Why the output of this op-amp amplifier has distortion? - Page 1
Forum section: electronics / projects

author: ali_asadzadeh
author_rank: Super Contributor
timestamp: July 06, 2023, 12:42:23 pm
content: Hi,I want to amplify the output of a signal gen (Vg1) with Power MOSFET, so I can measure the current with a second op-amp and an external ADC, In this design the VG1 is set to 2.5mv pp @ 50Hz,But the output has lots of distortion!what's the source of the problem?You can find the simulation file in Tina-TI software.

author: Shay
author_rank: Regular Contributor
timestamp: July 06, 2023, 01:15:07 pm
content: Looks like a crossover distortion. Notice the clipping always happens when the mosfets switch positions. Try to reduce the gate resistors, and reduce the frequency. Maybe the op amp can't sink and source the gate charge fast enough.Although your application looks odd to me, what are you trying to achieve and why?

author: EEEnthusiast
author_rank: Frequent Contributor
timestamp: July 06, 2023, 01:17:24 pm
content: This is the classic cross-over distortion. At very low input voltages, the current output is also very low and the MOSFET does not provide enough gain (gm*Rs) to be able to close the loop with the opamp.Can you try to have some bias current (few 10s of mAs) flow through both MOSFETs and re-simulate.

author: EEEnthusiast
author_rank: Frequent Contributor
timestamp: July 06, 2023, 01:22:40 pm
content: Quote from: Shay on July 06, 2023, 01:15:07 pmHe is already using a low frequency. 50 Hz. So it does not look like the gate charge problem at least.

author: EEEnthusiast
author_rank: Frequent Contributor
timestamp: July 06, 2023, 01:51:10 pm
content: Add some bleeder resistor (say 200 ohms) at the output of the push-pull driver to -5V. That should provide enough current for the MOSFET to be alive.You may add an error of 0.1% in your measurement, if that can be ignored.

author: macaba
author_rank: Regular Contributor
timestamp: July 06, 2023, 03:36:06 pm
content: OP07 is too slow to minimise the class B crossover distortion, ideally you would use a faster op-amp or do some Class AB biasing. A faster op-amp will reduce the class B crossover distortion but can never eliminate it entirely.If you are stuck with OP07, then a 470 resistor between Vgate and Vout will let the op-amp do some of the work whilst it's slewing to turn on the opposite FET but the waveform still looks ugly.

author: Terry Bites
author_rank: Super Contributor
timestamp: July 06, 2023, 03:44:18 pm
content: Macaba speaks the hard truth.This typical "crossover distortion". Its few mV and thats in the similar in magnitude to your signal. At higher signal levels it would be far less noticable.When the waveform is positive, the feedback loop is closed via T1. When the waveform is negative, the feedback is closed via T2.Near zero there is a period when both transistors are off and the circuit is briefly open loop.There is no control via negative feedback control until the opamp has swung its output to reclose the loop.An OP-07 does not have the necessary speed to swing its output in this period, it can't close the loop quickly enough.This is caused by the low "slew rate" of the opamp; it's only 0.3-V/μs for an OP-07.Try a faster opamp in your simulation. eg OP-16 and see how that changes things.BTW thats a voltage source with a voltage monitor. Its not a current source.For fun: Work out or simulate how high voltages of the gate capacitances can be charged via a 1K resistor with a drive voltage changing at 0.3V/uS. That adds to your problems.

author: Vovk_Z
author_rank: Super Contributor
timestamp: July 06, 2023, 04:57:07 pm
content: Add some bias to Mostets gates (e.g. 3.3 V Zeners), or use just much faster opamp.

author: Marco
author_rank: Super Contributor
timestamp: July 06, 2023, 05:08:08 pm
content: Biasing a MOSFET AB stage is a classical problem, not an entirely trivial one.BJTs are much easier.

