**Summing Amplifier Circuit Diagram:**

Figure 14.17 shows an Summing Amplifier circuit diagram in inverting configuration with three inputs V_{a}, V_{b}, V_{c}. Depending on the relation between R_{a}, R_{b}, R_{c} and R_{F}, the circuit can be used as a Summing amplifier, Scaling amplifier or Average amplifier.

Using Kirchoff‘s circuit equation, we have l_{a} + l_{b }+ l_{c }= I_{B} + I_{f}. But I_{B} ≡ 0 and V_{1} ≡ V_{2} ≡ 0

Therefore

i.e.

In this circuit R_{a} = R_{b} = R_{c} = R_{F}.

Therefore

Hence the output voltage is the negative sum of all the input voltages. If each input voltages is amplified by a different factor, i.e. weighted differently at the output, the circuit is called a **scaling** or **weighted amplifier** (Fig. 14.17). The condition can be obtained by making R_{a}, R_{b}, and R_{c}, different in value. The output voltage of the scaling amplifier is then

where R_{F}/R_{a} ≠ R_{F}/R_{b} ≠ R_{F}/R_{c}.

Figure 14.17 can be modified to be used as an average amplifier. In this amplifier, the output voltage is the average value of the input voltages. This modification can be obtained by making R_{a} = R_{b} = R_{c} = R. Also, the gain by which input is amplified must be equal to 1 over the number of inputs, i.e. R_{F}/R = 1/n where n is the number of inputs. Therefore the output voltage is given by V_{o} = V_{a} + V_{b} + V_{c}.

Therefore the output voltage for three inputs is R_{F}/R = 1/3. The output voltage is given by