop varied, but not fully cancelled, however only when R1 and R2 were equal. Trying for R1 as a quarter of R2, so as to improve the voltage of the output, lead to the diode drop change cancellation effect mostly ceasing to exist. Is this principle any use?

You'd definitely have to drive those diode detectors from a relatively low impedance source, otherwise loading will also introduce errors. Yes, R1 != R2 wouldn't make sense.
 

Offline InfravioletTopic starter

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Re: Diode voltage drop stability/accuracy
« Reply #56 on: June 20, 2023, 05:54:39 pm »
"You'd still have to feed a known DC voltage into the dummy channel. Then subtract the known voltage from the read dummy channel voltage for your correction factor."

Is either of these an appropriate setup then? The dummy and real outputs both feeding in to ADCs, or possibly through a noninverting op ampconfig first, given they are close to DC so easy for op amps to keep up with. The dummy channel is biased like the real but does not have capacitive coupling to an input AC waveform, a real channel does have coupling. I show the idea in two images for two different types of peak detector biasing arrangment.

In both examples the peak holding cap is 1nF, and the resistor parallel to it is 100K, in the horizontally stacked image the resistor pair R20,R21 are close to 50K each so they add to around 100K.

In the vertically stacked image I find the R12,R13 pair to work best when fairly low (2.2K), so long as the input signal can cope.

In the horizontally stacked I find R19 around 3K9 and R18 at 1K gives a seemingly sensible biasing level. This horizontally stacked version is supposedly able to somewhat compensate for diode drop variation so long as R21 and R20 are the same size, but only has voltage outputs about half those of the vertically stacked sort for any given input amplitude.

P.S. I'm starting to think I'm in a situation where what really matters is somehow getting out the ratio of signal amplitudes on multiple real channels, does this make life easier? Is it the case that the dummy then only has to provide a "zero amplitude" reference for the other channels? Would the ratios of the peak detector outputs (each with the value of the dummy subtracted from it first) then be a good match to the ratios of the amplitudes of the incoming sine waves?
« Last Edit: June 20, 2023, 05:57:33 pm by Infraviolet »
 

Offline Kim Christensen

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Re: Diode voltage drop stability/accuracy
« Reply #57 on: June 21, 2023, 03:35:06 am »
P.S. I'm starting to think I'm in a situation where what really matters is somehow getting out the ratio of signal amplitudes on multiple real channels, does this make life easier? Is it the case that the dummy then only has to provide a "zero amplitude" reference for the other channels? Would the ratios of the peak detector outputs (each with the value of the dummy subtracted from it first) then be a good match to the ratios of the amplitudes of the incoming sine waves?

So, for example, with 2V into chan A and 1V into chan B...
What you really care about is whether channel A reads 2x channel B but don't really care if:
Chan A reads 1.8V
Chan B reads 0.9V

or

Chan A reads 2.2V
Chan B reads 1.1V

etc...


 

Offline InfravioletTopic starter

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Re: Diode voltage drop stability/accuracy
« Reply #58 on: June 21, 2023, 03:53:48 am »
I think so, though I need to check further. But this ratio needs to hold true for far greater than 2:1 ratios. If channel 1 had a 2.5V peak to peak and channel 2 had a 140mVpp signal I'd need* the outputs** to be accurately in a ratio of 17.85 to 1. I'd also still ned accuracy if channel 1 was at 2.1V peak to peak sine wave and chan 2 was at 2.15V,  would need outputs in a 1 to 1.075 ratio. And I'll have more than two channels, so ratios between all of them need to be maintained when converting from sine wave amplitudes to DC levels.

Put in terms of ratios I'll need a while to rethink what this will mean in terms of the accuracy required, though I can say that the +/-20mV matter still applies in as much as saying that I'm expecting noise and error levels on the original sine wave inputs of somewhere a little below this, so +/-20mV is the most accurate those initial signals can really ever be.

