Op Amp Analysis Question

priscared · 14th June 2008, 02:35 AM

Hey all, long time between posts.
I have a op-amp circuit im trying to analyse. I need to know the transfer function for it. I know the gain is -15. But id love to know how you get that value. Circuit is attached
Cheers

Conrad Hoffman · 14th June 2008, 05:19 AM

It's a trick question, right? The 75k resistor goes back to the input, so its effect will be entirely dependent on the driving impedance of the source, which isn't shown. Driven from a low impedance source, it makes no contribution, so just analyze based on the gain of the two stages. If the impedance of the source is high, set that as a resistor to ground, then choose some arbitrary voltage at some point, say V3=1V, and just calculate around the circuit based on that. I'm too lazy, so would just put the circuit in LTSpice.

sawreyrw · 14th June 2008, 05:35 AM

The gain of the first stage is -Rf/Ri where Rf=18K and Ri=6K. The gain of the second stage is 1+Rf2/Ri2 where Rf2=8K and Ri2=2K. So yes, the overall gain is -15.

The 75K resistor doesn't do anything. The 10K resistor is irrelevant from a DC point of view. It may effect the ac performance of the circuit.

priscared · 14th June 2008, 05:46 AM

thanks for that, i was thinking the 75k was doin nothing too relevant. Bastards, this was a past exam question.

14th June 2008, 05:19 AM	#2
Conrad Hoffman diyAudio Member Join Date: Mar 2007 Location: Canandaigua, NY USA	It's a trick question, right? The 75k resistor goes back to the input, so its effect will be entirely dependent on the driving impedance of the source, which isn't shown. Driven from a low impedance source, it makes no contribution, so just analyze based on the gain of the two stages. If the impedance of the source is high, set that as a resistor to ground, then choose some arbitrary voltage at some point, say V3=1V, and just calculate around the circuit based on that. I'm too lazy, so would just put the circuit in LTSpice. __________________ I used to be an audiophool like you but then I took an arrow to the knee.

14th June 2008, 05:46 AM	#4
priscared diyAudio Member Join Date: Dec 2003 Location: Victoria Australia	thanks heaps thanks for that, i was thinking the 75k was doin nothing too relevant. Bastards, this was a past exam question.

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14th June 2008, 05:35 AM	#3
sawreyrw diyAudio Member Join Date: Apr 2006 Location: Minnesota	The gain of the first stage is -Rf/Ri where Rf=18K and Ri=6K. The gain of the second stage is 1+Rf2/Ri2 where Rf2=8K and Ri2=2K. So yes, the overall gain is -15. The 75K resistor doesn't do anything. The 10K resistor is irrelevant from a DC point of view. It may effect the ac performance of the circuit.

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