author: SiliconWizard
author_rank: Super Contributor
timestamp: July 06, 2023, 09:10:22 pm
content: You're never going to have acceptable crossover distortion levels with such a simple scheme with the gates directly connected to the output of the opamp (the series resistors won't change the matter one bit.)The MOSFETs are not properly biased this way.You'll need to add diodes, get the idea there:https://www.petervis.com/Amplifiers/mosfet-push-pull-amplifier-circuit/mosfet-push-pull-amplifier-circuit.html

author: Vovk_Z
author_rank: Super Contributor
timestamp: July 07, 2023, 03:05:43 am
content: 1 diode voltage drop is about 0.6-0.7 VDC so it is nothing for a Mosfet. Typical Mosfet needs not less than 3V gate bias voltage (so I'm talking about a pair of 3.0, 3.3 or even 3.6 V Zeners).

author: ali_asadzadeh
author_rank: Super Contributor
timestamp: July 07, 2023, 12:55:18 pm
content: Thanks for all the feedback guys.QuoteQuoteHow should I do it? please share some circuits.QuoteI have used op07 because it's super cheap and I have a lot of them, also It has low offset input, so the output error or ofset can be lower.Please show some circuits regarding using zeners and bias.QuoteI know, But my goal is using MOSFET, since I have a ton of these IRF540 and IRF9540, and They can tolerate good power dissipation, I want to parallel (4-5)of them, so I can sink 30A from them.

author: CaptDon
author_rank: Super Contributor
timestamp: July 07, 2023, 01:53:36 pm
content: How will you get 30 amps of current flow from only a +/- 5 volt power rails to the FET's? I think the RDS of those FET's is too high? The RDS is also very poorly matched between the Pchannel and Nchannel. Look at the Peavey Deca 528 schematic. They used a single NChannel Mosfet on the low side, but a pair of parallel Pchannel Mosfets on the high side just to get a bit of RDS symmetry but then the extra gate capacitance on the high side means more attention to the driver circuit design. You simulation circuit is nothing more than an over simplified 'textbook example' to show theory. It is improper to use in the real world. Your design is going to need a few more parts to bias the Mosfets into more of an AB1 or AB2 operating environment and to get 30 amps of current flow you will probably need to hit more like +/- 10 volts for the FET supply rails. At 5 volts D.C. your 200 milliohm resistor will only flow 25 amps and that is with zero Mosfet loss.

author: ali_asadzadeh
author_rank: Super Contributor
timestamp: July 08, 2023, 11:25:33 am
content: QuoteThanks for the feedbacks, The 200 milliohm resistor is the maximum resistor that I will use and it's for simulation only, the actual resistor is around 50m. I agree that I need to have more P channel MOSFETs, also the 5V is to limit the MOSFET power dissipation. do you suggest circuits for biasing the MOSFETS? any links, schematics, etc..

author: ali_asadzadeh
author_rank: Super Contributor
timestamp: July 08, 2023, 02:10:00 pm
content: I have updated the circuit, changing the op-amp to op-27 that I have already and also adding 5.6V Zener's, Now it seems its working much better, The problem with lower end currents (VG set to 2.5mv) is that the current is not that symmetrical (has DC bias), But as I increase the VG1 to 2.5V it would be symmetrical,The updated circuitwhen VG1 is set to lower values(like 2.5mv 50Hz) it's not symmetrical (has DC bias)So how can I remove the DC bias?The VG1 would be replaced with a DAC, and I want to Replace the R3 with a Digital pot IC, so I can change the output amplitude digitally, Now the problem is finding a digital pot IC that would accept negative voltages on its input, I have MCP41010 in the junk box and it would not accept negative on it's terminals, DO you suggest a POT IC or any other method to control and change the output amplitude?

author: Vovk_Z
author_rank: Super Contributor
timestamp: July 08, 2023, 03:16:48 pm
content: Quote from: ali_asadzadeh on July 07, 2023, 12:55:18 pmI meant exactly what you showed as 'The updated circuit' (near this post).But 5.6 V Zeners is too much. I guess you'll be surprized how much current they'll vaste with such a large gate bias voltage. Use 3V0 or 3V3 Zeners in a real circuit or you'll need a very good heatsinc and a very powerful pover supply.

