# Need help to understand the amplifiers circuit

#### Lee Peter

Joined Jun 2, 2016
4
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.

#### dannyf

Joined Sep 13, 2015
2,197
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.

#### shteii01

Joined Feb 19, 2010
4,644
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.

#### ifixit

Joined Nov 20, 2008
652
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.

#### Lee Peter

Joined Jun 2, 2016
4
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.
Thanks. I think I get the same result with you.

#### Lee Peter

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