Op-amp Comparator

A comparator, in electronics, is a circuit configuration that compares two voltages (or currents) and indicates which one is larger. Thus, the inputs to a comparator should be different in nature. Comparators can be easily configured using op-amps, since the op-amps have high gain and balanced difference inputs.

Theoretically, an op-amp in open-loop configuration (no feedback) can be used as a comparator. When the input voltage at the non-inverting terminal V₊ is greater than the voltage at the inverting input terminal V_–, the output of the op-amp saturates at its positive extreme. When the non-inverting input voltage drops below the inverting input voltage, the op-amp output switches to its negative saturation level. Comparator circuits are most widely used in analog-to-digital converters (ADCs) and in oscillators.

Op-amp Inverting Comparator

In an inverting comparator, the input voltage V_in is applied to the inverting input terminal of the op-amp and the non-inverting input terminal is connected to reference voltage, through resistors R₁ and R₂. As long as the input voltage V_in is lesser than the reference voltage V_ref, the output of the op-amp remains positively saturated. When V_in goes above the reference voltage, the output of the op-amp switches to its negative saturation level and remains negatively saturated as long as V_in is less than V_ref. The circuit of a comparator using op-amp is shown in the figure below

By choosing the values of resistors R₁ and R₂, the reference voltage V_ref can be adjusted and comparator can be used to compare input voltage with the corresponding reference voltage.

Op-amp Non-Inverting Comparator

In the case of an op-amp non-inverting comparator, the input voltage V_in is applied to the non-inverting input terminal and the reference voltage, V_ref, is connected to the inverting input terminal. When the input voltage V_in is greater than the reference voltage V_ref, the op-amp output is positively saturated. In practice, the difference (V_in-V_ref) will be a positive value. Since there is no feedback to the op-amp input, the open-loop gain of the op-amp will be infinity. Hence the output will swing to its maximum possible value, +Vsat. When the input voltage falls below the reference voltage, the output switches to its negative saturation voltage.