In another thread, LvW says:
Let's use it to analyze this simple opamp circuit:
All we have to do is find 4 voltage divider ratios. I get this result:
This circuit is so simple that it can be analyzed visually. Assuming an ideal opamp, the voltage across R2 is zero, so the current in R2 is zero. Therefore the current in R1 and R3 is also zero, so the voltage across R1 and R3 is zero. Thus, Vp = Vn = Vin = Vout, so Acl = 1, not -1.
LvW, can you explain where the sign error comes from? Is there a mistake in the algebra?
This a powerful method to analyze opamp circuits, but I think there's a sign error somewhere.The closed-loop gain of an idealized opamp (Aol infinite) with feedback can be expressed as Acl=Hf/Hr
with Hf=(Vp-Vn)/Vin for Vout=0
and Hr=(Vp-Vn)/Vout for Vin=0.
Let's use it to analyze this simple opamp circuit:
All we have to do is find 4 voltage divider ratios. I get this result:
This circuit is so simple that it can be analyzed visually. Assuming an ideal opamp, the voltage across R2 is zero, so the current in R2 is zero. Therefore the current in R1 and R3 is also zero, so the voltage across R1 and R3 is zero. Thus, Vp = Vn = Vin = Vout, so Acl = 1, not -1.
LvW, can you explain where the sign error comes from? Is there a mistake in the algebra?
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