What is Comparator?

A comparator serves the purpose of determining if an input voltage surpasses a certain level. In noncritical applications, general-purpose op-amps can fulfill this function due to their high open-loop gain. When configured as a comparator, op-amps operate without negative feedback, causing the output to saturate at either a positive or negative voltage state. Even a small input voltage can drive the op-amp into saturation, given its high open-loop gain.

For specialized comparison tasks, dedicated integrated circuit comparators are often preferred. These comparators are optimized for comparison functions, featuring low bias current and rapid switching times (as low as 5 ns). The low bias current is crucial as it prevents changes in switching points when present in external resistors. Additional features of comparators may include hysteresis with adjustable thresholds and a stable internal reference voltage compensating for temperature variations. Some comparators offer dual opposite-polarity outputs.

Zero-Level Detection

The op-amp as a zero-level detector.

One application of a comparator is zero-level detection, depicted in Figure 1(a). The inverting input is grounded, serving as a 0 V reference, while the signal is applied to the non-inverting input. When the input signal crosses the 0 V reference point, the output abruptly switches between saturated states. Figure 1(b) illustrates the result of a sinusoidal input, demonstrating the zero-level detector’s ability to convert a sine wave into a square wave.

Nonzero-Level Detection

Nonzero-level detectors

The zero-level detector can be adapted to detect voltages other than zero by introducing a fixed reference voltage to the inverting input, as shown in Figure 2(a). A more practical configuration, depicted in part (b), involves a voltage divider to set the reference voltage. The reference voltage (VREF) is determined by the voltage divider equation:

Where is the positive op-amp supply voltage? As long as the input voltage is below the reference voltage, the output stays at the maximum negative level. When the input voltage exceeds the reference voltage, the output transitions to the maximum positive state, as shown in Figure 2(c) with a sinusoidal input voltage. This configuration allows the comparator to detect and respond to nonzero reference voltages.

In summary, comparators, whether implemented with general-purpose op-amps or specialized integrated circuits, find utility in various applications such as zero-level detection and nonzero-level detection for tasks like square wave generation.

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