DC servo questions

[mikep]

[quote author="RogerFoote"]

Did you look at the "Hybrid Op Amp" schem?

With it, you can run the unit with or without the servo. And you will most definately have more than a few mV of offset with the servo disengaged.[/quote]

yes I looked at it. I have built similar output stages. as long as you AC couple the input there should be very little DC offset. *much* less than .6V. IMO this topology can be even be DC coupled with no servo. how much closed loop gain did you have when you measured that offset? just trying to help. based on my experience, that sounds screwy.

edit: I didn't mention, I only like this topology for low gain INVERTING. up to 20dB gain. that could be part of the explanation. roger, are you using it non-inverting?

 

[bcarso]

The hybrid ought to work best in noninverting apps. But you do need a d.c. return for the noninverting input.

Where the hybrid as drawn would have problems would be in the four-resistor differential amp configuration, where the 47k resistor from the servo section needs to be accounted for.

In the noninverting-only configuration the 47k just looks like it's in parallel with the feedback divider resistor, in the signal passband---so it increases the gain to some extent. If the divider Z is low to begin with the 47k will have a small effect.