Author Topic: EEVblog #1157 - Transistor Zener Clamp Circuit  (Read 5015 times)

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Offline EEVblog

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EEVblog #1157 - Transistor Zener Clamp Circuit
« on: December 13, 2018, 09:10:37 pm »
A look at the dual bipolar transistor zener clamp circuit, and it's usefulness as a fast low leakage overload clamp.
The first part is lifted from video #1000 with 12 minutes of extra commentary.

Interview with Dave Taylor, designer of the famous Fluke 8060A multimeter:
https://theamphour.com/180-an-interview-with-dave-taylor-multi-talented-meter-maker/

 
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Offline MT

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Re: EEVblog #1157 - Transistor Zener Clamp Circuit
« Reply #1 on: December 13, 2018, 11:22:26 pm »
IMX9/25/26 Vebo are mentioned in the electrical characteristics. High Vebo trannies is used by the audio industry in AC mode to mute popping on outputs when power up/down, base needs to be floating (PNP driver) else it will acts a diode and clip out signal.
« Last Edit: December 13, 2018, 11:35:37 pm by MT »
 

Offline johnlsenchak

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Re: EEVblog #1157 - Transistor Zener Clamp Circuit
« Reply #2 on: December 14, 2018, 12:05:59 am »

Great informative  video, would like to see more like this  !  Thumbs up!

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Offline Dr. Frank

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Re: EEVblog #1157 - Transistor Zener Clamp Circuit
« Reply #3 on: December 14, 2018, 12:23:07 am »
Dave,

using a bipolar transistors reversed  BE diode, or n-JFETs to protect sensitive amplifier inputs, with ultra-low leakage currents ~pA, has been very common in metrology circuits.
Fluke and also Keithley often use that, maybe hp also, but I could not tell, who was the earliest adopter.
Flukes 845A null detector/µV-amplifier is from 1965, but still uses these ultra low leakage diodes, like CD12599, which all cost a fortune.

I think, I've seen that transistor solution already in early 1970ties circuits.

What's missing in your nice video, is a measurement on this leakage current at higher voltages, and a measurement on the zener voltage of the transistors.
In the 121GW, on its 50MOhm and 5 MOhm ranges, it's  important to keep these currents really low, in the low pA range.

If high break-down voltage transistors are used, there may be higher leakage currents than on a 'normal' 5..7V break down-type. 

I've already investigated on that 121GW problem a few months back: https://www.eevblog.com/forum/testgear/eevblog-121gw-multimeter-issues/msg1770224/#msg1770224, and my final solution, using a jelly-bean n-JFET, which D-G or D_S diode withstands up to 30V, has a few pA leakage only:
https://www.eevblog.com/forum/testgear/eevblog-121gw-multimeter-issues/msg1790081/#msg1790081

So this protection, either using n-JFETs, or (leakage - selected) bipolar transistors, will cure the 5M / 50M problem, but requires a re-calibration of the 121GW, as the temperature dependent leakage currents
of the former 1N4001s had been included in the former calibration.

Frank

PS: And yes, I'm glad, that UEI did NOT use TVS in that place!

PPS: Does this circuit now really protect the input of U9?

It looks like it replaces D8 only, that means that R_RLD is clamped at an (unspecified) BE-zener voltage of the transistor of maybe +/-25V, or more.
If IC U9 is operated  at either 4V or 15V, that might damage its input pin15. And this voltage should not go below -0.6V.

I think, that circuit is a complete fail in this application.
« Last Edit: December 14, 2018, 01:19:37 am by Dr. Frank »
 
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Offline TinyMirrors

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Re: EEVblog #1157 - Transistor Zener Clamp Circuit
« Reply #4 on: December 14, 2018, 04:07:01 am »
I tried simulating this in lt spice and no dice! Well no clamping anyways. I guess I'll have to build it in real life.
 

Offline pc2005

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Re: EEVblog #1157 - Transistor Zener Clamp Circuit
« Reply #5 on: December 14, 2018, 06:12:05 am »
Hi,

It seems a modified version of the circuit was used in the eastern bloc probably from like late '60 to early '90. In my country it was in every TV made and it was used for stabilization of ~30V tuning voltage (varicap in superheterodyne tuner). The original device seems to be TAA550.

The most common problem was degradation of the part so a tuned channel became really unstable.
 

