Applications of Operational Amplifiers: Comparator

Comparators:

An analog comparator has two inputs one is usually a constant reference voltage V_R and other is a time varying signal v_i and one output v_O. The basic circuit of a comparator is shown in fig. 5.

When the noninverting voltage is larger than the inverting voltage the comparator produces a high output voltage (+V_sat). When the non-inverting output is less than the inverting input the output is low (-V_sat). Fig. 5, also shows the output of a comparator for a sinusoidal.

Fig. 5

v_O = -V_sat if v_i > V_R

    = + V_sat if v i < V_R

If V_R = 0, then slightest input voltage (in mV) is enough to saturate the OPAMP and the circuit acts as zero crossing detector as shown in fig. 6. If the supply voltages are ±15V, then the output compliance is from approximate – 13V to +13V. The more the open loop gain of OPAMP, the smaller the voltage required to saturate the output. If v_drequired is very small then the characteristic is a vertical line as shown in fig. 6.

Fig. 6

If we want to limit the output voltage of the comparator two voltages (one positive and other negative) then a resistor R and two zener diodes are added to clamp the output of the comparator. The circuit of such comparator is shown in fig. 7, The transfer characteristics of the circuit is also shown in fig. 7.

Fig. 7

The resistance is chosen so that the zener operates in zener region. When V_R= 0 then the output changes rapidly from one state to other very rapidly every time that the input passes through zero as shown in fig. 8.

Fig. 8

Such a configuration is called zero crossing detector. If we want pulses at zero crossing then a differentiator and a series diode is connected at the output. It produces single pulses at the zero crossing point in every cycle.