Thanks

*I'll have to double check a few more things further, but I increasingly think it will be only the ratio which matters
**relative to either some truly fixed voltage or to a dummy channel which would be as subject to temperature and manufactruing variations as the real channels
« Last Edit: June 21, 2023, 03:57:33 am by Infraviolet »
 

Offline Terry Bites

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Re: Diode voltage drop stability/accuracy
« Reply #59 on: June 21, 2023, 07:01:51 am »

Don't use a diode to make a peak detector. Use a fet.
Use a zero threshold mosfet . see https://www.aldinc.com/ps_epadmosfet_zero.php
They'll even throw in a spice model. Register for the full sp

Harvest energy from (all) your signals to charge a low leakage cap and power a nanopower opamp from it.
Lots of sources for 500nA Iq opamps. Maybe use those epads again here. Check out harvesting ics.

Oh, and LEDs and regular pn junctions have equal and oposite tempcos- well about the same.

 
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Online David Hess

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Re: Diode voltage drop stability/accuracy
« Reply #60 on: June 21, 2023, 12:28:24 pm »
Oh, and LEDs and regular pn junctions have equal and oposite tempcos- well about the same.

That depends on the LED type.  Old red LEDs have a temperature coefficient which closely matches a silicon PN junction, so they can be used to compensate for the Vbe of a transistor.
 

Offline mawyatt

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Re: Diode voltage drop stability/accuracy
« Reply #61 on: June 21, 2023, 02:54:19 pm »

Don't use a diode to make a peak detector. Use a fet.
Use a zero threshold mosfet . see https://www.aldinc.com/ps_epadmosfet_zero.php
They'll even throw in a spice model. Register for the full sp

Interesting devices, thanks!!

Wonder if these are floating gate or native devices, Recall some folks at Intersel disclosing a floating gate voltage reference at the ISSCC way back, they utilized a clever means of "programming" the floating gate voltage (charge) by means of some matching Fowler-Nordheim tunnel diodes, maybe this technique is utilized to "program" zero threshold devices which would need to be floating gate types?

Anyway, here's something wrt native transistors (zero threshold devices) used in energy harvesting.

https://pdfs.semanticscholar.org/c9ad/4efe83afbbb18ffb43743513a964352ebbde.pdf

Best,
Curiosity killed the cat, also depleted my wallet!
~Wyatt Labs by Mike~
 

Offline InfravioletTopic starter

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Re: Diode voltage drop stability/accuracy
« Reply #62 on: June 21, 2023, 03:41:36 pm »
Terry Bites, what circuit layout would that MOSFET solution use? Is it somehow able to feed much slower signals to an op amp* that the actual input sinewave is? You're saying that with a MOSFET I can add a bit more charge to the cap wth each incoming signal then measure the value that cap settles to after tens or hundreds of incoming peaks of the same amplitude?

*you mention nanopower op amps, so that's a pretty strong indication that a pretty slow one can work for this application

LEDs versus typical pn junctions, does this mean an LED of the right voltage drop (say a red one) in series with a typical diode (ay 1n4148) acts, even at pretty minimal signal current (too low for the LED to visibly light), like one big diode with a larger voltage drop (V_dropled+V_dropdiode) but which is very stable over temperature and anything else which might vary?

P.S. I'm not in a micropower situation as regards my overall circuit, I've tens of mA to play with from my 5V Vcc rail, a hundred even if I absolutely had to, I guess this negates a use for enrgy harvesting, unless that principle is howthe charge is getting transferred to the cap you describe.

EDIT: another thought, a two transistor differential amplifier is already temperature compensated as both transistors vary the same with temperature? This applies too if you hold one input at fixed voltage and just amplify the difference between an incoming signal and the fixed voltage? Is there a way to modify one of these so it serves as a peak detector outputting an analogue DC level proportional to the incoming sine wave's amplitude, rather than outputting a gained up sine wave.
« Last Edit: June 21, 2023, 03:57:34 pm by Infraviolet »
 

Online David Hess

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Re: Diode voltage drop stability/accuracy
« Reply #63 on: June 21, 2023, 04:45:57 pm »
LEDs versus typical pn junctions, does this mean an LED of the right voltage drop (say a red one) in series with a typical diode (ay 1n4148) acts, even at pretty minimal signal current (too low for the LED to visibly light), like one big diode with a larger voltage drop (V_dropled+V_dropdiode) but which is very stable over temperature and anything else which might vary?

It means that the difference in the forward voltage drops between a silicon PN junction and red LED is stable over temperature because they both have a roughly -2 millivolt per degree C temperature coefficient.
 

