Author Topic: How hard is it a make a transistor opamp like LM358 ? Chances of working ?  (Read 4501 times)

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Online Zero999

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Re: How hard is it a make a transistor opamp like LM358 ? Chances of working ?
« Reply #25 on: January 21, 2018, 05:19:42 am »
Yes, it's effectively three transistors with the bases and emitters connected together. In real life it's probably just one transistor, but with multiple collectors.
 

Offline David Hess

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Re: How hard is it a make a transistor opamp like LM358 ? Chances of working ?
« Reply #26 on: January 21, 2018, 09:16:15 am »
Yes, it's effectively three transistors with the bases and emitters connected together. In real life it's probably just one transistor, but with multiple collectors.

But usually schematics do not show the ratio of emitter areas so some reverse engineering to determine the operating point is needed.
 

Offline edavid

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Re: How hard is it a make a transistor opamp like LM358 ? Chances of working ?
« Reply #27 on: January 21, 2018, 09:57:50 am »
Where are you going to get those dual- and triple-collector transistors?  Look at Q19 in your schematic.

That's IC shorthand for "B and E wired in parallel". :)

Similarly if you see multiple emitters (but you'll never see multiple bases), though those can actually be constructed as a single device, which means you get some hFE between emitters (in a half-inverted configuration, so to speak), whereas you get no hFE from discrete devices wired in that way.  This is probably never used as a feature.

No, they are lateral PNPs, so they really are single devices with multiple collector contacts.  See page 1-23 of Hans Camenzind's book: http://www.designinganalogchips.com/

 

Online T3sl4co1l

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Re: How hard is it a make a transistor opamp like LM358 ? Chances of working ?
« Reply #28 on: January 21, 2018, 10:08:59 am »
No, they are lateral PNPs, so they really are single devices with multiple collector contacts.  See page 1-23 of Hans Camenzind's book: http://www.designinganalogchips.com/

Well, that's basically what I said about emitters, using a pile of emitters, isn't it? ;)

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Bringing a project to life?  Send me a message!
 

Online Zero999

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Re: How hard is it a make a transistor opamp like LM358 ? Chances of working ?
« Reply #29 on: January 21, 2018, 10:28:17 am »
Yes, it's effectively three transistors with the bases and emitters connected together. In real life it's probably just one transistor, but with multiple collectors.

But usually schematics do not show the ratio of emitter areas so some reverse engineering to determine the operating point is needed.
The simplified schematics on the other data sheets are quite helpful.
http://www.st.com/content/ccc/resource/technical/document/datasheet/61/46/87/01/98/ed/44/c5/CD00000464.pdf/files/CD00000464.pdf/jcr:content/translations/en.CD00000464.pdf
http://www.ti.com/lit/ds/symlink/lm358.pdf
 

Offline amspire

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Re: How hard is it a make a transistor opamp like LM358 ? Chances of working ?
« Reply #30 on: January 21, 2018, 12:02:40 pm »
A thing that has always annoyed me a little is you see components of cheap IC's that are almost impossible to get as discrete devices. Things like super-beta transistors.

In the case of the LM358, those input PNP's have a reverse base emitter breakdown voltage over 30V. This is nice because it is one of the few opamps that can have to the inputs connected permanently to, say, a 12v or 24v voltage even when the opamp power is off.

I don't remember ever seeing a junction transistor with a breakdown voltage more then 8V. Sure the 30V breakdown transistor probably has much lower gain, but it would still be good to have it available.
 

Offline edavid

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Re: How hard is it a make a transistor opamp like LM358 ? Chances of working ?
« Reply #31 on: January 21, 2018, 12:37:19 pm »
A thing that has always annoyed me a little is you see components of cheap IC's that are almost impossible to get as discrete devices. Things like super-beta transistors.
Buy a bag of 2N5961/2N5962/2N5963 while they are still around.

Quote
In the case of the LM358, those input PNP's have a reverse base emitter breakdown voltage over 30V. This is nice because it is one of the few opamps that can have to the inputs connected permanently to, say, a 12v or 24v voltage even when the opamp power is off.

I don't remember ever seeing a junction transistor with a breakdown voltage more then 8V. Sure the 30V breakdown transistor probably has much lower gain, but it would still be good to have it available.
You could use a regular PNP in inverted mode.

