🚀 go-pugleaf

>I just finished an Electric Circuits Lab and one of the circuits
>involved an op-amp where the op-amp had its two bipolar power supply
>connections, and a signal generator was feeding a sine wave into the
>non-inverting input of the op-amp.  Then, a direct connection was made
>from the output to the inverting input and the gain was measured.  It
>was found that there was no change in the gain.  Then, the same
>circuit was used but instead of the piece of wire going from the
>output to the inverting input a 27k resistor was used and the gain
>remained the same although one would have thought it would
>decrease.(At least I did anyway!)  Is this because there was no second
>resistor that would normally have gone from the inverting input to
>ground?  That is all I can come up with because the formula to
>calculate gain is Afb=(Rf)/(R1) + 1 for closed loop gain.
>Can anyone verify that my assumption is correct or not?
>                                  Thanks, Rob

Your last statement is correct.  Since there is no resistor you call
R1, it is effectively a resistor of infinite resistance.  If you 
substitute infinity into your equation, you'll find that the (Rf)/R1
term -> zero and you are left with Afb=1, just like the case when you
used a wire and Rf=0.  A qualitative way of looking at this is as
follows.   First assume an ideal op-amp.  i.e., there should be no 
current flowing into the inverting terminal.  With the wire, the voltage
at the inverting terminal is the same as the output since they are
shorted with the wire.  The same conditions exist with the 27k resistor
since with the ideal op-amp no current flows into the inverting 
terminal, so there is no current flowing through the resistor, so there
is no voltage drop accross the resistor, so the output voltage equals
the voltage at the inverting terminal.  The results are identical to the
results when the wire was used.

Randy