Adder using Operational Amplifier

Adder:

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 or adder.

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

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, 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.

I=  V₁/R1+V₂/R2…..+Vn/Rn;

Vo= - Rf I=Rf/R( V₁+V₂+….V_n).

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.

Non-Inverting Summing Amplifier:

A summer that gives a non-inverted sum is the non-inverting summing amplifier of figure Let the voltage at the (-) input terminal 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