4.10 COMPARATOR

A comparator compares a signal voltage applied at the input of an op-amp with a known reference voltage V _ref given at the other input. It is an open-loop operation, i.e., there is no feedback path in the case of a comparator.

Comparators can be classified into two types, namely noninverting and inverting. In both cases, V _ref may be either positive or negative. The output of a comparator can be in one of two states, + V _sat or ? V _sat. If the voltage at inverting input terminal is greater than the voltage at non-inverting input terminals then V _o = ? V _sat, otherwise V _o = + V _sat.

Non-Inverting Comparator

The signal input is given to the non-inverting input terminal and the reference voltage is given to the inverting input terminal.

Figure 4.27: Non-inverting comparator.

As long as input V _i < V _ref, the output will be ? V _sat. If input V _i > V _ref, the output voltage will be + V _sat.

Figure 4.28: Input-output for non-inverting comparator.

In a practical comparator, V _ref is obtained by using a potentiometer between two power supply terminals. By adjusting the potentiometer, the V _ref voltage is varied.

Inverting Comparator

The signal input is given to the inverting input terminal and the reference voltage is given to the non-inverting input terminal.

Figure 4.29: Inverting comparator.