# Differential Op-Amp

Discussion in 'Electronics Homework Help' started by kamalmouawad, Apr 27, 2020.

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Apr 27, 2020
What are the formulas of the following in an non-ideal differential op-amp in terms of resistors:

Ac=?

2. ### Harald KappModeratorModerator

11,662
2,697
Nov 17, 2011
What is Ad? What is AC?
Might be differential gain and common mode gain, but might be something completelx different.

Show us the related circuit diagram. Equations are useless without a reference.

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Apr 27, 2020
Yes the differential and common gain in NON IDEAL in terms of Resistors

4. ### Harald KappModeratorModerator

11,662
2,697
Nov 17, 2011
You can expect good guidance from us, but we will not do your homework.
The relevant equations can be found all over the internet. What work/research have you done so far? Show us your efforts and results.
It is simply not possible that you haven't found these equations. relevan serach terms "op amp differential amplifier common mode gain" will turn up results.

5. ### hevans1944Hop - AC8NS

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2,154
Jun 21, 2012
@kamalmouawad: You have provided an application circuit that uses an "ideal" op-amp to convert the differential signals VI1 and VI2 to the single-ended output signal Vo, referenced to ground. This circuit is commonly used with two other op-amps to create an instrumentation amplifier with high-impedance differential inputs, low-impedance differential output, high common mode rejection ratio, and adjustable gain that can be set by the value of a single resistor. Your circuit then performs a differential to single-ended conversion, usually without additional gain.

When non-ideal (un-matched) resistors are used, the CMRR (Common Mode Rejection Ratio) of the three op-amp circuit can be destroyed because an unbalanced Wheatstone bridge can exist, formed by R1, R2, R3, and R4, whose unbalanced output at the junctions of R1, R2 and R3, R4 (as a result of a common-mode signal provided at the two inputs) is amplified by the open-loop gain of the "ideal" op-amp.

As @Harald Kapp has stated, we will help you but we will not do your homework. The three op-amp instrumentation amplifier is a classic circuit that has been studied in great detail with results published all over the Internet. It's greatest implementation fault occurs in the circuit you provided when so-called "NON IDEAL" or unmatched resistors are used.