author: Vovk_Z
author_rank: Super Contributor
timestamp: July 08, 2023, 03:21:29 pm
content: Quote from: ali_asadzadeh on July 08, 2023, 02:10:00 pmOP27 doesn't have a lot of DC offset but if it is still too much for you I can remember you that OP27 has a nulling circuit exact for this purpose.

author: Marco
author_rank: Super Contributor
timestamp: July 08, 2023, 03:39:11 pm
content: You probably have an amp worth of shoot through current.

author: Wolfram
author_rank: Frequent Contributor
timestamp: July 08, 2023, 07:03:13 pm
content: Zener diodes used to bias MOSFETs for class AB operation is not going to work very well in theory or practice. The required Vgs for a given bias current has a huge spread to begin with, and it also changes with temperature. There are ways to bias MOSFETs for this configuration, but it's significantly easier with BJTs.

author: Marco
author_rank: Super Contributor
timestamp: July 08, 2023, 10:59:03 pm
content: 4 bjts and a pot will work well enough for a lab device. Have a current mirror up top and on the bottom, connect the pot between the mirrors to set the current, drive the mirrored currents into the MOSFET gates with separate gate resistors to the opamp.Also add a shunt on the source or drain of one of the MOSFETs so you can measure the bias current to adust the pot.

author: David Hess
author_rank: Super Contributor
timestamp: July 08, 2023, 11:44:15 pm
content: Either the MOSFET about stage need to be properly biased into class-AB, or a more suitable design is required. I would do something like figure 3 shown below, but modified to use MOSFETs by trimming the values of R1 and R2 to a point just below conduction:https://circuitcellar.com/resources/quickbits/boosting-op-amp-output-drive/

author: EEEnthusiast
author_rank: Frequent Contributor
timestamp: July 09, 2023, 12:45:33 pm
content: If you could share what is the final purpose of the design, then there may be a more elegant solution.

author: ali_asadzadeh
author_rank: Super Contributor
timestamp: July 09, 2023, 01:12:01 pm
content: QuoteThanks for the feedback, would you please share some ciruits?QuoteThe final goal is to build something precise enough for generating AC currents, ranging from 1mA to 30A, frequency of 50 or 60Hz, that can be used with harmonics, say 10th harmonic, so 600Hz or 1KHz is the max frequency, it should be able to generate arb signals, Mostly AC with Harmonics, so I can use them to calibrate some Measurement CT's in a Customer product.QuoteI do not get your point.QuoteI think they would get at most (12-5.6)/1k which is around 6.4mA and it's not that huge!?QuoteThanks for the feedback.

author: Vovk_Z
author_rank: Super Contributor
timestamp: July 09, 2023, 02:43:30 pm
content: Quote from: ali_asadzadeh on July 09, 2023, 01:12:01 pmI haven't stated it clearly that I'm talking about Mosfets current. With 5 VDC at it's gate almost any Mosfet will be quite open and will pass a lot of current through itself. So, with 5V6 at gates you'll have 6 mA through Zeners and several amperes (or more) through Mosfets. And you don't wan't so much bias current. You need somewhere between 10-100 mA of Mosfet bias current. That's enough with fast enough opamp. More accurately - you need more close to 10 mA mosfet bias current with a fast opamp (e.g. OPA189), and possibly close to 100 mA with a slower opamp (e.g. OP07).And consider a temperature too. Mosfets wan't be cold if they work. The hotter - the more 'open,' they are. And it increases current and so on (more current -> more temperature -> more current). That's why I'm talking about adequate heatsinking.(I have similar prototype of a Howland current pump. I used 10 mR shunt for 10 A rated output current, OAR5R010). 100-200 mR shunt will be too hot with currents 5-10 A or more.I would suggest to use more sophisticated (more stable) Mosfets bias current control circuit if you don't want to play 'hot games' with every pair of Mosfets (but I don't have goid example at the moment).

