Hi everyone!

I was looking for a good reference fullwave rectifier circuit and came across the one below which is referenced in this TI publication. I've seen a similar circuit before but without the capacitor C1. What exactly does C1 do in this circuit?

The article says it's a "compensation capacitor" and that it "ensures the circuit is stable with A2 in the feedback loop". I'm not sure what that means nor how what value it should be either. The article says to "set the C1 * R1 pole equal to ~1/4 the unity gain of A2".

 

When the diode becomes reversed biased that would open the
feedback loop if there were not C1 closing the loop for AC.

Regards, Dana.

 

Here is my precision rectifier circuit.

 

Here is my precision rectifier circuit.

View attachment 153737

Thanks but I prefer the one in the OP since it only requires one diode and is ideal for single supply applications

 

When the diode becomes reversed biased that would open the
feedback loop if there were not C1 closing the loop for AC.

Regards, Dana.

I've used the circuit many times without the capacitor and it seems to work just fine. What kind of behavior would I see when feedback loop is opened?

 

They both act the same in this simulation

 

Feedback open potential oscillations. The loop would open when
part goes into slew rate limiting preventing OpAmp to servo diode
into conduction.

Also that cap is inferred in article to have a compensation role for the
other OpAmp, you have to do an ac analysis on its impact on phase margin.

Regards, Dana.

 

Feedback open potential oscillations. The loop would open when
part goes into slew rate limiting preventing OpAmp to servo diode
into conduction.

Also that cap is inferred in article to have a compensation role for the
other OpAmp, you have to do an ac analysis on its impact on phase margin.

Regards, Dana.

OK. I think I understand now. How do I figure out what value cap to use for C1?

My input signal frequency range is 20-500 Hz.

 

Thanks but I prefer the one in the OP since it only requires one diode and is ideal for single supply applications

This one requires no diodes:

 

This one requires no diodes:
View attachment 153793
View attachment 153794

Thanks but I'm really just looking for insight on how the circuit in the OP works rather than alternative configurations. The fact that the circuit seems to work without the cap is confusing.

 

I just noticed that C1 isn't used for any of the op-amps in the example table