Offline NANDBlog

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Re: EEVblog #1157 - Transistor Zener Clamp Circuit
« Reply #6 on: December 14, 2018, 11:27:42 am »
Anyone knows transistors that are defining the base-emitter voltage not in the "Absolute maximum ratings" part of the datasheet? Or where they define that this is a normal operation of the transistor? I'm asking because I'm designing intrinsically safe circuits ( ATEX ) and it would be nice to use these techniques, for high power (or low leakage) zener circuits. But the notified body would just throw this back that we exceed 2/3 of the maximum rating (requirements for such circuits) so we are not compliant. 
 

Offline santiall

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Re: EEVblog #1157 - Transistor Zener Clamp Circuit
« Reply #7 on: December 14, 2018, 12:16:52 pm »
Japanese use to mute the outputs of professional audio devices with BJTs. It is a bit puzzling the first time you see that and even more once you try the circuit with a normal transistor and doesn't work till you learn about the reverse EB voltage :D

For example Yamaha has been using 2SD1915 (25V) BJTs in many products and lately the INC2002AM1 (50V). I guess no wonder that those 'muting' specific transistors are mostly Japanese.
 

Offline AndersJ

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Re: EEVblog #1157 - Transistor Zener Clamp Circuit
« Reply #8 on: December 14, 2018, 12:21:44 pm »
I don't get it.
How do you mute audio using the reverse EB voltage?
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Offline santiall

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Re: EEVblog #1157 - Transistor Zener Clamp Circuit
« Reply #9 on: December 14, 2018, 12:27:17 pm »
you mute when the transistor is conducting, what you want is a high reverse voltage so when the transistor is off, when not muting, you don't clip half of the audio signal.
I'm attaching the output of the 0V96 digital mixer so you can see how they do it

EDIT, an interesting link with more details: https://www.electroschematics.com/9660/muting-transistor-attenuator-circuits-2sc2878/
« Last Edit: December 14, 2018, 12:36:46 pm by santiall »
 
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Offline mrpackethead

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Re: EEVblog #1157 - Transistor Zener Clamp Circuit
« Reply #10 on: December 14, 2018, 01:53:36 pm »
Anyone knows transistors that are defining the base-emitter voltage not in the "Absolute maximum ratings" part of the datasheet? Or where they define that this is a normal operation of the transistor? I'm asking because I'm designing intrinsically safe circuits ( ATEX ) and it would be nice to use these techniques, for high power (or low leakage) zener circuits. But the notified body would just throw this back that we exceed 2/3 of the maximum rating (requirements for such circuits) so we are not compliant.

Not sure about how those rules work, but are you saying that if you had a 15V zener you would only be able to use it up to 10V?
On a quest to find increasingly complicated ways to blink things
 

Online Kleinstein

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Re: EEVblog #1157 - Transistor Zener Clamp Circuit
« Reply #11 on: December 14, 2018, 03:00:48 pm »
There is another downside on using a transistor as a zener: the tolerance of the break down voltage. The absolute maximum specs are kin dof minimum values. The actual value can be quite a bit higher.

The maximum limits rule applies to the absolute maximum ratings only, as exceeding these could damage the part. So if a 15 V zener would have a maximum rating of lets say 18 V you are stuck. It's stupid rules because it's just a question of having the numbers there or not.
Using the transistor as a zener is not an intended function - so no normal specs on this.
 

Offline ggchab

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Re: EEVblog #1157 - Transistor Zener Clamp Circuit
« Reply #12 on: December 14, 2018, 04:00:04 pm »
Very interesting video  :-+
Happy to see again this type of content  :)
 

Offline NANDBlog

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Re: EEVblog #1157 - Transistor Zener Clamp Circuit
« Reply #13 on: December 14, 2018, 05:21:49 pm »
Anyone knows transistors that are defining the base-emitter voltage not in the "Absolute maximum ratings" part of the datasheet? Or where they define that this is a normal operation of the transistor? I'm asking because I'm designing intrinsically safe circuits ( ATEX ) and it would be nice to use these techniques, for high power (or low leakage) zener circuits. But the notified body would just throw this back that we exceed 2/3 of the maximum rating (requirements for such circuits) so we are not compliant.

Not sure about how those rules work, but are you saying that if you had a 15V zener you would only be able to use it up to 10V?
I need to de-rate anything for current, voltage and power (amongst others) to 2/3 of it's original rating. That is for stuff which is in the "absolute maximum rating" part, and Vbe is there. So "normally" i wouldn't be able to use these transistors in this configuration, because it is exceeding that rating.

