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 smit...@c-c-i.com http://www.C-C-I.Com 281-331-2744 =========================================================================
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