The comparator is typically made up of an integrated operational amplifier. Unlike the standard operational amplifier circuit, the integrated amplification in a comparator is often in an open-loop or positive feedback state. As soon as a small signal is applied to both inputs, the op-amp enters the non-linear region and falls within part of the non-linear application range of the integrated op-amp. When analyzing a comparator, the principle of "virtual break" still applies, while the concepts of "virtual short" and "virtual ground" are only used when determining critical situations.
Firstly, there's the zero-level comparator (or zero-crossing comparator). A voltage comparator is essentially a circuit that compares and identifies an analog input signal, \(u_i\), with a fixed reference voltage \(U_R\). If the reference voltage is zero, it’s called a zero-level comparator. Depending on the input mode, there are two types of zero-level comparators: inverting input and non-inverting input, as illustrated in Figures 1(a) and (b).
[Image description: Diagrams showing inverting and non-inverting zero-level comparators]
The threshold voltage and transmission characteristics are commonly used to describe the operational characteristics of the comparator. The threshold voltage (also known as the threshold level) is the input voltage value when the comparator's output voltage flips, referred to as the threshold value, denoted by \(U_{TH}\).
Transmission characteristics refer to the relationship between the output voltage (\(u_o\)) of the comparator and the input voltage (\(u_i\)) on a rectangular coordinate plane. Generally, the process for drawing transmission characteristics involves first finding the threshold and then analyzing the input voltage from lowest to highest (forward process) and from highest to lowest (reverse process) based on the specific circuit of the voltage comparator. Next, the output voltage changes accordingly, allowing us to draw the transmission characteristics.
Secondly, there's the any-level comparator (or capture-zero comparator). In the zero-level comparator, the ground terminal is connected to a reference voltage (\(U_R\)), set to a DC voltage. Since the size and polarity of \(U_R\) can be adjusted, this transforms the circuit into an any-level comparator or a zero-trap comparator.
[Image description: Diagram of an any-level comparator with its transmission characteristics]
The level voltage comparator has a simple structure and high sensitivity, but its anti-interference capability is weak. That means that if the input signal changes due to interference near the threshold, the output voltage might repeatedly jump between high and low levels, potentially causing the output state to malfunction. To enhance the anti-interference capability of the voltage comparator, we’ll discuss a hysteresis voltage comparator with two distinct thresholds below.
Thirdly, there’s the hysteresis voltage comparator, also known as a Schmitt trigger. A key feature of this comparator is that when the input signal (\(u_i\)) gradually increases or decreases, it has two thresholds that are unequal, and its transmission characteristic forms a "hysteresis" curve. The hysteresis comparator also comes in two varieties: inverting input and non-inverting input.
\(U_R\) is a fixed voltage, and adjusting its value can change the threshold and the magnitude of the hysteresis. Taking the inverting hysteresis comparator shown in Figure 4(a) as an example, we can calculate the thresholds and plot the transmission characteristics.
[Image description: Diagram of an inverting hysteresis comparator with its transmission characteristics]
1. Threshold during the forward process.
2. Threshold during the reverse process.
[Image description: Diagram illustrating the forward and reverse processes on the voltage transfer characteristic]
It’s straightforward to calculate the two thresholds of the non-inverting hysteresis comparator shown in Figure 4(b) using the threshold condition and the superposition principle method.
[Image description: Diagram of a non-inverting hysteresis comparator]
The difference between the two thresholds, \(\Delta U_{TH} = U_{TH1} – U_{TH2}\), is called the hysteresis. From the above analysis, it’s clear that changing the \(R_2\) value can alter the magnitude of the hysteresis, and adjusting \(U_R\) can change \(U_{TH1}\) and \(U_{TH2}\), but it won’t affect the magnitude of the hysteresis. In other words, the transmission characteristics of the hysteresis comparator will shift to the right or left, but the width of the hysteresis curve remains unchanged.
[Image description: Diagram showing the waveform transformation of the comparator]
For instance, the transmission characteristics of the hysteresis comparator and the waveform of the input voltage are depicted in Figures 6(a) and (b). Based on the transmission characteristics and the two thresholds (\(U_{TH1}=2V, U_{TH2}=-2V\)), the waveform of the output voltage (\(u_o\)) can be drawn, as shown in Figure 6(c). From Figure (c), it’s evident that \(u_i\) changes between \(U_{TH1}\) and \(U_{TH2}\) without causing \(u_o\) to jump. However, the hysteresis also leads to hysteresis in the output voltage, resulting in level discrimination errors.
[Image description: Diagram showing the strong anti-interference ability of the hysteresis comparator]
Finally, there’s the window voltage comparator. Both the level comparator and the hysteresis comparator share a common trait: when \(u_i\) changes in one direction (either the forward process or the reverse process), \(u_o\) only jumps once. This allows the detection of only one input signal level, making this comparator a single-limit comparator.
The double-limit comparator is also called the window comparator. Its unique feature is that a single-direction change in the input signal (for example, \(u_i\) rising from a low enough level to a high enough level) makes the output voltage (\(u_o\)) jump twice, and its transmission characteristics resemble a window. The window comparator provides two thresholds and two stable output states, enabling the determination of whether \(u_i\) falls between two specific levels.
[Image description: Diagram of a window comparator circuit and its transmission characteristics]
In summary, each type of comparator serves different purposes depending on the application requirements. Understanding these differences helps in selecting the appropriate comparator for various electronic circuits.
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