John Woodgate said:
I read in sci.electronics.design that John Popelish <
[email protected]>
The resistance looking into the virtual ground is R/Ao. Since Ao is sort
of big, you need an awful lot of grounded inputs to get to a shunt
resistance equal to that! OK, if you are pushing a 741 op-amp to 20 kHz,
there is a problem, but who does that these days .....? (;-)
I can remember designing a number of op-amp circuits
where loop gain was a concern, some fairly recently.
Of course, the frequency at which loop gain became
a concern was higher than 20 kHz.
In any configuration where loop gain is needed to get
accuracy (which includes driving down the natural,
open-loop distortion of the amplifier), and where
adequate loop gain is not a foregone conclusion, if you
add inputs then you have likely degraded loop gain and
need to reevaluate its adequacy. This can happen in
real situations; you should not defer that kind of analysis
for (nominally) unity gain DC summers with hundreds of
thousands of inputs.
In an earlier post on this subthread, you wrote: "The
voltage at the inverting input is zero for an ideal op-amp
and is V/Ao for an open loop gain of Ao." Very true,
but if you look at the named subject, and consider the
formula for closed-loop gain (which I posted earlier):
Avcl = G / (1 + GH) == 1 / (1/G + H)
you should be able to see what happens as H (which
designates the gain of the feedback network) is made
so small that it approaches 1/G. The closed loop gain
is no longer well approximated as 1/H, which is the
result that obtains with an ideal op-amp.