I'm not sure how much elaboration is needed, but here's some:
If you take all of the feedback from the output terminal, that's better for DC accuracy by eliminating the voltage drop of the series resistor, while still providing some overload protection to the opamp. But, it also decreases phase margin so that it will be more prone to oscillate with capacitive load. If the series R becomes zero, the voltage drop and the extra loss of phase margin are eliminated, but the inability to drive large capacitive loads remains - it is a limitation of the amplifier.
Usually a small amount of series R can help a lot with capacitive loading stability, but even when small it can drop enough DCV to be a problem. A common way to solve both problems is to sense the DC right at the output to eliminate the drop in the series R as above, but to increase stability by taking some AC ahead of the resistor - usually at the output of the amplifier.
If the amplifier has an integrating feedback capacitor, it's usually already connected that way, so only the resistive part of the feedback needs to go to the terminal. If there is no feedback capacitance, then a small amount can be added from the amplifier output to the effective inverting input.
I don't know what the output stage of the 731A looks like, but it must be an inverting (integrator) amplifier or a buffer, if using an opamp. In either case there should be a way to modify the feedback network. However, whatever is changed or added may affect the overall frequency response and noise.
Ed _______________________________________________ volt-nuts mailing list -- [email protected] To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/volt-nuts and follow the instructions there.