author: Marco
author_rank: Super Contributor
timestamp: July 09, 2023, 03:42:27 pm
content: The classical way is with a VGS multiplier, it's a bit of a pain.https://www.powersystemsdesign.com/articles/boosting-output-in-high-voltage-op-amps-with-a-current-buffer/36/6563Even that isn't really ideal, ideally want to keep the MOSFET from shutting off completely. So then you get to something like the LT1166 (discrete solutions start to get annoying, though the HiFi crowd does try).https://www.analog.com/en/products/lt1166.html

author: ArdWar
author_rank: Frequent Contributor
timestamp: July 09, 2023, 06:15:03 pm
content: Biasing MOSFET into proper class B/AB operation is such a pain you might as well going back and use BJT instead. There's practically no easy way to keep them from running away without using fairly complex current sensing. Even more so with modern MOSFET manufacturer not even trying to make Vgs characteristic even remotely uniform anymore.Why pick MOSFET power stage anyway?

author: David Hess
author_rank: Super Contributor
timestamp: July 09, 2023, 06:59:13 pm
content: Quote from: ArdWar on July 09, 2023, 06:15:03 pmBiasing MOSFETs for class-AB is more difficult because the Vbe multiplier must be modified to have a lower temperature coefficient before multiplication, but plenty of designs manage it. I would have used bipolar transistors in this case, but see below.QuoteI could not think of any reason to use MOSFETs instead of bipolar transistors in this application, however in theory higher performance is possible with MOSFETs because they do not suffer from Ft and hfe fading at high currents. On the other hand, their lower transconductance produces more crossover distortion.One other advantage MOSFETs have, albeit at higher cost, is that they are available with much higher power ratings than the largest bipolar transistors, simply because they are available with larger dies which is what makes them more expensive.

author: Vovk_Z
author_rank: Super Contributor
timestamp: July 10, 2023, 08:20:15 am
content: I'm sure LT1166 is a good thing but it costs $10-$15. That's too far from cost of two resistors and two Zeners or couple of transistors.

author: ali_asadzadeh
author_rank: Super Contributor
timestamp: July 10, 2023, 08:34:54 am
content: QuoteThanks for the feedback, If I decrease the bias resistors to 470ohm I think we are in the 10mA Bias current, so do you mean this way?QuoteThanks for the feedback.Vovk_Z do you have aby suggestions about controlling the output Current, I mean scaling it from say 1mA to 30A.

author: Vovk_Z
author_rank: Super Contributor
timestamp: July 10, 2023, 08:56:10 am
content: A 'bias current' is a Mosfets current. They must be slightly open with some minimal opening gate voltage (about 3 V).To control an output current (to make a current sourse) I used classic Howland current pump with 10k/2k or 4k/1k or 10k/1k set of resistors and 0.01 R shunt (I wrote about a shunt abowe).

author: ali_asadzadeh
author_rank: Super Contributor
timestamp: July 11, 2023, 07:49:10 am
content: Vovk_Z can you share your Howland current pump circuit?

author: Vovk_Z
author_rank: Super Contributor
timestamp: July 11, 2023, 03:52:48 pm
content: I guess you want accurate final circuit, but unfortunately I don't have it.It exists as a prototype pencil design schematics.Here, I found it. I used INA163 first and it had a large temperature drift. Then I used OPA189 after, instead of A3 opamp (you may use op07, op27) in my last prototipe and it was fine.You don't need that A1, A2 opamps on the left side of my 'DaveCAD'.Actually you need only A3 opamp (it depends on a shunt relative resistance).C7, C8 capacitors don't help so may be eliminated.R10, R11 consist of series connected constant resistors (1-4k) and 10k potentiometer to adjust Zener current and so Zener voltage and so mosfet gate bias voltage and so mosfet initial (bias) current.Instead of 6k resistors shown (they are inside INA163) you may use any available precision (0.1%) ones. For example: 10k and 1k, or 10k and 2k, or 10k and 4k, or 4k and 1k, 1k and 100R etc.I used two MeanWell 5V 20A (or 30A?) power supllies for output stage and regulated +-12..15VDC power suplly for opamps.

