1 Working Principle

Generally, in electronics, the comparator is used to compare two voltages or currents which are given at the two inputs of the comparator. A comparator circuit compares two voltages and outputs either a 1 (the voltage at the plus side; VDD in the illustration) or a 0 (the voltage at the negative side) to indicate which is larger. The operational amplifier can be used as a comparator theoretically without negative feedback. However, the open-loop gain of the operational amplifier is very high, so it can only process signals with a very small input differential voltage. Moreover, in general, the delay time of the op amp is long, which cannot meet the actual requirements. The comparator can be adjusted to provide a very small time delay, but its frequency response characteristics will be limited. To avoid output oscillation, many comparators also have internal hysteresis circuits. The threshold of the comparator is fixed, some have only one threshold, and some have two thresholds.

2.1 Hysteresis Voltage

The voltage between the two input terminals of the comparator will change the output state when it crosses zero. Because the input terminal is often superimposed with a small fluctuation voltage, the differential mode voltage generated by it will cause the comparator output to change frequently. In order to avoid output oscillation, the new comparator usually has a hysteresis voltage of several mV. The existence of it requires two switching points of the comparator: one is used to detect the rising voltage, the other is used to detect the falling voltage. The difference of the voltage threshold (VTRIP) is equal to the hysteresis voltage (VHYST). The offset voltage of hysteresis comparator is the average of TRIP and VTRIP-. The input voltage switching point of the comparator without hysteresis is the input offset price comparator , not the zero of the ideal comparator. In addition, the offset voltage generally varies with temperature and power supply voltage. And the power supply rejection ratio is usually employed to express the influence of power supply voltage changes on the offset voltage.

2.2 Bias Current

The input impedance of an ideal comparator is infinite. Therefore, there is no effect on the input signal theoretically. However, the actual input impedance of the comparator cannot be infinite. There is a current at the input end that flows through the internal resistance of the signal source and flows into the comparator, thereby generating an additional voltage difference. The bias current (Ibias) is defined as the median of the input currents of the two comparators and is used to measure the effect of input impedance.

2.3 Super Power Swing

To further optimize the operating voltage range of the comparator, Maxim uses the parallel structure of the NPN tube and the PNP tube as the input stage of the comparator. Thus the input voltage of the comparator can be expanded. In this case, the lower limit can be lower to the lowest level, and the upper limit is 250mV higher than the power supply voltage to reach the Beyond-the-Rail standard. The input of this comparator allows a larger common-mode voltage.

2.4 Drain-source Voltage

The comparator has only two different output states (zero level or power supply voltage). Its output stage of the comparator with full power swing characteristics is an emitter follower, which makes its voltage difference smaller between input and output signals. The voltage difference depends on the emitter junction voltage under the saturation state of the internal transistor of the comparator, which is equal to the drain-source voltage of the MOSFFET.

2.5 Output Delay Time

It includes the transmission delay of the signal through the components and the rise time and fall time of the signal. For high-speed comparators, such as MAX961, the typical value of the delay time can reach 4.5ns and the rise time is 2.3ns. Pay attention to the influence of different factors on the delay time when designing, including the influence of temperature, capacitive load, input overdrive and so on.
Although the comparator has different types. The design and construction of each should take care of ordinary uses without affecting its measuring accuracy. The instrument should be very sensitive and withstand a reasonable ill usage without permanent harm.

3 Comparator Classification

Comparators are classified into various kinds, such as electronic, electrical, mechanical, optical, sigma, digital and pneumatic comparators. These are used in various applications. Here we are talking about electronic comparator.

3.1 Voltage Comparator

A voltage comparator is a circuit that discriminates and compares input signals, and is a basic unit that forms a non-sine wave generating circuit. Voltage comparators are commonly used including single-limit comparators, hysteresis comparators, window comparators, and three-state voltage comparators. Voltage comparator can be used as an interface between analog circuits and digital circuits, as well as waveform generation and conversion circuits.