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.

A
typical summing amplifier with three input voltages V_{1}, V_{2}
and V_{3} three input resistors R_{1}, R_{2}, R_{3}
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=
V_{1}/R1+V_{2}/R2…..+Vn/Rn;

Vo=
- Rf I=Rf/R( V_{1}+V_{2}+….V_{n}).

To
find Rcomp, make all inputs V_{1}
= V_{2} = V_{3} = 0.

So
the effective input resistance R_{i} = R_{1} || R_{2}
|| R_{3}.

Therefore,
Rcomp = R_{i} || R_{f} = R_{1} || R_{2} || R_{3}
|| R,f.

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
R_{1} = R_{2} = R_{3} = R = R_{f}/2, then V_{o}
= V_{1}+V_{2}+V_{3}

_{}

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