Lecture 11

The circuit of analog inverter is shown in fig. 1. It is same as inverting voltage amplifier.

Assuming OPAMP to be an ideal one, the differential input voltage is zero.

i.e. v_d = 0
Therefore, v₁ = v₂ = 0

Since input impedance is very high, therefore, input current is zero. OPAMP do not sink any current.

\ i_in= i_fv_in / R = - v_O/ R_fv_o = - (R_f/ R) v_in

If R = R_f then v_O = -v_in, the circuit behaves like an inverter.

If R_f/ R = K (a constant) then the circuit is called inverting amplifier or scale changer voltages.

Fig. 1

Inverting summer:

The configuration is shown in fig. 2. With three input voltages v_a, v_b & v_c. Depending upon the value of R_f and the input resistors R_a, R_b, R_c the circuit can be used as a summing amplifier, scaling amplifier, or averaging amplifier.

Again, for an ideal OPAMP, v₁ = v₂. The current drawn by OPAMP is zero. Thus, applying KCL at v₂ node

This means that the output voltage is equal to the negative sum of all the inputs times the gain of the circuit R_f/ R; hence the circuit is called a summing amplifier. When R_f= R then the output voltage is equal to the negative sum of all inputs.

v_o= -(v_a+ v_b+ v_c)

Fig. 2

If each input voltage is amplified by a different factor in other words weighted differently at the output, the circuit is called then scaling amplifier.

The circuit can be used as an averaging circuit, in which the output voltage is equal to the average of all the input voltages.

In this case, R_a= R_b= R_c = R and R_f / R = 1 / n where n is the number of inputs. Here R_f / R = 1 / 3.

v_o = -(v_a+ v_b + v_c) / 3

In all these applications input could be either ac or dc.