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measuring open loop gain in lab

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booshkkaman

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Can anybody explain to me how to set up an operational amplifier to measure its open loop gain and also explain why it works?
 
ya, i actually found this too
but im not sure what the mathematics behind it is and why it works
can you explain it to me?
like, the 1k resistor is between ground and a virtual ground (ideally) so how does it come into play in the analysis?
i just don't understand how they got open loop gain = 101*Vout/Vx
 
Normally an inverting amplifier will not have the 100k and 1k resistors. 100k= short, 1k=open. When the signal generator outputs 1 volt the amplifier will output –1 volt to return the input back to ground. Because the amplifier does not have infinite gain the output is slightly less than 1 volt and there is a very small signal the (-) input. If the amplifier has an open loop gain of 10,000 then the signal at (-) input will be 1/10,000 volts. That is hard to measure.

By making a 1/101 in front of the amplifier (– input) we can see the effect of open loop gain multiplied 101 times. (The signal at (-) input is still 1/10,000. The signal at the junction of the two 10k resistors is 101 times bigger. Approx 1/100.
 
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