Circuit to boost the output current of an opamp, with large capacitors on the output?

Hello.

I have been thinking about making my own SMU like device(Source Measure Unit) and for that I need to take a signal from an op-amp and boost the current capability a huge amount. This isn't a new problem for me though since from very early in my electronics interest I have wanted a power amplifier to be put at the output of my arbitrary function generator, but the best I could come up with was a unity-gain follower using an BUF634 - 250-mA High-Speed Buffer which was considering my experience with electronics at that time a smashing success but I was then as now not happy about ±250mA.

But today I am thinking about an SMU circuit which should be capable of supplying maximum ±5V @ ±1A, but this thread is just as much about the creation of an similar circuit that can output far higher currents, something like ±5A.

I have found a solution for boosting the current up to ±5A with the following circuit(all the pictures of waveforms that I might put in this post later will all have been generated with this simulation):


The circuit can manage a load of 0.85Ω at 10V_p-p up to 100kHz(but at higher currents there are bumps around the zero-crossing which I beilive has to do with the...don't know what it's called but has to do with the transistors 0,6V drop), any lower load/higher current will produce clipping of the negative part of the waveform and below 0,2Ω the positive part of the waveform starts to clip as well.

I have neglected to use the two resistors following each Darlington collector that was included in the original schematic that I found somewhere because they caused some weird behaviour that I didn't understand. The first problem with this circuit is that it is completely unprotected(those missing resistors was said to be some small measure of over current protection)and I don't know how to fix that but the even bigger problem is it's inability to manage any capacitance placed across the buffers output and 0V/GND. And I want my SMU circuit to be able to handle a lot of capacitance but I have no idea of how to do that.
The SMU circuit would be able to output DC or any waveform you'd like but probably most often sinewaves.

Look what happens if I add 10nF at the buffers output:


or 0,1µF:


The waveform looks similar as the picture above with increasing capacitances until it looks more like this next picture which is made with 47µF at the output:


The last picture which is attached but not shown is a picture of the bumps around the zero-crossing.

So, why is this?
And more importantly what can I do about it?

I tried inserting a small resistor in the path of the output which seems to calm it down but it feels as an inappropriate place for a resistor.
And what is up with the frequency of the oscillations?
It seems to decrease with increasing capacitance, but what other parts of the circuit is it that is causing this?

I would want a buffer that can output 100kHz and not mind being loaded with 10µF, but look at this picture of a 10kHz and a 100kHz sinewave while they are loaded by 10µF:


Any and all advice would be greatly appreciated.

Regards

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Using an op-amp which is rated for a much higher load capacitance than the one used in the simulations that was shown in my first post did decrease the problem but it's a long way from being gone.

Is it impossible to use an op-amp in an application like this?

Hi,

Why don't you use an audio amp?

They are designed for higher currents, and some if them go beyond 100kHz.

Klaus

I think that is a stability problem because there is feedback circuit from output to OPAMP and phase shift is not clear.
As KlausST said, a AF Power Amplifier can be used, they are stable.

I also agree with the audio power amplifier opion.

But if you want, as a personal challenge to design your own, study how audio amplifiers bias the output stage in class AB.

Right now, you are operating the output stage on class B, and relying on the opamp's open loop gain to diminish crossover distortion.
Since it is open loop during this transition, it will take some time to settle once that the loop has regained control. That may somehow explain the ringing.

That, plus the loop instability in itself.

I have at many times while searching for this kind of solutions thought that it would appear as an audio amp looks like what I am looking for, but I lack pretty much any insight what so ever into audio and such signals and I think(not sure though) that I have been deterred by those circuits by fear that they aren't suitable at all only that I can't understand why or how.

But I will take a look at those circuits, in determining from a datasheet if a IC is suitable for my purpose or not is there an specification for bandwidth or gain bandwidth product that I can use to determine it's useful frequency range, or is there some other critical parameter that someone whom has no experience with "audio" amplifiers should be aware of?

In any case I guess I should start googling and reading datasheets, I'm sure I will return with some questions(would be weird if I didn't).

Thanks.

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Oh right, I had a question to ask but I asked another one for some reason...

A audio amp you say, ok. Is it possible to define what sort of capacitances such IC's can drive?
I mean only a sort of ball park value since I sure it depends on the specific IC, but I am sitting here wondering "am I going to be able to drive several µF of capacitance or is that an unrealistic goal?".

At least I assume you referred to a integrated amplifier.

Hi,

It seems you ask us for specifications.
This is difficult, because we don't know your application in detail.

From your posts:
Frequency range, stability, voltage range, current range, capacitive load, protection...

You need to use the internet, go to manufacturer's and distributor's sites and use the interactive selection tools.
In detail you additionally need to read datasheets.

Klaus

Using an off-the-shelf audio power amplifier chip is the obvious and best way to go as you get a lot of features in one package.

If you have the time and like to experiment, you can look at this design as a start.

It appears (according to simulation) to be very stable over a wide frequency range with different load combinations. It looks ok up to 100 kHz with 4.7uF load capacitance.
It starts to get really distorted driving 10uF at 100 kHz.

It gets tricky driving high capacitances at high frequency while maintaining stability.
The compensation values I picked is sort of a compromise between performance and stability.

Like with every high-power amplifier, layout and grounding are very important if you want an amplifier and not a power oscillator. Components T4, T5 should be on a good heat sink and D1-D4 thermally coupled to the heat sink to prevent thermal runaway.

You need to use the internet, go to manufacturer's and distributor's sites and use the interactive selection tools.
In detail you additionally need to read datasheets.

Click to expand...

Right, but I fear, it doesn't help much to design a buffer amplifier driving capacitive loads. Neither usual OPs nor audio amplifiers are designed for this load case.

Instead you need to learn about feedback amplifiers, loop gain, stability criteria, phase margin, compensation methods.

If you drive loads with electrolytic capacitors that will have some ESR component the amplifier copes much better.

Below is the result of driving a 100uF with a typical 0.5 Ω ESR component.

So called zero ESR capacitors is the problem.

I have also used a design with similarity to E-design's posted schematic for driving primarily capacitive loads - the key thing is that there be *some* real resistance. But it doesn't have to be a large amount. Bandwidth up to ~300KHz was possible with sufficiently fast current mirror and VAS feeding it with constant current sources.