author: Marco
author_rank: Super Contributor
timestamp: July 11, 2023, 05:17:59 pm
content: As the schematic shows, a Howland current pump has the exact same problem biasing MOSFETs as a volt-volt amplifier.

author: ali_asadzadeh
author_rank: Super Contributor
timestamp: July 13, 2023, 08:21:46 pm
content: Vovk_Z Really thanks for sharing.

author: ali_asadzadeh
author_rank: Super Contributor
timestamp: July 13, 2023, 08:57:19 pm
content: Vovk_Z, you mean I should eliminated the A1 and A2 op-amps and only use A3 op-amp, so How should I feed my DAC output to this op-amp, (since my DAC would generate the sine waves), also Do you have any Idea or way of controlling the output current amplitude, maybe by using another DAC or external Digital POT Chips? since controlling it digitally is needed, since I need to have a control range of around 1mA to 30A

author: MathWizard
author_rank: Super Contributor
timestamp: July 14, 2023, 12:45:41 am
content: I'm surprised any modern op-amp is too slow for anything at 50Hz. If the gate resistor and gate capacitance time constant was 1/5 of the period 1/50, so 4ms, that would be like the gate capacitance was a massive 4uF. And if the slew rate of the OP07 is 0.3V/us, and if the gate was driven to only 5V, by itself it the op-amp would only take 16.7us to go from 5 to GND.Even if the gate really was 4uF, at say 20mA discharge current, that's only 1ms from 5V to 0V. So what makes this op-amp too slow ? I bought a bunch of them a few months ago, but never bothered to try them yet, I've been just learning textbooks all spring.

author: Vovk_Z
author_rank: Super Contributor
timestamp: July 14, 2023, 08:02:03 pm
content: Quote from: ali_asadzadeh on July 13, 2023, 08:57:19 pmMy task was to work mainly with 50 Hz signal. Actual frequency range was chosen as 5 Hz - 5 kHz (+-%). So I was fine with usual DC blocking capacitor at this module input (f3 = (1..2) Hz). Working with frequency range from DC to some kHz is another task, a bit more difficult. If I had to do it I would look at some opamp literature, starting with "The Art if Electronics", and another opamp application literature and app notes.

author: Vovk_Z
author_rank: Super Contributor
timestamp: July 14, 2023, 08:05:31 pm
content: Quote from: ali_asadzadeh on July 13, 2023, 08:57:19 pmIt depends, I'm not good at digital part, this in not my area of knowledge. If your DAC has enough resolution to make good undistorted (?) 1 mA then you don't need anything else. But I guess there may be several different approaches.It takes 15 bits to go from 1 mA to 30 A (full scale). And you need several bits 'under' 1 mA to have not very distorted low-level sine, IDK, 8-10 bits more. So it gives us you need 24 bit DAC to have such a large current range 'in one range' without dividing into two or more ranges (for example, 16 bit DAC and two ranges 1 mA - 0.125 A, 0.125 A - 30 A, etc).

author: ali_asadzadeh
author_rank: Super Contributor
timestamp: July 14, 2023, 08:21:21 pm
content: I want to use audio coded DAC's , since they are pretty cheap and have 16bit or 24Bit resolution, they usually would act like this, VCC/2 is their Zero input, and 2's compliment input would generate like GND to VCC voltages, so I have a 0 ~ 5V or 0~3.3V control voltage with an audio DAC, if I supply the DAC with a REF voltage IC, then It would behave like a 50$ DAC!

author: Vovk_Z
author_rank: Super Contributor
timestamp: July 14, 2023, 08:31:28 pm
content: I guess it would be helpful to start another topic to discuss this exact task - how to get DC frequency range from usual audio DAC (this is an interesting task). I'm sure somebody will go there with required knowledge (because this topic is too far from what you ask now).

