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Chapter: Linear Integrated Circuits : Applications of Operational Amplifier

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Adder using Operational Amplifier

Op-amp may be used to design a circuit whose output is the sum of several input signals.

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 Rf 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+….Vn).

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 || 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 = Rf/2, then Vo =  V1+V2+V3


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