In the comparator circuit, the output voltage Vo is what relationship to the input Vin?

In the comparator circuit, the output voltage Vo is what relationship to the input Vin?

• A. In phase with Vin
• B. 180 degrees out of phase with Vin
• C. Slightly out of phase with Vin
• D. Independent of Vin

Answer: (C)

A comparator doesn’t produce a scaled version of the input; it output saturates high or low depending on whether Vin is above or below a reference. For a sinusoidal Vin, the output becomes a square wave that switches at the threshold crossing. In an ideal, instantaneous comparator, the switching occurs exactly at the crossing with no delay. Real devices, however, have finite propagation delay and limited slew rate, so the output lags the input a tiny bit. That small delay shows up as a slight phase difference, hence the output is slightly out of phase with Vin. The other possibilities aren’t correct because the output isn’t a linear, in-phase replica of the input, nor is it completely inverted at all times, and it certainly isn’t independent of Vin.

A comparator doesn’t produce a scaled version of the input; it output saturates high or low depending on whether Vin is above or below a reference. For a sinusoidal Vin, the output becomes a square wave that switches at the threshold crossing. In an ideal, instantaneous comparator, the switching occurs exactly at the crossing with no delay. Real devices, however, have finite propagation delay and limited slew rate, so the output lags the input a tiny bit. That small delay shows up as a slight phase difference, hence the output is slightly out of phase with Vin. The other possibilities aren’t correct because the output isn’t a linear, in-phase replica of the input, nor is it completely inverted at all times, and it certainly isn’t independent of Vin.