author: Zero999
author_rank: Super Contributor
timestamp: July 14, 2023, 09:46:15 pm
content: Quote from: MathWizard on July 14, 2023, 12:45:41 amI'm not surprised. The output of the op-amp has to change very quickly to avoid crossover distortion. Biasing both MOSFETs on slightly helps, but isn't perfect.

author: ali_asadzadeh
author_rank: Super Contributor
timestamp: July 15, 2023, 08:12:32 am
content: I have made a new thread for controlling the output, you can see it in this linkhttps://www.eevblog.com/forum/projects/how-to-control-the-output-of-a-current-pump-with-the-help-of-an-audio-dac/new/#new

author: MegaVolt
author_rank: Frequent Contributor
timestamp: July 24, 2023, 08:59:14 am
content: Quote from: MathWizard on July 14, 2023, 12:45:41 amThe problem is not speed. The problem is that the transistors are closed. And the op amp just doesn't know what's going on at the output to fix it. It turns off and on only when the input signal becomes large enough to open at least one transistor. We see this step on one side because the op-amp has a bias voltage that helps in one case and interferes with the second.In short. Then there is no constant path for direct current from the output of the op-amp to its input.

author: tooki
author_rank: Super Contributor
timestamp: July 24, 2023, 10:37:41 am
content: Quote from: ali_asadzadeh on July 07, 2023, 12:55:18 pmEven so, those are ordinary switching MOSFETs, which aren’t optimized for a well-characterized Vgs curve. So even if you manage to design a circuit thatsimulatesperfectly, it’s unlikely it will perform properly if you build one in hardware, and one of the failure modes is self-destruction of the MOSFET.This application note from IXYS (now Littelfuse) does a really good job of explaining the issues:https://www.ixys.com/Documents/AppNotes/IXAN0068.pdfUpdated version:https://www.littelfuse.com/~/media/electronics/white_papers/littelfuse_mosfets_withstand_stress_of_linear-mode_operation_en_white_paper.pdf.pdfKeep your switching MOSFETs and use them for switching purposes. But build your amplifier out of something that makes sense for an amplifier, like BJTs or a proper linear-region MOSFET. (The Littelfuse application note lists some they still make. But bear in mind that they cost far more than BJTs.)Edit:This application note from Nexperia goes into even more detail of the failure modes and explains why modern low-RDSonMOSFETs are more likely to fail when kept in the linear region:https://assets.nexperia.com/documents/application-note/AN50006.pdf

author: David Hess
author_rank: Super Contributor
timestamp: July 24, 2023, 12:37:14 pm
content: Quote from: tooki on July 24, 2023, 10:37:41 amSwitching MOSFETs can still be used in linear applications, but it requires careful attention to the safe operating limit modified by the thermal instability curve. Secondary breakdown in bipolar transistors has a different cause but the same result, and they are regularly used this way. Switching MOSFET datasheets often leave the thermal instability area off of their safe operating area curve which will make things more difficult.The temperature coefficient of Vgs is a little more difficult to handle because it varies significantly with Vgs, whereas Vbe of a bipolar transistor is relatively stable, but it can still be done.

author: tooki
author_rank: Super Contributor
timestamp: July 24, 2023, 12:49:21 pm
content: Quote from: David Hess on July 24, 2023, 12:37:14 pmI didn’t say it can’t be done, I merely stated that it’s not what I’d recommend doing. Do you think OP has the skills/knowledge/experience needed to successfully design a linear MOSFET amp, especially using switching MOSFETs? I don’t think they do (nor do I think I could). Hence my recommendation to use those switching MOSFETs for switching and use something else for the linear amplifier.

