Opamp Vo Vin lag: Cancelled using another lag

[eblc1388]

In a NS webcast "Use and abuse of Opamp" by Bob Pease, the following diagram was shown.

I had trouble understand the mechanism why additional lag which was induced by adding the second Opamp, would cancel the lag with just one opamp.

Any ideas? Thanks

 

[audioguru]

Hi, L. Chung,
I think you missed the last part:
"No problem with lag, except for, wait a minute, one little thing. Is it a good idea to put a lag in the feedback loop? If you have a 30 MHz op amp and you want to cancel the lag with the 30 MHz op amp, putting a lag in the feedback is not necessarily a good idea. I went out and got some of these op amps. Well, it’s not a disaster; it’s a caution that when you actually build it, you look and it shows you can see the ringing at about 22 MHz, with a Q of about 2.
Now, if you don’t mind that, you could probably use the circuit. You know, it probably won’t oscillate. But I’m nervous because I don’t like things that ring that badly. I think this approach would work better. If you've got to use a reasonably quick op amp, use an R and a C. This has a lag. You want to put a lag in the feedback. Let the capacitance here give you the immediacy so it don’t oscillate."

 

[eblc1388]

No, I did watch the whole webcast.

Of course there are cons with the technique, but doesn't Bob implied that the technique works, apart from some ringings. He also shown a slightly modified version(with a resistor and capacitor added) later.

What I am not understand is how one can cancel a lag, using another lag.

 

[theimperia]

Actually, you can't cancel the lag (unless you know how to travel back in time...do you?)

If it takes 5uS for the signal to get from input to output, then whatever you do, there's no way you can make it faster...otherwise, you'll have a signal at input AND output AT THE SAME TIME, which defies all logic (from non-relativistic point of view ). Of even better, having signal at the output BEFORE you apply it to the input...this one, I can't even imagine it...

Am I right?

 

[Russlk]

The lags don't cancel. The output will have an overshoot until the negative input catches up with the input. If there is much capacitance at the output or negative input, the output will also undershoot ad nausium.

 

[Roff]

No, I did watch the whole webcast.

Of course there are cons with the technique, but doesn't Bob implied that the technique works, apart from some ringings. He also shown a slightly modified version(with a resistor and capacitor added) later.

What I am not understand is how one can cancel a lag, using another lag.

I think you misunderstood Bob Pease. He doesn't imply that it works. He's WAY smarter than that. Go back and read **broken link removed**, which Audioguru quoted from above.

 

[eblc1388]

Thanks everyone for their input on this one.

The lags don't cancel. The output will have an overshoot until the negative input catches up with the input. If there is much capacitance at the output or negative input, the output will also undershoot ad nausium.

That's it Russlk. Spot on and thanks. What I now understand it is an extra lag in the feedback means that the corrective action of the opamp would be "steeper" than without the lag. This would then led to several overshoots naturally. I should have looked more closely at the waveforms in the initial diagram I posted which was amusingly accurate and Pease already explained the underlying reasons there. My salute to Bob.

What Pease offered/suggested later is a better design to reduce the overshooting.