a basic problem of the opamp

[ddrr]

Hi All,

I have a question here. Does the slide below means that the gain peak can enlarge the closed-loop bandwidth? How can I understand this qualitatively?

Many thanks!!

 

[keith1200rs]

Gain peaking will give a higher bandwidth but with it comes instability so it is not necessarily useful. It is often a sign of an opamp that is not unity gain stable.

Keith.

 

[FvM]

Strictly spoken, an unstable OP would show peaking to infinity. You'll notice that most wide band (e.g. > 100 MHz)
unity gain compensated VFB OPs show considerable peaking and respective overshoot in pulse response. The above
shown gain diagram looks rather common for this device class.

 

[LvW]

Gain peaking will give a higher bandwidth but with it comes instability so it is not necessarily useful. It is often a sign of an opamp that is not unity gain stable.
Keith.

For my opinion, this sounds a bit misunderstanding (sorry: misleading)
One could have the impression that the peaking is the CAUSE of higher bandwidth.
Of course, this is not the case.
The shown picture gives two NORMALIZED gain responses (unity and gain of two).
Lower gain means more negative feedback - which has two effects:
Wider bandwidth and less phase margin. The latter is the reason for peaking - nothing else.

Added after 2 minutes:

One additional comment: If you want absolutely no peaking (for a second order system) you need a margin of app. 65 deg.

 

[FvM]

One could have the impression that the peaking is the CAUSE of higher bandwidth.
Of course, this is not the case.

The statement from the above shown slide is correct anyway:

The bandwidth at a gain of 1 looks more than twice as wide as the bandwidth at a gain of 2

Because (closed loop) bandwidth is simply defined by a -3 dB drop compared to DC gain, it's actually larger than GBW product
for A=+1 due to peaking. Manufacturers love to to print this apparent bandwidth on top of there high speed OP data sheets.