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I am trying to design an op-amp but because it is going to be used at a max frequency of 10kHz, I only need to make sure the phase margin is good enough at this frequency. Most of the methods for designing op-amps that I found compensate the op-amp for the worst case scenario which is when it used as a buffer. However, in my case, it will never be used as a buffer. Is there a method I can follow that shows how to design an op-amp that is compensated at a particular frequency ?

Please see an image of the circuit: https://imgur.com/a/5IwOVge

The maximum frequency the input signal is 10kHz. The high-pass circuit sets the closed-loop gain at 40dB.

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  • \$\begingroup\$ Start with the open loop bode plot. \$\endgroup\$
    – Andy aka
    Feb 2, 2020 at 10:31
  • \$\begingroup\$ Frequency compensation generally is thought to serve the amplifier the expected stability when a negative feedback network is applied - not just for a 'buffer' configuration. That's why you don't compensate for one frequency, which honestly doesn't make much sense to me. \$\endgroup\$
    – edmz
    Feb 2, 2020 at 11:18
  • \$\begingroup\$ The stability margin (phase margin) does not only depend on the frequency range you intend to work with. You also must specify the mimnimum closed-loop gain for your application. \$\endgroup\$
    – LvW
    Feb 2, 2020 at 14:38
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    \$\begingroup\$ The signal you care about may be at 10kHz, but if it starts oscillating madly at 500kHz you'll care -- so it needs to be compensated for the whole frequency band. \$\endgroup\$
    – TimWescott
    Feb 2, 2020 at 14:59
  • \$\begingroup\$ Your question is too broad. Could you edit your question to tell us what voltage gain you want, and whether its inverting or not? Perhaps show us your candidate circuit? Most modern op-amps come pre-compensated and stay stable in most op-amp circuits. Unless you need super speed or super precision, or are asking for super-high gain, chances are great that you're fine just whipping up a cookbook solution. \$\endgroup\$
    – TimWescott
    Feb 2, 2020 at 15:01

1 Answer 1

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If an amplifier is inadequately compensated it may oscillate even with no input signal applied.

Plot loop gain (open loop and feedback network combined) and phase and adjust the compensation capacitor's value to make sure that the loop gain gets down below 1 well before the loop phase reaches -180 degrees, (say 50 degrees before.) This should then give stability when the loop is subsequently closed.

Usually such a high value of closed loop gain would give you a big advantage of being able to compensate for a high bandwidth. (With a lower beta open loop gain can be higher giving increased bandwidth). But high bandwidth is not important to you. So with a beta of 1/100 you can compensate for an unusually high phase margin giving very good stability and not worry about the reduced bandwidth that results.

But bear in mind that the bigger the required phase margin, the lower the open loop gain will need to be (for any particular beta) and the lower the feedback factor (1 + B.Aol) will be resulting in reduced performance such as higher distortion levels.

So it is a compromise between stability and performance.

beta, B = 1/(Noise Gain)

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