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1. Inverting Summing Amplifier
2. Non-Inverting 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 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,
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 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}.

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

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