**Summing Amplifier Circuit:**

Figure 14.17 shows an Summing Amplifier Circuit 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 amplifer 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

ThereforeIn this circuit R_{a} = R_{b} = R_{c} = R_{F}. Therefore V_{o} = — (V_{a} + V_{b} + V_{c}). (14.7) 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

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 volt-age is given by

**Subtractor**

A subtractor circuit using a basic differential amplifier is as shown in Fig. 14.19.

By selecting the appropriate values for the external resistance, the input signal can be scaled (attenuated) to the desired value. If this is done, the circuit is referred to as a scaling amplifier.

As in Fig. 14.19, all values of the external resistance are equal, and the gain of the amplifier is unity.

Therefore, the output voltage of differential amplifier with unity gain is

Hence the circuit is called a subtractor.