# Need help to understand the amplifiers circuit

Discussion in 'The Projects Forum' started by Lee Peter, Jun 2, 2016.

1. ### Lee Peter Thread Starter New Member

Jun 2, 2016
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Recently I am reading a paper including the circuit below. Could anyone help me analysis this circuit? The paper said the first stage is to get the voltage difference and the second stage is to compare with the power ground. I do not know why the the first stage get the voltage difference.

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2. ### dannyf Well-Known Member

Sep 13, 2015
2,196
417
A very non sensical circuit. Take the top one for example. Vac = Va - (Vc - Va) / R x R, or 2Va - Vc. The last stage is a comparator so Sac=V+ if Va iss greater than Vc / 2, and V- otherwise.

3. ### shteii01 AAC Fanatic!

Feb 19, 2010
3,518
515
The voltage difference has to do with the use of both inputs of the opamp. You are giving it Va at the none-inverting input and Vc at the inverting input, opamp then amplifies the difference.

4. ### ifixit Distinguished Member

Nov 20, 2008
639
110
Assume Va, Vb, & Vc are 60Hz sine waves, 2 Volts peak2peak. Assume also that Vb leads Va by 120 degrees & Vc leads Vb by 120 degrees. Analyse the function now.

5. ### Lee Peter Thread Starter New Member

Jun 2, 2016
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Thanks. I think I get the same result with you.

6. ### Lee Peter Thread Starter New Member

Jun 2, 2016
4
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Thanks. I got the same result with danny. Do you think the analysis is right?

7. ### Lee Peter Thread Starter New Member

Jun 2, 2016
4
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But I got the result Vac= 2Va-Vc. Is that correct?