capacitornegative feedbackoperational-amplifier
I have the following circuit:
I think I understand pretty much all the aspects of it except for the small capacitor in the feedback. What is the need for it? Judging by the frequency of 1 MHz that is the inverse product of the capacitance and the resistance, I can guess the need for this capacitor arises at high frequencies, when the gain of the amplifier tends to unity. However, I can't figure out the purpose.
How about this? You do give up very high impedance input, but the resistors do not have to all be 1K.
Cipocip - the phase of the loop gain does not assume a value of 180deg between those two frequencies. You must not look at the asymptotic lines only. The real magnitude of the loop gain does not have a roll-off of -40dB/dec. This is true because (a) the open-loop gain of the opamp and (b) the rise of 1/beta both never reach 20 dB/dec (corresponding to a 90deg shift). As you know, each first-order system has a phase shift of 90 deg for infinite frequencies only. In the present case, the situation is even less critical because the 1/beta function has a pole (following the zero). Hence, the maximum phase shift will be (perhaps) only 75...80 deg. - dpending on the distance between both frequencies.
More than that, a clear picture of the stability margin can be gained looking at the NYQUIST plot for the loop gain. The critical point never is encircled.
Best Answer
The input capacitance of this opamp is a few pF which, combined with the rather high value feedback resistor, creates a pole at a frequency that's low enough to add enough phase shift in the feedback to reduce phase margin. This may make the opamp unstable or marginally stable.
The 10pF cap compensates for this. If you draw the opamp's input capacitance on the schematic, for example 2pF to ground on the "-" input, then it becomes clear how the 10pF cap forms a capacitive divider with this parasitic cap and restores phase margin.
Note the input capacitance is the parasitic capacitance of the input transistors, protection diodes etc, in the chip itself.