Summer or Adder Circuit

Summer or Adder Circuit:

As the input impedance of an op-amp is extremely large, more than one input signal can be applied to the inverting amplifier. Such circuit gives the addition of the applied signals at the output. Hence it is called summer or adder circuit.

Depending upon the sign of the output, the summer circuits are classified as inverting summer and noninverting summer.

Inverting Summer

In this circuit, all the input signals to be added are applied to the inverting input terminal, of the op-amp. The circuit with two input signals is shown in the Fig. 2.25.

Summer or Adder Circuit
As point B is grounded, due to virtual ground concept the node A is also at virtual ground potential.
Summer or Adder CircuitNow from input side,

Summer or Adder Circuit

Applying at node A and as input op-amp current is zero,

Summer or Adder Circuit

From output side,

Substituting (5), (2) and (3) in (4),

Summer or Adder Circuit

If the three resistances are equal, R1 = R2

Summer or Adder Circuit

By properly selecting R f, R1 and R2, we can have weighted addition of the input signals like aV, + bV2, as indicated by the equation (6).
Infact in such a way, n input voltages can be added.

Thus the magnitude of the output voltage is the sum of the input voltages and hence circuit is called as summer or adder circuit. Due to the negative sign of the sum at the output it is called inverting summer. It shows that there is phase inversion.

Non-inverting Summing Amplifier

The circuit discussed above is inverting summing amplifier, which can be noticed from the negative sign in the equation (6). But a summer that gives non-inverted sum of the input signals is called non-inverting summing amplifier. The circuit is shown in the Fig. 2.26.

Summer or Adder Circuit
Let the voltage of node B is VB. Now the node A is at the same potential as that of B.

Summer or Adder Circuit

From the input side,

Summer or Adder Circuit
But as the input current of op-amp is zero,

Summer or Adder CircuitSummer or Adder CircuitSummer or Adder CircuitEquating the two equations (5) and (6),

Summer or Adder Circuit

Substituting equations (4) in (7) we get,

Summer or Adder CircuitThe equation (8) shows that the output is weighted sum of the inputs.

Summer or Adder CircuitAs there is no phase difference between input and output, it is called non-inverting summer amplifier.

Average Circuit

If in the inverting summer circuit, the values of resistance are selected as,

Summer or Adder Circuitthen from the equation (Vo = V1 + V2) we get,
Summer or Adder CircuitThus the magnitude of the output voltage is the average of the two input voltages. So circuit acts like an averager.
Similarly average of n inputs can be calculated by selecting

Summer or Adder Circuit