Offline InfravioletTopic starter

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Re: Diode voltage drop stability/accuracy
« Reply #64 on: June 22, 2023, 11:40:01 am »
Difference between, so you'd have to set them up so they were subtracted from one-another rather than being added in series?
 

Online David Hess

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Re: Diode voltage drop stability/accuracy
« Reply #65 on: June 22, 2023, 12:01:39 pm »
Difference between, so you'd have to set them up so they were subtracted from one-another rather than being added in series?

That is right, but it is not difficult to do.  For instance the LED could control the base voltage of a transistor, so now the emitter voltage is constant, about 2 volts, despite changes in Vbe over temperature.  Put a resistor between the emitter and common and you have a temperature compensated constant current source.
 

Offline InfravioletTopic starter

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Re: Diode voltage drop stability/accuracy
« Reply #66 on: June 22, 2023, 05:10:17 pm »
How would that constant current source then get me a temperature compensated peak detector? Do you mean running an LED from the incoming sine signal, then in to a transistor's base, putting the collector to the high voltage rail and using then having the emitter run to the parallel cap and res of a peak detector?

Wouldn't the big voltage drop from an LED plus the B-to-E of a transistor also mean the peak detector would have very poor linearity and sensivity for the smalelr amplitude input signals in my plausible range?

Also, I keep wondering about whether a two transistor differential amplifier can be modified to form a peak detector. My thoughts are that if it could then the use of two transistors in that manner would compensate temperature effects? I can't find any search results about the idea though.

Thanks
« Last Edit: June 22, 2023, 05:16:54 pm by Infraviolet »
 

Online David Hess

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Re: Diode voltage drop stability/accuracy
« Reply #67 on: June 22, 2023, 05:32:25 pm »
How would that constant current source then get me a temperature compensated peak detector? Do you mean running an LED from the incoming sine signal, then in to a transistor's base, putting the collector to the high voltage rail and using then having the emitter run to the parallel cap and res of a peak detector?

It does not, unless your peak detector requires a temperature compensated current source.

Quote
Wouldn't the big voltage drop from an LED plus the B-to-E of a transistor also mean the peak detector would have very poor linearity and sensivity for the smalelr amplitude input signals in my plausible range?

There is an alternative peak detector design which replaces the diodes with bipolar transistors.  The advantage is that the collector supplies the emitter current instead of the base.  See figure 97 on page 44 of Linear Technology application note 47.  The transistor Vbe voltages still need to be matched.

Quote
Also, I keep wondering about whether a two transistor differential amplifier can be modified to form a peak detector. My thoughts are that if it could then the use of two transistors in that manner would compensate temperature effects? I can't find any search results about the idea though.

An operational transconductance amplifier can be used to make a closed loop peak detector without the slew rate limitations of an operational amplifier.
 

Offline InfravioletTopic starter

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Re: Diode voltage drop stability/accuracy
« Reply #68 on: June 22, 2023, 10:20:01 pm »
Thanks for the app note idea. It looks like the following works as a peak detector, with pretty decent linearity with input voltage as compared to a simple diode based detector.

R28 and R29 give a low quiescent biasing point to the incoming signal, R30 affects the quiescent point of the output and is set large enough to prevent "clipping"* on the largest plausible input waveform, C16 and R31 set the decay rate (1n with 100K works nicely for my signal).

To temperature compensate it and protect against manufacturing variation all I have to do is have both the NPNs be in the same physical package? As long as both NPNs for each channel's copy of this are within a dual IC, that will be enough, wouldn't need to have the transistors of allchannels share an IC I hope? If its that simple then I think this could be the solution to my problems, guess I would no longer need a dummy channel to compare to it either with this method in use?

Thank you indeed

P.S. completely forgot t attach image, sorry, Also is this a circuit which ought to have emitter degeneration resistors added, or is this topology immune from needing them to improve stability? r_e_prime on this is, in theory, about 130 ohms on the left hand transistor and 1.3K on the right hand.
« Last Edit: June 22, 2023, 11:23:22 pm by Infraviolet »
 

Online David Hess

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Re: Diode voltage drop stability/accuracy
« Reply #69 on: June 23, 2023, 01:28:33 am »
R30 could be replaced with a current source for better performance.