author: David Hess
author_rank: Super Contributor
timestamp: July 24, 2023, 03:08:42 pm
content: Quote from: tooki on July 24, 2023, 12:49:21 pmI somewhat agree since the OP also did not take my advice about circuit topology. On the other hand, the OP is limiting themselves to what they have available. I had enough 20 year old power MOSFETs to build my recent solid state relay project if I used like 8 in parallel, but still chose to buy new parts.If the switching MOSFETs are derated enough, then they will work. This shows why more expensive linear MOSFETs could be cheaper to use; they will not need to be derated because of thermal instability.

author: ali_asadzadeh
author_rank: Super Contributor
timestamp: July 25, 2023, 10:43:43 am
content: I have made the final schematic in Altium, Please checkout the schematics and give me your feedback, Note that I plan to use an ATX power supply for the board, also I have added additional connectors for external Industrial powers, I have breadboarded the simplified design using only one MOSFET and was able to generate 1A on breadboard successfully, so Now I'm aiming for 30A,The controller would be an MCU or FPGA, I have not decided yet, But it would connect to P7 and control it.

author: Vovk_Z
author_rank: Super Contributor
timestamp: July 26, 2023, 05:38:27 pm
content: My 5 cents:1. Use individual gate 10R resistors but not one for all gates. That's more safe and reliable in terms of stability.2. I would try more sophisticated gate bias control circuit instead of simple Zeners (it depends on heatsinking and so on general device design). If you can afford enormous heatsink then ok.3. You may get rid of 1:1 shunt amplifier range and use faster OP37 instead of slow OP07. This may give you a bit more stability.4. I'm not sure you need a buffer after current shunt amplifier. But if it's for external connections then it's a good decision.

author: PCB.Wiz
author_rank: Super Contributor
timestamp: July 26, 2023, 08:22:26 pm
content: Quote from: ali_asadzadeh on July 25, 2023, 10:43:43 amYou should update the first post with 1A progress , and again when 30A is workingAddit: as already mentioned you should ditch the 3v6 zeners, and use a smarter threshold adjustment. See the curve below, for why. That's about -8mV/'C, which needs to be compensated.Since you already consume a drawer full of power mosfets, just use two more (one of each) for the biasing/threshold tracking.Wire drain to source, and bolt the added biasing mosfets to the middle of the heatsink groupings, so they track the temperature. You've basically then made 'MOSFET Zeners', that do track.You may also want a parallel C across the biasing mosfet, tho I think this is not high frequency use ?Also, coarsely match the MOSFETS clusters with a meter, as you have two sides, so you can group them.

author: ali_asadzadeh
author_rank: Super Contributor
timestamp: July 27, 2023, 09:14:26 am
content: QuoteThanks for the feedback Vovk_Z,I will use a fairly big heatsink, I will post dimensions once I decided on that, But do you suggest any circuits for the gate bias control?QuoteThanks PCB.Wiz for the feedback, would you please share some circuit picture's, so I would not mess it up.thanks

author: Vovk_Z
author_rank: Super Contributor
timestamp: July 27, 2023, 12:10:11 pm
content: Quote from: PCB.Wiz on July 26, 2023, 08:22:26 pmMy experiments showed when frequency range is several kHz max capacitors across Zeners almost doesn't help. I measured THD at 1 kHz (voltage across a shunt) and the most impact on THD has Mosfets bias current (gates bias voltage). I tried 10 and 47 uF caps and they almost do nothing at such a 'low' frequences. If we have to work at 10 kHz or more - then possibly caps would help (I haven't tested it on 10 kHz, because I don't need 10 kHz. And it is harder to make it work a 10-20 kHz or more with a siple opamp - you need additional drivers etc).

author: PCB.Wiz
author_rank: Super Contributor
timestamp: July 27, 2023, 08:53:10 pm
content: Quote from: ali_asadzadeh on July 27, 2023, 09:14:26 amIt's simple enough, as below.RG can be zero and a small RS allows you to run the power stage quiescent a bit higher than the bias fet current. RD is the 10k you have already on the zeners.You can also group your fets by VGS and get some IQ control that way.