[volt-nuts] 731A output impedance

Bob Smither smither at c-c-i.com
Wed Nov 28 04:18:58 UTC 2012

On 11/27/2012 07:37 PM, ed breya wrote:
> 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.

Figure 9 of the TI data sheet shows exactly what you are suggesting.

> 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.

Excellent points Ed.  The output stage is a non-inverting amplifier with a small
gain (about 1.3).  The compensation of the lm301A is OK but I think it could be
improved to better tolerate load capacitance.

I have not looked at what would be required to move the op-amp sense point to
the 731A output and leave the 1K inside the loop.  I would prefer to not butcher
the board.  Clearly shorting the 1K is pretty easy!  The lm301 is protected
against shorts to ground.

Thanks to all who added to this thread.

Bob Smither, PhD                                   Circuit Concepts, Inc.

I've come to realize that protecting freedom of choice in our everyday
lives is essential to maintaining a healthy civil society.
   -- George McGovern

Smither at C-C-I.Com            http://www.C-C-I.Com            281-331-2744
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