The transistor Vbe voltages still need to be matched for best accuracy.  Usually just having them in close proximity is good enough for temperature tracking.
 

Offline InfravioletTopic starter

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Re: Diode voltage drop stability/accuracy
« Reply #70 on: June 23, 2023, 02:56:35 pm »
"The transistor Vbe voltages still need to be matched for best accuracy"
Matching, in this context, is mostly thermal. Two transistors in close proximity, or ideally in the same IC, even if it isn't truly monolithic inside, are all that is required, not selecting specially manufactured matched transistors? And no need for emitter degeneration resistors here either? Am I right in understanding the only property of a transistor which has a strong effect on this circuit and is variable is Vbe, and with this topology the two transistors cancel any changes they both undergo.

P.S. are there any keywords I can use online to find more information about the circuit type I show in reply #68. The app note didn't give it a name as such, and while I think it is related to a current mirror circuit, those don't seem to take input signals on to NPN's emitters.
« Last Edit: June 23, 2023, 03:26:36 pm by Infraviolet »
 

Offline TimFox

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Re: Diode voltage drop stability/accuracy
« Reply #71 on: June 23, 2023, 03:48:07 pm »
The actual VBE of an individual transistor of a given part number is a (presumably weak) function of the actual geometry, which is going to be different during manufacture from unit to unit.
Monolithic dual transistors are inherently matched, since the two geometries are formed together and should match each other well.
Measuring VBE of individual transistors (at a given collector voltage and current) from a batch received from Mouser (probably from the same production lot) is straightforward.
Thermal matching thereafter is a matter of design:  you can epoxy TO-92 packages together on their flat side, for example.
 

Offline wizard69

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Re: Diode voltage drop stability/accuracy
« Reply #72 on: June 23, 2023, 08:15:05 pm »
Cn I quickly check how stable the 0.7V drop across a typical small signal diode is, and how accurate between diodes of the same model.
If this is production size devices then you have to consider difference between batches and manufactures.
Quote
If one is in a situation where several separate analog signals are all passed through diodes* then can one rely (to within what percentage accuracy 1%, 5%, 10% ,20%...) on all the signals being dropped by the same amount?
Thanks
Quote
That all depends upon what you mean by the same amount, the imple answer is NO.  I'm not sure why you are running signals through diodes but this sounds like a device demanding calibration.
Quote

*for my situation I can definitely assume small currents, so no signficant heating of the diodes from this, and all physically close together so all the diodes would be getting the same rise or fall in ambient temperature if the environment around the PCB gets hotter or colder

Like I said above not sure what you are doing here but this sounds like an instrument that would need calibration and characterization.   However that depends a lot on what sort of error you can tolerate or correct.
 

Offline InfravioletTopic starter

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Re: Diode voltage drop stability/accuracy
« Reply #73 on: June 23, 2023, 11:13:27 pm »
I think its probably got to the point where the only thing to really do is some temperature testing. I don't have much in the way of equipment to get things to precise temperatures, let alone to keep them there when in the breadboard, but...

I'll take a particular pair of transistors, see how they peform in this application at room temperature, then let them heat sitting on a 3d printer bed for a while and try again, then pop them in the fridge for a few hours and another try. That gives a far larger temperature range than I'll actually need the finished peak detector to work for, so it will show an extreme limit of how bad temperature variation can be. Maybe it won't be so bad afterall, I might be over-worrying about these sorts of effects simply becaue I haven't done anything analogue and precise with discrete transistors before, it just I see temperature and manufacturing variation effects mentioned in the context of discretes where you never hear them warned of for op amps or digital chips. If it is enough to be troublesome, calibration is the way to go I guess, I can think of an easy way to make it automatic* and provide a dummy channel for reference which can give datapoints for a signal of known size and for a DC state.

*I'm not mass producing these, but I am making a sort of module which can be included in a variety of future projects and I like to ensure any I build can be quickly put in to use and not need huge amounts of probing for individual calibration, but I think there might be a nice way to make it self-calibrating.
 

