[volt-nuts] 731A output impedance

[Bob Smither]

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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