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.