Op-Amp Adder and Subtractor

Applications of Operational Amplifiers in Tamil

Op-Amp Adder

  • Op-amp is used to design a circuit whose output is the sum of several input signals. Such a circuit is called a summing amplifier or a summer or adder.
  • If the input resistors are equal in value (R1= R2 = R) then the summed output voltage is as given and the gain is +1. If the input resistors are unequal then the output voltage is a weighted sum and becomes:
Adder Circuit

Inverting Summing Amplifier

Inverting Summing Amplifier
  • A typical summing amplifier with three input voltages V1, V2 and V3 three input resistors R1, R2, R3 and a feedback resistor Rf.
  • The following analysis is carried out assuming that the op-amp is an ideal one, AOL= ∞. Since the input bias current is assumed to be zero, there is no voltage drop across the resistor Rcomp and hence the non-inverting input terminal is at ground potential.
Inverting Summing Amplifier

To find Rcomp, make all inputs V1 = V2 = V3 = 0.
So the effective input resistance Ri = R1 || R2 || R3. Therefore, Rcomp = Ri || Rf = R1 || R2 || R3 || Rf.

Non-Inverting Summing Amplifier

Non Inverting Summing Amplifier

Non-Inverting Summing Amplifier

  • A summer that gives a non-inverted sum is the non-inverting summing amplifier. Let the voltage at the (-) input terminal be Va which is a non-inverting weighted sum of inputs.

Let R1 = R2 = R3 = R = Rf/2, then V0 = V1+V2+V3

Op-Amp Subtractor

Non Inverting Summing Amplifier

Op-Amp Subtractor

  • The Subtractor also called a differential amplifier, uses both the inverting and non-inverting inputs to produce an output signal which is the difference between the two input voltages V1 and V2 allowing one signal to be subtracted from another.
  • If resistances are equal (R = R3 and RA = R4) then the output voltage is as given and the voltage gain is +1.
  • If the input resistance are unequal the circuit becomes a differential amplifier producing a negative output when V1 is higher than V2 and a positive output when V1 is lower than V2.


  • It is possible to perform addition and subtraction simultaneously with a single op-amp using the circuit.
  • The output voltage Vo can be obtained by using superposition theorem.
  • To find output voltage V01 due to V1 alone, make all other input voltages V2, V3 and V4 equal to zero.
Non Inverting Summing Amplifier

Adder Subtractor Circuit

  • This is the circuit of an inverting amplifier and its output voltage is, V1= -R/(R/2) * V 1/2= - V1
  • Similarly, the output voltage V02 due to V2 alone is, V02= - V2
  • Now, the output voltage V03 due to the input voltage signal V3 alone applied at the (+) input terminal can be found by setting V1, V2 and V4 equal to zero. V03=V3
Non Inverting Summing Amplifier

Non-inverting Amplifier

  • The circuit now becomes a non-inverting amplifier. So, the output voltage V03 due to V3 alone is V03 = V3
  • Similarly, it can be shown that the output voltage V04 due to V4 alone is V04 = V4

Thus, the output voltage Vo due to all four input voltages is given by
Vo = V01 = V02 = V03 = V04
Vo = -V1 -V2 +V3+ V4
V o = (V 3 +V 4) – (V1 +V 2)
So, the circuit is an adder-subtractor.

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