Miller compensation - pros/cons?

[Joel]

This type of loop stabilizing compensation cap is seen all the time inside opamps, and SS audio amps, but very rarely in tube amps - and almost never in vintage gear. This has me wondering if there's a reason for that.



It seems to be a good technique. It works. This example is from the amp I've got on the bench right now, and the 47pF cap was able to stabilize the amp with 20dB of NFB, while still retaining a closed-loop bandwidth of -3dB at 100kHz. Whereas it took a much larger cap (330pF) from plate to ground of the first stage to get similar stability... but the amp was then -3dB at <60kHz.

I didn't measure distortion at 20kHz with both methods, but I would bet that it was MUCH lower with the Miller cap.

Subjectively, after an all-too-brief listening session, I have to say that it sounded better with Miller compensation as well.

So, what, if anything, am I missing??

Joel

[tubetvr]

This has me wondering if there's a reason for that.

Yes, there are several reasons.

First is that you only need phase compensation if you use feedback, most classic audio amplifier use limited feedback and open loop response is determined by the output transformer, using Miller compensation in such an amplifier would seriously limit the bandwidth. For preamplifiers it was not so common to use heavy feedback so compensation was not needed. In a single stage amplifier or a multistage amplifier with a limited number of stages stability is achieved without compensation as long as the feedback is reasonable.

2nd reason, Miller compensation waste gain. Early it was discovered that this method for loop compensation was not optimal, there are much better methods available where you can keep the open loop gain higher and therefore achieve better closed loop performance but still have the same stability as with Miller compensation. In order to do this you need to design networks that is specific for an amplifier and have the effect of compensating phase at critical frequencies so as to keep stability. It is possible to show that using this kind of networks is always better than using Miller compensation. For good examples of this method see Valley and Wallman, Vacuum tube amplifiers chapter 9 "Low frequency amplifiers with stabilised gain" I use this method in my OTL and achieve very wide bandwidth with good stability in spite of using heavy feedback.

Phase compensating networks have been used in some classic audio amplifiers like the Williamson

Summary: There are better methods available than Miller compensation if stabilisation is needed and in many classical circuits very little compensation is needed or used.

Regards Hans

[Tom Bavis]

What's wrong with it? It isn't TRADITIONAL... Norman Crowhurst never mentioned it to my knowledge. I assume you've read Norman Koren's page? http://www.normankoren.com/Audio/ esp. "Feedback and Fidelity".

One thing I'd suggest... use a NPO ceramic capacitor - if you use mica, it shouldn't have DC across it - a friend told me it was a design rule at Harris Radio years ago. Silver + DC = silver migration. It may take years, but leakage develops across mica caps.

[Joel]

Hans,

The step networks, like in the Williamson, also shunt down the load resistance - ie. throw away the gain. So, why is this superior?

And, I was under the impression that -3dB at 100kHz was pretty good with an output transformer in the loop! Apparently you think otherwise?

Phase compensation will stabilize the amp (150pF across the 15k feedback resistor), but the output has large overshoot and ringing on square waves. The whole point of dominant pole compensation is to make the gain of the amplifier significantly down at the point where the phase shift equals 360 degrees. So, then it's necessary to make the open loop bandwidth "poor".

But, hey, feedback is a complex subject, and I am NO master of it, by any means.

regards,
Joel

[EC8010]

It strikes me that you are achieving much the same thing by adding a capacitor from anode to grid as you are by adding it from anode to AC ground. Yes, your capacitor is smaller, but that's because its value is being multiplied by the gain of the stage - Miller effect.

However, it also seems to me that Miller compensation will be dependent on the resistance of the source driving the amplifier and will cause the entire amplifier to have a higher input capacitance than if the capacitor had simply been placed across the anode load.

I like to use an RC network across the anode load and another RC network across the feedback resistor and juggle them for the best result. It would help if I had four hands, but two are surprisingly effective. My best amplifier manages -3dB at 200kHz, but I've got a silvered mica capacitor in the anode step network...

