[volt-nuts] 731A output impedance
eb at telight.com
Wed Nov 28 01:37:44 UTC 2012
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.
More information about the volt-nuts