 
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Offline amspire

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Re: How hard is it a make a transistor opamp like LM358 ? Chances of working ?
« Reply #32 on: January 21, 2018, 01:12:59 pm »
In the case of the LM358, those input PNP's have a reverse base emitter breakdown voltage over 30V. This is nice because it is one of the few opamps that can have to the inputs connected permanently to, say, a 12v or 24v voltage even when the opamp power is off.

I don't remember ever seeing a junction transistor with a breakdown voltage more then 8V. Sure the 30V breakdown transistor probably has much lower gain, but it would still be good to have it available.
You could use a regular PNP in inverted mode.
I did think of that, but you need 30v breakdown for both junctions.
 

Offline David Hess

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Re: How hard is it a make a transistor opamp like LM358 ? Chances of working ?
« Reply #33 on: January 21, 2018, 01:24:25 pm »
I don't remember ever seeing a junction transistor with a breakdown voltage more then 8V. Sure the 30V breakdown transistor probably has much lower gain, but it would still be good to have it available.

I linked an entire list of currently available parts like this earlier but most are surface mount now.  They used to be known as "chopper" transistors but they were also used in audio muting applications.

I don't remember ever seeing a junction transistor with a breakdown voltage more then 8V. Sure the 30V breakdown transistor probably has much lower gain, but it would still be good to have it available.

You could use a regular PNP in inverted mode.

This will only work for low voltage where the transistor would not need to be inverted anyway.  This swaps breakdown of the base-emitter junction at high differential input voltages for breakdown of the base-collector junction at high common mode voltages.  If your operational amplifier is only going to run on a 5 volt supply voltage, then nothing needs to be done.

What will work is adding a diodes in series with the emitters which is likely needed in one form or another anyway for a single supply input but this sacrifices gain and the offset voltage of the diodes add to the input offset voltage.  Matched diodes or matched diode connected transistors will ameliorate the problems with precision.  I have seen integrated and discrete operational amplifier designs which did this.
 

Offline David Hess

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Re: How hard is it a make a transistor opamp like LM358 ? Chances of working ?
« Reply #34 on: January 21, 2018, 01:25:45 pm »
Yes, it's effectively three transistors with the bases and emitters connected together. In real life it's probably just one transistor, but with multiple collectors.

But usually schematics do not show the ratio of emitter areas so some reverse engineering to determine the operating point is needed.

The simplified schematics on the other data sheets are quite helpful.

Oh, they are very helpful.  And in the case of the LM324 and many other parts, complete schematics are available.  But even the complete schematics do not usually show emitter ratios.

https://www.onsemi.com/pub/Collateral/LM324-D.PDF

Note that the above example shows the value of the 5pF compensation capacitor with goes along with Gm reduction of the differential input which is also shown; the later allows the former.  Implementing Gm reduction in a discrete copy of the integrated design without compromising performance will be very difficult because now you need *four* matched PNP transistors.

It is trivial to do by adding emitter degeneration resistors however this compromises offset voltage and noise.  Video amplifiers including the old LM318 did this and audio power amplifiers do this for increased slew rate.  Without Gm reduction, the compensation capacitance would be about 30pF making the chip larger and more expensive because the compensation capacitor takes up a lot of space.
« Last Edit: January 21, 2018, 03:42:48 pm by David Hess »
 

Offline Richard Crowley

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Re: How hard is it a make a transistor opamp like LM358 ? Chances of working ?
« Reply #35 on: January 21, 2018, 02:34:51 pm »
In pro retro audio circles, there are a several discrete component op-amp circuits still quite popular and sourced by multiple vendors.  And some of them available as kits.





 
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Offline RandallMcRee

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Re: How hard is it a make a transistor opamp like LM358 ? Chances of working ?
« Reply #36 on: January 21, 2018, 03:21:41 pm »
It seems to me that this thread has wandered off from the OP's original intent.

First, Mr. OP you should not be trying to build a discrete opamp using a schematic for an integrated circuit opamp. The strengths of integrated circuits are not the same strengths available to you. Unless you are one of those students who is designing a circuit for some CMOS fab. But, no, this is 'Beginners'.

Therefore, you should simply google "discrete opamp" and follow your nose. There are some good designs out there. There is a sort of competition to make the best discrete opamp using the fewest transistors. Some of them seem to be quite good.

Samuel Groner has some good low-noise designs, for example, e.g. http://www.nanovolt.ch/resources/discrete_opamps/
 
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Offline David Hess

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Re: How hard is it a make a transistor opamp like LM358 ? Chances of working ?
« Reply #37 on: January 21, 2018, 03:46:56 pm »
In pro retro audio circles, there are a several discrete component op-amp circuits still quite popular and sourced by multiple vendors.  And some of them available as kits.