That's the first I've heard of silver migration in silvered mica capacitors - any more detail?

[kevinkr]

It works, but one of the reasons why it was not commonly used is that I suspect it raises the input capacitance in exactly the same way as the miller capacitance in a triode does. Using a high mu triode with miller compensation could result in very large effective input capacitances .

[tubetvr]

The step networks, like in the Williamson, also shunt down the load resistance - ie. throw away the gain. So, why is this superior?

It throw away less gain and the gain is only reduced above a certain point, it is a fact that this kind of compensation is vastly superior to Miller compensation, pls read Bodes work where he explains about optimal frequency/gain behavior.

And, I was under the impression that -3dB at 100kHz was pretty good with an output transformer in the loop! Apparently you think otherwise?

No, it is quite good but I know that it is possible to achieve better performance with other methods.

Phase compensation will stabilize the amp (150pF across the 15k feedback resistor), but the output has large overshoot and ringing on square waves.

This is wrong, it doesn't have to be like that, it only depends on how you design the phase compensation network, you can achieve any degree of overshoot you want or no overshoot.

The whole point of dominant pole compensation is to make the gain of the amplifier significantly down at the point where the phase shift equals 360 degrees. So, then it's necessary to make the open loop bandwidth "poor".

Yes but that is what is bad, with dominant pole compensation you will get a gain that drops 6dB per octave but with phase compensating networks ala Bode you can achieve the optimum 9dB per octave asymptotic resulting in higher open loop bandwidth.

The key question is why Miller is used in most opamps and in many SS audio amplifiers when other type of compensation is superior?, there is a simple answer to this riddle even though there are quite many opamps that allow advanced compensation schemes and where it is also described in data sheets.

[Joel]

It strikes me that you are achieving much the same thing by adding a capacitor from anode to grid as you are by adding it from anode to AC ground.

Well, not really though, because with the Miller cap there is negative feedback. That has to linearize things to some extent. With shunt capacitors, and/or step networks, there is no NFB.

As for the input capacitance, let's see:
with the 12AX7 as drawn, I figure a total input C of about 137pF including strays and the 47pF cap. With the typical Zo of a good preamp, -3dB would be somewhere near 900kHz. Isn't that too high to even be a factor?

Joel

[Joel]

This is wrong, it doesn't have to be like that, it only depends on how you design the phase compensation network, you can achieve any degree of overshoot you want or no overshoot.

Hans,

My understanding is that, if you solely relied on 'phase compensation', you would be sending quite large voltage spikes back to the input of the amp, since the amp has not been bandwidth limited to below the point where the feedback loop has become positive feedback - and that this will not only cause momentary overloads of the input, but also slew-rate limiting.

Joel

[aletheian]

Originally posted by Joel
This type of loop stabilizing compensation cap is seen all the time inside opamps, and SS audio amps, but very rarely in tube amps - and almost never in vintage gear. This has me wondering if there's a reason for that.



It seems to be a good technique. It works. This example is from the amp I've got on the bench right now, and the 47pF cap was able to stabilize the amp with 20dB of NFB, while still retaining a closed-loop bandwidth of -3dB at 100kHz. Whereas it took a much larger cap (330pF) from plate to ground of the first stage to get similar stability... but the amp was then -3dB at <60kHz.

I didn't measure distortion at 20kHz with both methods, but I would bet that it was MUCH lower with the Miller cap.

Subjectively, after an all-too-brief listening session, I have to say that it sounded better with Miller compensation as well.

So, what, if anything, am I missing??

Joel

I'm no expert on any of this by any means... usually I'll go by ear and scope, and just call it a day when I like what I hear, but I have used that Miller compensation scheme several times, as well as adding resistance in series with the cap, or RC plate snubbing OR grid to ground caps OR Williamson style compensation. I'll just try them all and see what works best.

But as a result, I probably don't use the 'absolute best' method possible, but the cap feedback loops has worked wonders on occasion where other methods sounded dull. (I guess if my designs were better, then I might not HAVE to use so many tricks on occasion... LOL)

my $ .02