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What is the difference between a comparator and an operational amplifier

elecfans | 02/04/2021

Comparators and operational amplifiers are circuits often used in analog circuit design. They are often used in the basic circuits of analog ICs, such as current mirror circuits, constant current circuits, and operational amplifier circuits. According to the engineer of Yingrui's MCU development, due to the configuration of the internal circuit, the operational amplifier circuit and the comparator circuit are basically composed of a differential amplifier using differential inputs, and they are very similar.

So what is the difference?

(1) The comparator is used for open loop control.

(2) Operational amplifiers are basically used for feedback control.

This is the difference between a comparator and an operational amplifier.

For operational amplifiers, feedback is basically used. Therefore, considering stable operation, usually two-stage amplification + phase compensation is basic. This is because the phase of each amplification stage is usually about 90° and the phase of the second stage is usually 180°. Therefore, phase compensation is performed to generally set the phase margin to about 45° to 60° so that the total phase rotation does not oscillate at about 120° to 135°.

For the above reasons, in the case of operational amplifiers, considering stability, two or more stages of amplification are rarely used except for special applications (for example, when the gain is greatly increased or gain cannot be obtained).

On the other hand, the comparator is basically used for open-loop control and does not return feedback, so even if any number of amplification stages is added, basically no oscillation will occur. Some people don't know how to distinguish them, but basically, if there is a differential pair input, there is no capacitor for phase compensation, and of course there is no feedback.

In addition, since the terminal structure of the comparator is the same as that of the operational amplifier, it is composed of 5 terminals: +input terminal, -input terminal, positive power supply terminal, negative power supply terminal and output terminal. The input terminal of one side is used as the reference terminal to fix the voltage, and the difference between this reference voltage and the voltage input from the other terminal is amplified to output a high or low circuit.

(1) When +potential of input terminal" -potential of input terminal, high output.

(2) When-the potential of the input terminal" + the potential of the input terminal, low output.

In application, the biggest difference between operational amplifiers and comparators is the presence or absence of phase compensation capacitors. The operational amplifier uses a negative feedback circuit, and there must be a phase compensation capacitor inside the IC to prevent vibration. On the other hand, the comparator is not composed of a negative feedback circuit, and there is no built-in phase compensation capacitor. Phase compensation capacitors limit the response time between input and output, and comparators without phase compensation capacitors are more reactive than operational amplifiers.

In addition, if the operational amplifier is used as a comparator depending on the presence or absence of a phase compensation capacitor, because the phase compensation capacitor limits the reactivity, the reactivity is very poor compared with the comparator.

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Operational Amplifiers

Comparators

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