Summing Amplifier

Summing Amplifier:

Op-amp may be 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.

An inverting summer or a non-inverting summer may be discussed now.

1. 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 R_f is shown in figure 2.

The following analysis is carried out assuming that the op-amp is an ideal one, that is, A_OL = ∞. Since the input bias current is assumed to be zero, there is no voltage drop across the resistor R_comp and hence the non-inverting input terminal is at ground potential.

To find Rcomp, make all inputs V₁ = V₂ = V₃ = 0. So the effective input resistance R_i = R₁ || R₂ || R₃. Therefore, Rcomp = R_i || R_f = R₁ || R₂ || R₃ || R,_f.

2. Non-Inverting Summing Amplifier:

A summer that gives a non-inverted sum is the non-inverting summing amplifier of figure 3. Let the voltage at the (-) input teriminal be Va.

which is a non-inverting weighted sum of inputs.

Let R₁  = R₂  = R₃  = R = R_f/2, then V_o = V₁+V₂+V₃