Question

The following is a circuit of weighted summer capable of summing coefficients of both sign. The expressions for the output voltage v0 is

(a) v0 = v1 (R1/Ra ) (Rc/Rb ) + v1 (Ra/R2 ) (Rc/Rb ) – v1 (Rc/R3 ) – v1 (Rc/R4 )

(b) v0 = – v1 (R1/Ra ) (Rc/Rb ) – v1 (Ra/R2 ) (Rc/Rb ) + v1 (Rc/R3 ) + v1 (Rc/R4 )

(c) v0 = v1 (Ra/R1 ) (Rc/Rb ) + v1 (Ra/R2 ) (Rc/Rb ) – v1 (Rc/R3 ) – v1 (Rc/R4 )

(d) v0 = – v1 (Ra/R1 ) (Rc/Rb ) – v1 (Ra/R2 ) (Rc/Rb ) + v1 (Rc/R3 ) + v1 (Rc/R4 )

I have been asked this question in a job interview.

My doubt is from The Inverting Configuration topic in portion Operational Amplifiers of Electronic Devices & Circuits

Answer 1

The correct answer is (c) v0 = v1 (Ra/R1 ) (Rc/Rb ) + v1 (Ra/R2 ) (Rc/Rb ) – v1 (Rc/R3 ) – v1 (Rc/R4 )

To elaborate: The voltages are increased first by the left side of the portion and then are also magnified by the right side of the circuit. There are four inputs given out of which two are magnified twice and the other are magnified only once.