Noninverting configuration:

If the input voltages are connected to noninverting input through resistors, then the circuit can be used as a summing or averaging amplifier through proper selection of R₁, R₂, R₃ and R_f. as shown in fig. 3.

To find the output voltage expression, v1 is required. Applying superposition theorem, the voltage v1 at the noninverting terminal is given by Hence the output voltage is

Fig. 3

This shows that the output is equal to the average of all input voltages times the gain of the circuit (1+ R_f / R₁), hence the name averaging amplifier.

If (1+R_f/ R₁) is made equal to 3 then the output voltage becomes sum of all three input voltages.

v_o = v _a + v_b+ v_c

Hence, the circuit is called summing amplifier.

Example - 1

Find the gain of V_O / V_i of the circuit of fig. 4.

Solution:

Current entering at the inveting terminal . Applying KCL to node 1, Applying KCL to node 2, Thus the gain A = -8 V / VO

Fig. 4

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