Simplify the thinking a little here:

OP2 and its attached circuitry do not matter.

Why? Its a fixed positive gain.

Thats like saying the first op-amp has a bit more gain in it, and noting else beyond that.

Analyze it from there.

The second opamp has local feedback. In general that means less gain and a high frequency pole. The latter means that it most likely won't be dominant. So you don't really need to consider the 'phase'.

Of course, Raja.cedt is right in saying that the second opamp matters.
Contrary to loose-electron's statement ("fixed gain") the second opamp strongly influences the stability of the two-opamp combination (called "composite amplifier") due to its phase shift.
In this regard, the GBW of the second opamp as well as both resistor ratios are of importance. A simple ac simulations can reveal the problem areas.

Lex wrote on Nov 7^th, 2011, 12:59am:


I don't think that it is a question of "dominance"! The effect is simply that the frequency response of the second stage (with feedback) adds to the response of the open-loop gain of the 1st opamp.
Thus, this results in a composite opamp combination that has (at least) two poles within the active region. For a good phase margin the overall feedback should be sufficiently small - resulting in a relatively high overall gain. As mentioned already - details depend on resistor ratios and opamp properties (GBW in particular).

Hi Alexander,

I suppose you are somewhat more familiar with the secrets of the english language than I am. Thus, I agree with you.
However, for a reasonable design (that makes sense) and for a second opamp with a GBW that is not much larger than that of the 1st opamp there will be a 2nd pole in the active region of the resulting opamp combination.
Don`t forget, the composite opamp should have better properties (more gain for higher frequencies) if compared with a single opamp. Otherwise the whole combination makes no sense. Of course, I can give the 2nd opamp with feedback a gain of 0 or perhaps 6 dB. But, where is the improvement?  
Therefore, the 2nd opamp should provide an additional gain of - let's say - at least 12...20 dB. And if its own GBW is not larger at least by a factor of 10 (compared to opamp 1) there will be a second pole within the active region of the combination.
Don't you agree?  

buddypoor wrote on Nov 7^th, 2011, 3:37am:


Well first of all, the second opamp might have other features than gain. Say for example a good driving capability or so. You could mix a low noise opamp with a low distortion opamp in this way.

But assuming that the second opamp would purely be used for gain, why placing the local feedback at all? To me it would seem better to get rid of that local feedback (find a way of preserving the negative sign), and use some decent frequency compensation, e.g. placing a capacitor in parallel with the resistor in the fb network (i.e. placing a zero in the fb). Much more efficient, imho.

Thx for the info. I will take a look at it some time. =)

Hi Alexander,

Qote: Well first of all, the second opamp might have other features than gain. Say for example a good driving capability or so. You could mix a low noise opamp with a low distortion opamp in this way.

Yes, I agree with you. More gain - and thus - hopefully more bandwidth is only one of several possible reasons. For example, often the second opamp is replaced by a current feedback amplifier because of its good slew rate properties.

Quote: But assuming that the second opamp would purely be used for gain, why placing the local feedback at all? To me it would seem better to get rid of that local feedback (find a way of preserving the negative sign), and use some decent frequency compensation, e.g. placing a capacitor in parallel with the resistor in the fb network (i.e. placing a zero in the fb). Much more efficient, imho.

I doubt if this is really a good suggestion. If this would be true, you could compensate each opamp with 2 poles in the active region using such a parallel capacitor. Or do you speak about two completely different opamps with different GBW's?
But I know several other composite configurations that have similar (or even better) properties. Some are treated/investigated in the mentioned article (and eqipped with stabilizing features).

Also, if you want a special output stage, close the feedback path with that device in the feedback loop, that way the high gain of the opamp gets used to maintain linearity of the output device.