Offline InfravioletTopic starter

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Re: Diode voltage drop stability/accuracy
« Reply #74 on: June 25, 2023, 12:28:22 am »
The dual transistor peak detector design (see image in my reply #68) inspired by Linear App Note 47, Fig 97, doesn't seem to be anything like as linear* in the real world as it was in simulation. Any suggestions as to why? In sim, with the values shown, it gave a a straight line for inputs amplitudes above about 0.1V, which outperformed any form of diode of B-E junction based peak detector I tried. On breadboard it is behaving much worse than any diode detector, for larger input amplitudes the peak detector DC voltage falls far below that expected from a linear equation, and farther below even than from the diode of B-E junction detectors.

Why is this working so much worse in the real world than with simulations?

*in this context I mean how closely the output DC voltage follows a linear equation of the form Vo=A*Vpp_in+Offset

Thanks
 

Offline InfravioletTopic starter

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Re: Diode voltage drop stability/accuracy
« Reply #75 on: June 26, 2023, 09:42:04 pm »
Ok, so I did a test with the basic single diode (or single b-e junction of an NPN transistor) detector. It LOOKS like temperature barely affects the result at all, I chilled some transistors and diodes over night in a freezer, got them out during the day handling with tweezers so my wrm fingers wouldn't heat them too fast, plugged them in to the breadboard layout and watched whether the voltage drop between a fixed amplitude input sine wave an he detected DC output changed over time after they were plugged in as the parts were warming up to about the 25 degrees C summer room temperature. No noticable change over time from plugging them in for many tens of minutes, nothing more than the 10mV or so of range which the signal flucuated over anyway (probably from not-quite perfect conatct on a breadboard wire somewhere).

...either that or the heat capacity of the through hole components I breadboard tested with is so tiny they reach room temperature within seconds of being lifted from a -10C freezer and popped in to a breadboard.
 

Offline Vovk_Z

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Re: Diode voltage drop stability/accuracy
« Reply #76 on: June 27, 2023, 07:04:58 am »
1. Yes, small components change temperature quite fast.
2. It is often easier to test temperature dependence by heating component with  available instrument. E.g. heat-gun.
 

Offline InfravioletTopic starter

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Re: Diode voltage drop stability/accuracy
« Reply #77 on: July 01, 2023, 02:39:51 pm »
I've had some more thoughts, can anyone suggest if they make sense:

I need 3 channels of signal, I need to accurately know the ratio of the peak-to-centre height of all of them.

I'm thinking with 3 channels plus one dummy channel solely doing the peak detection section I should have enough stability?

All sections of the 3 channels' signal processing before the peak detection can be done within a quad transistor IC for each section (MMPQ3904 looks to be about the only quad NPN made any more, but seems to be from multiple manufacturers so fairly generic and not too likelyto stop being available, CA3046 was popular in things I've read but looks to be out-of-production, MMPQ2222 looks to be nearing end-of-life, many other transistor array ICs aren't independent transistors at all, but rather arrays of darlingtons with certain pins commoned) of processing. With 3 signals undergoing any give process within an IC containing the relevant transistor for each channel then any temperature variations or manufacuring differences between ICs ought to affect all channels equally. So the ratio between signal amplitudes ought to remain consistent regardless of environemtnal or manufactruing variations?

It is only at the peak detection stage that my signals get coupled to a fixed DC reference level, until then they could all be centred about different reference levels and it wouldn't change anything. So at this point if I use a fourth channel in the IC* as a dummy channel with a peak detection circuit around it, but just feed it the fixed level to which other channels get their signals coupled, then in theory any variations will affect all 4 channels of peak detection equally. If the "diode" drop of the peak detector changes then the dummy channel will drop by the same amount as the other 3, meaning the difference between each of them and the dummy would stay constant. 

Then as the difference between the real channels and the dummy should stay constant, and the ratio of signal strengths given in earlier stages ought to be kept constant by having shared ICs, I would get stable output values when comparing the ratios of (signalXpeakdetected-dummyChannelVoltage) vs (signalYpeakdetected-dummyChannelVoltage) vz (signalZpeakdetected-dummyChannelVoltage)?

And my guess is this would even make thos ratios stable against slight variation in supply voltage too?

Thanks

*as NPN's can be used like diodes, and I couldn't find a quad diode array which didn't have commoning of one pin or the other
 


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