My preference would be to use an integrated operational amplifier with a discrete output stage and maybe a discrete input stage if necessary.  In precision applications, self heating from the output stage is what ultimately limits performance.

First, Mr. OP you should not be trying to build a discrete opamp using a schematic for an integrated circuit opamp. The strengths of integrated circuits are not the same strengths available to you. Unless you are one of those students who is designing a circuit for some CMOS fab. But, no, this is 'Beginners'.

Therefore, you should simply google "discrete opamp" and follow your nose. There are some good designs out there. There is a sort of competition to make the best discrete opamp using the fewest transistors. Some of them seem to be quite good.

Most discrete audio power amplifiers use the traditional 3 stage operational amplifier topology so a lot can be learned from them.  As they get more complex, they include the discrete equivalents of what more advanced integrated operational amplifiers use.

« Last Edit: January 21, 2018, 03:49:54 pm by David Hess »
 

Online Zero999

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Re: How hard is it a make a transistor opamp like LM358 ? Chances of working ?
« Reply #38 on: January 22, 2018, 04:03:22 am »
In pro retro audio circles, there are a several discrete component op-amp circuits still quite popular and sourced by multiple vendors.  And some of them available as kits.
They're mostly audiophool products, aimed at silly people who blindly replace op-amp ICs in pieces of equipment, in the hope it will magically improve the sound quality, when in reality, it's more likely to bugger it up.

It seems to me that this thread has wandered off from the OP's original intent.
Fair point. Awhile ago I did an LTSpice simulation of the LM358, made with discrete transistors, in the hope of predicting whether the power consumption increases or decreases, when the output saturates to the positive rail. Thread linked below:
https://www.eevblog.com/forum/beginners/unused-opamps-within-a-multi-opamp-package/msg1074284/#msg1074284

I've modified it to work with discrete transistors. Note that this will still not behave exactly like the LM358. The input bias currents will be much lower, as the Hfe of the BC557 is higher than the transistors used inside the IC. The maximum input voltage will also be limited to 10V. If you  need more, add diodes such as the 1N4148, in series with the inputs.

« Last Edit: January 22, 2018, 05:13:05 am by Hero999 »
 

Offline schmitt trigger

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Re: How hard is it a make a transistor opamp like LM358 ? Chances of working ?
« Reply #39 on: January 22, 2018, 12:09:00 pm »
In these days where a diy project usually means some form of Arduino-based circuit, I applaud the individuals who have the desire to tackle a discrete component project.

So I say: of course give it a try!
In the end, with all the money you will spend, you would have purchased a cutting edge premium opamp and still have some spare change, yet most likely its performance will be lower than the lowliest opamp.

However, the learning process will be priceless.
 

Offline David Hess

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Re: How hard is it a make a transistor opamp like LM358 ? Chances of working ?
« Reply #40 on: January 22, 2018, 01:00:02 pm »
In the end, with all the money you will spend, you would have purchased a cutting edge premium opamp and still have some spare change, yet most likely its performance will be lower than the lowliest opamp.

The only places where performance cannot be better or at least close is where matched transistors are required and even there, hybrid and integrated duals, quads, and arrays can make up for this and where construction limits high frequency performance.  The trade offs are a little different but in most cases, better performance is possible for selected characteristics.

One big advantage of a discrete design is that you can add functionality like clamping, compensation, and high voltage operation easily.  Usually though it is better to add discrete parts to an existing integrated operational amplifier.  For instance the input and output stages can be replaced or bootstrapping can be used for high voltage operation or to improve common mode and power supply rejection.
 

Offline iampoor

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Re: How hard is it a make a transistor opamp like LM358 ? Chances of working ?
« Reply #41 on: January 22, 2018, 04:37:28 pm »
In pro retro audio circles, there are a several discrete component op-amp circuits still quite popular and sourced by multiple vendors.  And some of them available as kits.
They're mostly audiophool products, aimed at silly people who blindly replace op-amp ICs in pieces of equipment, in the hope it will magically improve the sound quality, when in reality, it's more likely to bugger it up.

In pro audio retro circles (I like that phrase! :D) They are used for mic preamps, and other circuits where the non linear characteristics, and sonic effects are desirable. They are also used for driving low impedance loads, or output transformers. The distortion characteristics of some discrete opamps are desirable. This is very different from someone ripping apart a modern piece of gear, and putting in discrete opamps willy-nilly.

