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# subtractor circuit using OP AMP

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Can some one explain how it is working

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According to me it should be -4.5 Vdc

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Your math is off. :wink: You apparently forgot about how R7 and R8 affect the non-inverting gain.

The voltage at the op amp (+) input is 2.5V, multiplied by the gain of the op amp, which is +2 (due to R7 and R8), giving an output of +5V for that input

The +7V at the minus input is multiplied by the op amp gain of -1, giving an output of -7V for that input.

Using superposition, the op amp output is thus -2V, exactly as you simulated.

I do it with Ohm's Law like this:

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• subtractor circuit.png
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Normally all values are equal for a unity gain differential amp ( 10K) to minimize offset from input bias current.

inverting gain= -R7/R8 *7V = (-1) * 7V
Non Inv gain = 1+ |inverting gain| * V+in= (1+1) * 5V*R5/(R5+R6)= (1+1) 5v * 1/2= 5v

Using superposition rules the output-7+5 =2 the difference.

As a check, the Vin-=Vin+ whenever the output is in the linear range ( not saturated)

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