For a zener, I only need to investigate for current and power of course, because the zener voltage is not a rating, it is "electrical characteristics".

Basically I'm asking if there are transistors specifically designed for this, with reliably doing this breakdown, with the manufacturer stating that it is made for it.

There is another downside on using a transistor as a zener: the tolerance of the break down voltage. The absolute maximum specs are kin dof minimum values. The actual value can be quite a bit higher.
That doesnt sound too good for my application.
 

Offline T3sl4co1l

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Re: EEVblog #1157 - Transistor Zener Clamp Circuit
« Reply #14 on: December 14, 2018, 06:00:11 pm »
I tried simulating this in lt spice and no dice! Well no clamping anyways. I guess I'll have to build it in real life.

SPICE doesn't model breakdown of BJTs, you need to put a zener in parallel with the respective junctions.  (The zener can be an ideal type, i.e., N = 20 and CJO=0 so it doesn't interfere with normal operation, and RS, BV and IBV set to model breakdown.)

I'm not sure anyone has modeled punch-through breakdown, i.e., the effect that gives rise to relaxation oscillators using E-B or C-B breakdown (the latter of which is famous for giving sub-nanosecond edges).  Extreme nonlinearities are not easy to model, or to simulate in SPICE.

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Offline artag

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Re: EEVblog #1157 - Transistor Zener Clamp Circuit
« Reply #15 on: December 14, 2018, 07:19:35 pm »
HP1740A .. that was the pride of my bosses' workshop back in 1980.
 

Offline imo

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Re: EEVblog #1157 - Transistor Zener Clamp Circuit
« Reply #16 on: December 14, 2018, 08:02:31 pm »
I'm not sure anyone has modeled punch-through breakdown, i.e., the effect that gives rise to relaxation oscillators using E-B or C-B breakdown (the latter of which is famous for giving sub-nanosecond edges).  Extreme nonlinearities are not easy to model, or to simulate in SPICE.
There is such a model with the 2N2369A for LTSpice in "Bordodynov library"
 
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Offline SiliconWizard

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Re: EEVblog #1157 - Transistor Zener Clamp Circuit
« Reply #17 on: December 14, 2018, 08:24:07 pm »
I tried simulating this in lt spice and no dice! Well no clamping anyways. I guess I'll have to build it in real life.

Had to dig up a little on this one, but it turns out LTSpice does model the collector-base breakdown voltage and base-emitter BV (BVcbo and BVbe parameters) in its BJT model, but doesn't model the collector-emitter BV as far as I've seen (look up "Q. Bipolar Transistor" in LTSpice's help.) Some other Spice-based simulators may include breakdown voltage parameters as well, but as far as I've seen, ngspice doesn't (unless its doc. is not up to date).

Unfortunately, the base transistor models shipped with LTSpice haven't been updated to make use of those parameters. These models can be found in the "standard.bjt" file in LTSpice's "lib\cmp" subdirectory.

I added the following model (modified 2N3904):
Code: [Select]
.model 2N3904_BD NPN(IS=1E-14 VAF=100 Bf=300 IKF=0.4 XTB=1.5 BR=4 CJC=4E-12 CJE=8E-12 RB=20 RC=0.1 RE=0.1 TR=250E-9 TF=350E-12 ITF=1 VTF=2 XTF=3 Vceo=40 Icrating=200m BVcbo=60 BVbe=6)
(values taken from a Fairchild 2N3904 datasheet)

With the attached LTSpice schematic, the voltage gets clamped at about +/- 8.3V.

 
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Offline TinyMirrors

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Re: EEVblog #1157 - Transistor Zener Clamp Circuit
« Reply #18 on: December 14, 2018, 11:24:53 pm »

I added the following model (modified 2N3904):
Code: [Select]
.model 2N3904_BD NPN(IS=1E-14 VAF=100 Bf=300 IKF=0.4 XTB=1.5 BR=4 CJC=4E-12 CJE=8E-12 RB=20 RC=0.1 RE=0.1 TR=250E-9 TF=350E-12 ITF=1 VTF=2 XTF=3 Vceo=40 Icrating=200m BVcbo=60 BVbe=6)

This worked perfectly! I'm gunna dig up that datasheet and take a look. Great post!
 

Offline David Hess

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Re: EEVblog #1157 - Transistor Zener Clamp Circuit
« Reply #19 on: December 14, 2018, 11:38:49 pm »
Dave, you asked about the history of these transistors.