@OP Trying building an API 2520, or gar1731 opamp. Very easy circuits to build, and can be built for well under 10$ in parts.
 

Offline Wimberleytech

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Re: How hard is it a make a transistor opamp like LM358 ? Chances of working ?
« Reply #42 on: January 23, 2018, 01:35:39 am »
I built this long ago with the CD4007.  It is a great learning tool.  Vary the bias current and see the effect on open loop gain, GBW, slew rate, etc.  Vary the capacitor to play around with compensation.

I just sketched this out this morning and did not build it to make sure I got all the connections correct.
 
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Online Zero999

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Re: How hard is it a make a transistor opamp like LM358 ? Chances of working ?
« Reply #43 on: January 24, 2018, 09:53:20 pm »
How matched are the transistors in the CD4007?

I've read about using them in an op-amp before:
https://wiki.analog.com/university/courses/alm1k/alm-lab-ota
http://sites.bu.edu/engcourses/files/2016/08/mosfet-differential-amplifier.pdf
 
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Offline David Hess

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Re: How hard is it a make a transistor opamp like LM358 ? Chances of working ?
« Reply #44 on: January 24, 2018, 11:29:53 pm »
How matched are the transistors in the CD4007?

Since they are monolithic and have identical geometries, they are pretty good; I would expect offset voltages of 10s of millivolts which may not seem very good but MOSFETs do not match as well as JFETs which do not match as well as bipolar transistors.  The p-channel MOSFETs do not closely match the n-channel MOSFETs however.
 

Offline Wimberleytech

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Re: How hard is it a make a transistor opamp like LM358 ? Chances of working ?
« Reply #45 on: January 25, 2018, 01:29:25 am »
How matched are the transistors in the CD4007?

Since they are monolithic and have identical geometries, they are pretty good; I would expect offset voltages of 10s of millivolts which may not seem very good but MOSFETs do not match as well as JFETs which do not match as well as bipolar transistors.  The p-channel MOSFETs do not closely match the n-channel MOSFETs however.

Two critical parameters, oxide thickness and carrier mobility, will match well on a common die.  Even with these identical process parameters, matching (at the next level) depends on geometry.  Looking at the metal layer die image, it appears that no special care was taken to match.  The structures are serpentine (not best for matching), and not drawn identically (also adverse to matching).  Obviously no attempt to common centroid (why would they?)...etc.  Since the 4000 series are metal-gate CMOS, they will not give the matching performance of self-aligned Si-gate CMOS.

I designed an integrated pacemaker driver once and the company wanted to see a breadboard working (back when simulations were nascent).  So, I had to actually breadboard the amplifiers etc.  I did it using 4007s.  Wire wrapped no less.

Maybe today, I will do a test and see how well these match.  I suspect that 10s of millivolts is about right--prolly on the low side 10-20 mV though.

Oh, one more thing.  Since a design does not have independent control of W/L ratios, you cannot get an optimal first-order design, so there will be some systematic offset in the 7-transistor diff amp architecture.  With some gain in the diff stage, it is probably swamped by the diff pair mismatch though.

---here is an update on matching
I tested the nch devices using 135uA drain current in a diode configuration (VDS = VGS) and got the following VDS measurements:
1.588
1.600
1.594

12mV maximum spread for ONE (and only one) device.
---
« Last Edit: January 25, 2018, 02:30:05 am by Wimberleytech »
 

Offline Wimberleytech

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Re: How hard is it a make a transistor opamp like LM358 ? Chances of working ?
« Reply #46 on: January 25, 2018, 01:33:21 am »
How matched are the transistors in the CD4007?

I've read about using them in an op-amp before:
https://wiki.analog.com/university/courses/alm1k/alm-lab-ota
http://sites.bu.edu/engcourses/files/2016/08/mosfet-differential-amplifier.pdf

These are good links.  I had seen the first one, but not the second.
 

Offline Richard Crowley

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Re: How hard is it a make a transistor opamp like LM358 ? Chances of working ?
« Reply #47 on: January 25, 2018, 02:03:30 am »
My understanding is that those original (and extinct) matched-pair devices (LM394, etc.) were designed with 50 adjacent monolithic transistors (for each "transistor") all connected in parallel to average-out the characteristics. And putting them within microns of each other on the same silicon substrate makes them as tightly integrated as possible.
 


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