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The circuit here: https://www.circuitlab.com/circuit/m36h38/active-regulator/ is a simple active regulator taken from Art of Electronics. I initially drove this circuit with a 1khz input and, after running a transient analysis, got a very nice output on the first half cycle, nothing on the second, and junk from there on out. Driving this circuit with a 10hz output gives more what one would expect. Can anyone explain why this circuit doesn't work for moderate frequencies? Did I make a stupid mistake?

Unless you have edited it since you posted your question, your circuit seems to be simulating exactly as expected.

However, that may not be what you are expecting.

Your circuit is not a regulator of any sort. It is a precision half wave rectifier.

The reason that it gives silly results at higher frequencies is because the opamp output has to come up from somewhere beyond the southern edge of the heliopause to one diode drop above your input voltage just as your input crosses zero from a negative excursion.

If you look at the opamp slew rate (double click on the opamp symbol or right click > Edit parameters) you'll see it can only do that at a certain V/us.

To see this, probe the opamp output pin (the diode anode).

Using an opamp-with-supply-rails symbol (and setting up suitable supply voltages) will help because it limits the -ve opamp o/p swing.

:)

Try search CL for these tags:

precision rectifier

precision peak detector

peak detector

Thank you for your response. I had meant to say rectifier, though I said regulator. Oops! Sorry for the confusion.

Anyways, I didn't think to check the slew rate because most op amps have a slew rate far faster than necessary to accomodate a 1khz signal (something like 15V/1us)

However, at your suggestion I put a probe on the Vout of the op amp. I was very surprised to see it peaking at 260kV! Not what I expected, but this explains the performance.

In the future, when I think that a circuit will have the op amp hitting the rails, I will provide the op amp with rails to hit.

Many Thanks for your help,

Hillary