# Transfer function of an opamp

Discussion in 'Electronics Homework Help' started by jupin938, Feb 24, 2014.

1. ### jupin938

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0
Feb 24, 2014
How to find the transfer function (or gain) of the following circuit. Since no current can flow through inputs can R2 short circuited?

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2. ### duke37

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770
Jan 9, 2011
Yes you could short circuit R2. BUT

have you got the circuit wrong, surely C2 should be connected to the other end of R2. R2 would then have a reason and this would be the standard circuit for a low pass filter.

3. ### jupin938

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0
Feb 24, 2014
Thanks. Yes, the schematic was wrong. Here is the corrected one and it was possible to solve.

Solving procedure,
Apply KCL at V1
Apply KCL at V2
Remove V1 from above two equations and find V2-----(A)
Apply KCL at V3 and find V3----(B)
Apply Golden rules V2=V3
Make (A)=(B) and find (Vout/Vin) ratio from it.

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• ###### opamp.png
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5,165
1,087
Dec 18, 2013
This is a Sallen Key LP filter. Here is my drawing and transfer function, I think this is right.

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5. ### Laplace

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Apr 4, 2010
I was wondering just what is Ao? It must be the open-loop gain of the LT1001, but what is the value of Ao? Typically when deriving a transfer function for an op-amp circuit one uses an ideal op-amp where the gain is infinite. But the LT1001 has a DC gain of 120 dB with a 3 dB corner frequency of 1 Hz, and a slope of 20 dB per decade (for a gain of one at a frequency of 1 MHz). So if the solution is intended to account for the actual value of Ao, then how does the transfer function account for the frequency dependent nature of the LT1001 open-loop gain?

5,165
1,087
Dec 18, 2013
Ao in this formula is the pass band gain of the circuit G=1+R4/R3. If the circuit was a unity gain version you just replace the Ao with 1.
Thanks  