# Electronic Instrument Amplifier

foobag
Need some help guys

https://www.physicsforums.com/attachment.php?attachmentid=32237&d=1297816285

I began to analyze it and I got to this:

OA2 is a non inverting amplifer
OA1 is a voltage follower

thus:

V1(1+ (R1/R2) is the input for R3 into the final op-amp OA3 which is an inverting op-amp. And V2 is the second input that goes through R4 into the op-amp OA3.

Could someone please verify my logic, and tell me whether this is a summarizing op-amp in the end with 2 inputs into the negative terminal, with feedback resistor R5?

Thanks!!

## Answers and Replies

Mentor
Need some help guys

https://www.physicsforums.com/attachment.php?attachmentid=32237&d=1297816285

I began to analyze it and I got to this:

OA2 is a non inverting amplifer
OA1 is a voltage follower

thus:

V1(1+ (R1/R2) is the input for R3 into the final op-amp OA3 which is an inverting op-amp. And V2 is the second input that goes through R4 into the op-amp OA3.

Could someone please verify my logic, and tell me whether this is a summarizing op-amp in the end with 2 inputs into the negative terminal, with feedback resistor R5?

Thanks!!

Could you please re-post the attachment? Something happened with it. (not your fault, I don't think)

Mentor

Yes, I think your analysis of the circuit is right. The correct term for OA3 is "summing" amplifier, not summarizing amplifier. Everything else that you said is correct.

Last edited by a moderator:
foobag
thanks for your input

i got a funny output: V_out = -20(V1) - 1.961(V2)

the 2nd part of the question asks to design a new circuit with Vout = 25(V1) - 10(V2), so aside from changing the resistors would I have to change OA2 into an inverting op amp itself so that it potentially switches the sign of V1 from negative to positive?

thanks!

Mentor
thanks for your input

i got a funny output: V_out = -20(V1) - 1.961(V2)

That doesn't look right. The V2 gain should be about 20 as well, I think. Can you post your work?

the 2nd part of the question asks to design a new circuit with Vout = 25(V1) - 10(V2), so aside from changing the resistors would I have to change OA2 into an inverting op amp itself so that it potentially switches the sign of V1 from negative to positive?

thanks!

Yes, you would invert the first gain stage for V1.

foobag
i did v1(1+(r1/r2))/r3 + v2/r4 = -Vout/R5

so vout = -(r5/r3)V1(1+(r1/r2)) - (r5/r4)v2

pluggin in resistor values i got

vout = -20v1 - 1.961v2

is this wrong?

Mentor
i did v1(1+(r1/r2))/r3 + v2/r4 = -Vout/R5

so vout = -(r5/r3)V1(1+(r1/r2)) - (r5/r4)v2

pluggin in resistor values i got

vout = -20v1 - 1.961v2

is this wrong?

R5/R4 is not equal to 1.96...