Summing Amplifier

Summing amplifier produces an output that is equal to the sum of input signals multiplied by corresponding voltage gain values. There are two types of summing amplifiers - inverting summing amplifier and non-inverting summing amplifier.

Adder circuit is one where the output that is equal to the sum of input signals. Here, voltage gain being unity for all input signals. There are two types of adder circuits - inverting adder and non-inverting adder.

Figure below shows circuit diagram of three input inverting-type summing amplifier.

Inverting-type summing amplifier

The output voltage is given by

The expression for output voltage is derived as follows.

Also

Therefore

If in the inverting summing amplifier of Q3, , then it becomes an inverting adder circuit. The output here is given by

Figure below shows circuit diagram of three input non-inverting summing amplifier.

Non-inverting summing amplifier

As we can see from the figure, the non-inverting summing amplifier is constructed from its inverting counterpart by cascading it with an inverting amplifier.

If in the non-inverting summing amplifier of Q6, R1 = R2 = R3 = R4 = R and R6 = R7 = R¹₂, then it becomes a non-inverting adder circuit. The output voltage is given by

An alternative non-inverting adder circuit with two inputs, with a single opamp is shown in figure below. The circuit behaves like a non-inverting amplifier with a gain of 1 to both the inputs.

Non-inverting adder with single opamp

With only V1 present and V2 grounded, voltage V1 at non-inverting input is given by

This voltage gets amplified by a gain factor (1 + 2R/R) = 3 to produce V1 at the output. Similarly, with V1 grounded, V2 also appears as V2 at the output. When both are simultaneously present, output is V1 + V2. That is,

If the adder circuit of single opamp were to be used for adding n inputs, the feedback resistor value would be equal to nR.

An averager circuit produces an output that is equal to average of the amplitudes of the applied input signals. Figure below shows an inverting averager circuit for n inputs.

Inverting averager circuit

With only one input present at a time and all other inputs grounded, the gain value is 1/n. That is, each input is multiplied by a gain value equal to 1/n. When all the inputs are present simultaneously, the output is given by

A non-inverting averager may be built by connecting a unity gain inverting amplifier at the output of the inverting averager circuit.