Voltage comparators are usually composed of integrated op amps. Unlike ordinary op amps, the integrated op amps in comparators are mostly in open loop or positive feedback. As long as a very small signal is added to the two inputs, the op-amp will enter the nonlinear region, which belongs to the nonlinear application range of the integrated op-amp. When analyzing the comparator, the principle of virtual break still holds, and concepts such as virtual short and virtual ground are only adapted when judging the critical situation.
01 Over Zero Comparator
A voltage comparator is a circuit that compares and discriminates an analog input signal ui with a fixed reference voltage UR.
A comparator with zero reference voltage is called a zero-level comparator, also called an over-zero comparator. According to the different input methods can be divided into two types of zero-potential comparators with inverting input and in-phase input, as shown in Figure 1(a) and (b).
Figure 1 Over-zero comparator (a) Inverted input (b) In-phase input
The operating characteristics of a comparator are usually described in terms of threshold voltage and transfer characteristics.
The threshold voltage (also called the threshold level) is the input voltage value that causes the comparator output voltage to jump, referred to as the threshold value and denoted by the symbol UTH.
The transfer characteristic is the relationship between the output voltage uo of the comparator and the input voltage ui in plane right-angle coordinates.
The general procedure for drawing the transfer characteristic is to first find the threshold value, then analyze the change of the output voltage in the two cases from the lowest to the highest input voltage (positive process) and from the highest to the lowest input voltage (negative process) according to the specific circuit of the voltage comparator, and then draw the transfer characteristic.
02 Captured Zero Comparator
The ground terminal in the zero level comparator is connected to a reference voltage UR (set to DC voltage) instead. Since the size and polarity of UR can be adjusted, the circuit becomes an arbitrary level comparator or captive zero comparator.
Figure 2 Arbitrary level comparator and transmission characteristics
(a) Arbitrary level comparator (b) Transfer characteristics
Figure 3 Level detection comparator transmission characteristics
(a) Level detection comparator (b) Transmission characteristics
The level-voltage comparator has a simple structure and high sensitivity, but it has poor interference immunity. That is, if the input signal changes near the threshold due to interference, the output voltage will jump repeatedly between high and low levels, which may cause the output state to malfunction. In order to improve the immunity of the voltage comparator, the hysteresis voltage comparator with two different thresholds is introduced below.
03 Hysteresis voltage comparator
Hysteresis comparator is also known as Schmidt trigger, hysteresis comparator. The characteristic of this comparator is that when the input signal ui gradually increases or decreases, it has two thresholds and is not equal, and its transmission characteristics have the shape of a “hysteresis” curve.
The hysteresis comparator also has two types of inputs: inverting and in-phase.
UR is a fixed voltage, and changing the UR value changes the threshold and the hysteresis size.
Take the inverting hysteresis comparator shown in Figure 4(a) as an example, calculate the threshold value and draw the transmission characteristics.
Figure 4 Hysteresis comparator and its transmission characteristics
(a) Inverted input (b) In-phase input
Forward process
The threshold value of the forward process is
The abcd segment that forms the voltage transfer characteristic.
Negative process
The threshold value of the negative process is
forms the defa segment on the voltage transfer characteristics. It is called a hysteresis voltage comparator because it is similar in shape to the hysteresis line.
Using the threshold condition for finding the threshold and the superposition principle method, it is not difficult to calculate two thresholds for the in-phase hysteresis comparator shown in Figure 4(b).
The difference between the two thresholds ΔUTH=UTH1-UTH2 is called the return difference.
From the above analysis, it is clear that changing the value of R2 can change the size of the return difference, and adjusting the UR can change UTH1 and UTH2, but does not affect the size of the return difference. That is, the transmission characteristics of the hysteresis comparator will be shifted parallel right or left, and the width of the hysteresis curve remains unchanged.
Figure 5 Waveform transformation of comparator
(a) Input waveform (b) Output waveform
For example, the waveforms of the transfer characteristics and input voltage of the hysteresis comparator are shown in Figure 6(a) and (b). Based on the transfer characteristics and the two thresholds (UTH1 = 2V, UTH2 = -2V), the waveform of the output voltage uo can be drawn, as shown in Figure 6(c). As seen in Fig. (c), ui varies between UTH1 and UTH2, which does not cause a jump in uo. However, the return difference also leads to the hysteresis phenomenon of the output voltage, which makes the level discrimination error.
Figure 6 Diagram illustrating the high immunity of the hysteresis comparator to interference
(a) Known transmission characteristics
(b) Known ui waveform
(c) uo waveform drawn from transmission characteristics and ui waveform
04 Window voltage comparator
Level comparators and hysteresis comparators have a common feature that uo jumps only once when ui changes in one direction (positive process or negative process). Only one input signal level can be detected, and this comparator is called a single-limit comparator.
The double-limit comparator is also known as a window comparator. It is characterized by a unidirectional change in the input signal (e.g., ui rises monotonically from sufficiently low to sufficiently high) to jump the output voltage uo twice, and its transmission characteristics are shown in Figure 7(b), which is shaped like a window and is called a window comparator. The window comparator provides two thresholds and two output stabilization states available to determine whether ui is between two levels.
Figure 7 Window comparator circuit and transmission characteristics
(a) Window comparator (b) Transmission characteristics
