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To Calculate:
Let Ro = Open Loop Op Amp output impedance
. Ao = Open loop op amp gain
. Ac = Closed loop op amp gain
. Rc = Closed loop op amp output impedance
. Then:
Rc = Ro(Ac/Ao)
To measure, using the circuit that you propose;
Let Vs = the "battery" voltage
. Rc = Closed loop op amp output impedance
. Rl = the load resistor (the 210K resistor that you show)
1) Remove the load and battery
2) Tweek the input so that the output voltage = exactly 0
3) Re-apply the load, Vs
4) Adjust the load (Rl) so that the output current is just below the
. op amp rated load current for linear operation.
Let Vo = the output voltage under these conditions.
. Then:
Rc = VoRl / (Vs - Vo)
.
The problem with this method is that Vo is very small, making accurate measurements difficult.
.
Regards,
Jon
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