Voltage comparator is used to identify and compare input signalsCircuit, yes and nosine waveThe basic unit circuit of the generating circuit.Common voltage comparators include single limit comparator, hysteresis comparator, window comparator, tristate voltage comparator, etc.
The voltage comparator can be regarded as an operational amplifier whose amplification is close to infinity.
Function of voltage comparator: compare the size of two voltages (use the high or low level of output voltage to indicate the size relationship of two input voltages):
When the "+" input terminal voltage is higher than the "-" input terminal, the voltage comparator output is high level;
When the "+" input terminal voltage is lower than the "-" input terminal, the voltage comparator output is low level;
It can work in linear working area and nonlinear working area.
When working in a linear workspace, it is characterized by virtual shortness,Virtual disconnection；
When working in nonlinear working area, the characteristics are jump and virtual break;
Since the output of the comparator has only two states of low level and high level, the integrated operational amplifier usually works in the nonlinear region.From the perspective of circuit structure, the operational amplifier is usually in the open loop state, and in order to make the conversion of the output state of the comparator more rapid, so as to improve the response speed, it is generally connected with positive feedback in the circuit.[1]
Function improvement
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Function improvement
The simple voltage comparator has simple structure, high sensitivity, but poor anti-interference ability, so we need to improve it.The improved voltage comparators include hysteresis comparator and window comparator.[2]
characteristic
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Operational amplifier determines "operation parameters" through feedback circuit and input circuit, such as amplification factor. Feedback quantity can be part or all of output current or voltage.The comparator does not need feedback. It directly compares the quantities of the two input terminals. If the in-phase input is greater than the inverse, it outputs a high level, otherwise it outputs a low level.The input of the voltage comparator is a linear quantity, and the output is a switching (high and low level) quantity.In general applications, linear operational amplifiers can sometimes be used to form voltage comparators without negative feedback.[2]
Operational amplifier voltage comparator
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Operational amplifieruseDifferential input, and has a highgain, which is similar to the characteristics of the comparator, so it can be used as a low performance comparator in practical applications.
Theoreticallyopen loopConfiguration (Nonenegative feedback）The operational amplifier can play the role of low-end comparator.When the voltage at the positive phase input (V+) is higher than that at the negative phase input (V -)Open loop gain, output a forward saturation voltage+U at the output terminalsat。When the voltage at the inverting input terminal (V -) is higher than that at the normal input terminal (V+), a reverse saturation voltage - U is output at the output terminalsat。For the operational amplifier working in the negative feedback configuration of the linear section and powered by the separated voltage (± V), its transfer function can be written as follows:
, which is different from the nonlinear non negative feedback comparator.
In practice, compared with using a dedicated comparator, using an operational amplifier comparator has the following disadvantages:
one
The op amp is designed to work in the linear section with negative feedback, so the saturated op amp usually has a slower flip speed.Most operational amplifiers are equipped with aPendulum rateCompensation forcapacitance。This makes the operational amplifier comparator generally existMicrosecondGradePropagation delayIn contrast, the flip speed of the special comparator isnanosecondmagnitude.
two
The op amp has no built-in hysteresis circuit and requires a dedicated external network to delay the input signal.
three
The current at the static operating point of the operational amplifier is onlynegative feedbackStable under conditions.Appears when the input voltage is unequaldirectOffset.
four
The function of the comparator isdigital circuit When using the operational amplifier comparator to generate input signals, it is necessary to considercompatibility。
five
Multiple operational amplifiers may be generated between different frequenciesinterfere。
six
Many op amps have reverse inputsseries connectionOfdiode。The input of the two poles of the op amp is generally the same, which will not cause problems.However, the two poles of the comparator need to be connected with different voltages, which may lead to unexpected diode breakdown.[1]