They were what was used before JFETs became available for chopper and sampling circuits and they often had completely symmetrical construction so the Vcb and Veb were identical.  This yielded poor gain compared to an asymmetrical device however it improves the saturation voltage and I think the Early voltage also.

As mentioned above, they were used in audio muting applications and this was done even after JFETs became available because they can operate at very low voltages.  Note that when the emitter is forward biased, the transistor will conduct in either direction making them useful in AC applications.  This is where they are used as shunt choppers in audio muting applications and the Rigol DS1000Z schematics you made show the same configuration for the bandwidth limiting circuits which I have seen in some other oscilloscopes.  A normal transistor works for this but a chopper transistor works better.

Some analog circuits use the reverse connection of a standard bipolar transistor for its low saturation voltage to do things like reset an integration capacitor but earlier a chopper transistor would have been used.  Early low offset and offset drift chopper amplifiers used them.

Below is an except from the GE Transistor Manual discussing chopper transistors and their circuits.  I have this in paperback but copied it out of an existing PDF available online which shows they had a 50 microvolt matched device in the 1960s.

Tektronix used the dual series transistor chopper configuration twice in their 7T11 sampling sweep as shown below but I am not sure why since JFETs were readily available at that time.  My guess is that the transformer coupled bipolar chopper has less charge injection.  The chopper output drives a hold capacitor which is buffered by a JFET source follower or a very simple JFET operational amplifier.  In other places in the 7T11, a JFET transistor was used for chopping instead so Tektronix was certainly aware of their capability.

Q400 and Q402 are just 2N3904s demonstrating that there is nothing special required to use this configuration.  Q446 and Q448 use the 2N3563 is a 600-1500MHz 20-200hfe 12Vce 50maIc part designed as an RF amplifier but it is not gold doped; they might have used it for less feedthrough capacitance.

I'm not sure anyone has modeled punch-through breakdown, i.e., the effect that gives rise to relaxation oscillators using E-B or C-B breakdown (the latter of which is famous for giving sub-nanosecond edges).  Extreme nonlinearities are not easy to model, or to simulate in SPICE.

The GE book linked above mentions that this behavior is what makes avalanche switching work.  I have seen it several times on my curve tracer without realizing its significance which suggests an easy way to look for suitable transistors.
 

Offline floobydust

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Re: EEVblog #1157 - Transistor Zener Clamp Circuit
« Reply #20 on: December 14, 2018, 11:41:25 pm »
Audio muting transistors i.e. 2SC2878 have a high reverse-hFE rating and higher VEBO 25V.

A big issue is clamp leakage current verse temperature, you can measure a nice low leakage current at room temp but I usually find it's 10X higher with a 10°C temperature increase.
 

Offline SilverSolder

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Re: EEVblog #1157 - Transistor Zener Clamp Circuit
« Reply #21 on: December 15, 2018, 03:56:37 am »
I thought the BE junction gets damaged over time, with a reverse current?
 

Offline David Hess

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Re: EEVblog #1157 - Transistor Zener Clamp Circuit
« Reply #22 on: December 15, 2018, 04:33:21 am »
I thought the BE junction gets damaged over time, with a reverse current?

It does but this does not matter for a low leakage clamp or Vbe zener diode.
 

Offline EEVblog

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Re: EEVblog #1157 - Transistor Zener Clamp Circuit
« Reply #23 on: December 15, 2018, 05:42:38 am »
you mute when the transistor is conducting, what you want is a high reverse voltage so when the transistor is off, when not muting, you don't clip half of the audio signal.
I'm attaching the output of the 0V96 digital mixer so you can see how they do it

EDIT, an interesting link with more details: https://www.electroschematics.com/9660/muting-transistor-attenuator-circuits-2sc2878/

I'd never really given it a thought before, as line level stuff would be fine for a regular tranny. But yeah, higher voltage line signal need a special snowflake.
 

Offline santiall

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Re: EEVblog #1157 - Transistor Zener Clamp Circuit
« Reply #24 on: December 15, 2018, 06:28:48 am »
I'd think that when creating those type of transistors the designers were aiming for true symmetrical BJTs with a good negative hFE and the high reverse voltage is a necessary evil. They are specified as switches and mute transistors, not as clamps so I'd think that'd be their origin rather than the other way around.

As said, they have been extremely popular in Japanese electronics for muting due the very low on-resistance, lower than FETs, and probaly cheaper and more consistent too than FETs.

 


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