Op Amp Compensation

Aug 31, 2011 6 Replies

The out of loop resistive compensation is a bit confusing to me. We add a resistor to essentially slow down the op amp which prevents it from ringing.



But when one says "... driving a capacitive load" what one really means is driving a capacitive reactive load? Because if we are driving a low pass filter the resistor also compensates. So really the issue with op amps is purely capacitive loads and is what is meant by the terminology?



So as long as the impedance of the load is not close to being purely capacitive, even if it uses large capacitors, it is not considered a capacitive load?



e.g., 10k resistor in series with 1u is fine but driving a 1n capacitor directly is much different?



Yeah that's right. The simplist way of compensating for cap loading is just to put a bit of resistance between the opamp output and the cap.

I couldn't seem to find my favorite app note from AD about cap loading... Maybe someone else will have it?

George H.

Think about where the feedback is taken from, and imagine the output impedance of the op-amp as an internal resistor (that is

This one?

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Yes! Much thanks. There's also a trick where you increase the noise gain of the circuit. (I've never tried it though.)

George H.

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IOW you can turn a good op amp into a crappy one and make it stable that way. ;)

Cheers

Phil Hobbs

Dr Philip C D Hobbs Principal Consultant ElectroOptical Innovations LLC Optics, Electro-optics, Photonics, Analog Electronics 160 North State Road #203 Briarcliff Manor NY 10510 845-480-2058 hobbs at electrooptical dot net http://electrooptical.net

Have fun with this!

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