Practical | Talk about the essential difference between op amp and comparator

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Operational amplifiers and comparators are similar in appearance and on drawings, so what are the differences between them and how to distinguish them in practical applications? Today I will give a comprehensive analysis of them with pictures and texts to consolidate your foundation and help engineers reach a higher level.

Let's take a look at their internal differences first:

From the internal diagram, we can see that the difference between the operational amplifier and the comparator lies in the output circuit. The operational amplifier uses a dual transistor push-pull output, while the comparator uses only one transistor, with the collector connected to the output terminal and the emitter grounded.

The comparator requires an external pull-up resistor from the positive power supply to the output terminal, which is equivalent to the collector resistance of the transistor.

Operational amplifiers can be used in linear amplification circuits (negative feedback) or in nonlinear signal voltage comparison (open loop or positive feedback).

Voltage comparators can only be used for signal voltage comparison and cannot be used in linear amplification circuits (comparators have no frequency compensation).

Both can be used to compare signal voltages, but comparators are designed as high-speed switches with faster slew rates and shorter delays than op amps.

As a linear amplifier circuit, I will not say more here (there will be a need to discuss amplifiers separately in the future). This is very common in motherboard circuit diagrams. It is generally used in voltage stabilization circuits. Using a negative feedback circuit, it is equivalent to a three-terminal voltage regulator in conjunction with a transistor, but it is more flexible to use. As shown below:

In many cases, you need to know which of two signals is larger, or when a signal exceeds a preset voltage (used as a voltage comparison). It is easy to build a simple circuit using an operational amplifier to achieve this function. When the V+ voltage is greater than the V- voltage, the output is a high level. When the V+ voltage is less than the V- voltage, the output is a low level. As shown below:

Analyze the circuit, 2.5V is divided by resistors to get 1V input to V- terminal, when the bus voltage normally generates 1.2V, it is input to V+, at this time V+ voltage is higher than V- voltage, output a high level to the EN start pin of CPU power management chip. If the bus voltage is not output or abnormally less than 1V, at this time V+ voltage is lower than V- voltage, output low level.

When the voltage at the in-phase terminal (V+) of the comparator is lower than the voltage at the inverting terminal (V-), the output transistor is turned on and the output is grounded at a low level; when the voltage at the in-phase terminal is higher than the inverting terminal, the output transistor is turned off and the power supply output through the pull-up resistor is high. As shown in the figure below:

Let's analyze the circuit. When the comparator U8A above has VCC output, it is divided by the voltage divider resistor and input to the non-inverting terminal (V+). Its voltage is greater than the voltage of 5VSB input to the inverting terminal (V-) after voltage division. The internal transistor is turned off, and the power supply 12v is output through the pull-up resistor (at the same time, the voltage of the non-inverting terminal of the comparator U8B below is also greater than the inverting terminal, and the internal transistor is also turned off). The N-channel field transistor Q37 is turned on and outputs VCC5V. At the same time, the P-channel field transistor Q293 is turned off. Conversely, when the inverting terminal voltage is greater than the non-inverting terminal voltage, the internal transistor is turned on, the pull-up power supply 12V is pulled down to a low level, the N-channel field transistor Q37 is turned off, and the P-channel field transistor Q293 is turned on, and the output is 5VSB. This is the 5VDUAL generation circuit.

In practical applications, comparators require a pull-up power supply, while operational amplifiers generally do not.

The essential difference between op amp and voltage comparator:

(1) The main difference between an amplifier and a comparator is the closed-loop characteristic

Most amplifiers work in a closed-loop state, so they are required not to self-excite after closing the loop. Most comparators work in an open-loop state and pursue speed. For low frequencies, amplifiers can completely replace comparators (pay attention to the output level). Conversely, comparators cannot be used as amplifiers in most cases.

Because the comparator is optimized for speed, this optimization reduces the closed-loop stability range. The op amp is optimized for the closed-loop stability range, so it reduces the speed. Therefore, comparators and amplifiers of the same price range are best to perform their respective duties. Just as an amplifier can be used as a comparator, it cannot be ruled out that a comparator can also be used as an amplifier. But the price you pay for its closed-loop stability may exceed the price of adding an amplifier.

In other words, whether an op amp is used as a comparator or an amplifier depends on the depth of the circuit's negative feedback. Therefore, a shallow closed-loop comparator may work in an amplifier state without self-excitation. However, a large number of tests must be conducted to ensure that the product is stable under all working conditions. At this time, you must carefully calculate the cost/risk.

(2) Operational amplifiers and comparators are exactly the same. Simply put, comparators are open-loop applications of op amps, but comparators are designed for voltage threshold comparisons. They require precise comparison thresholds, short output edge rise or fall times after comparison, and outputs that comply with TTL/CMOS levels/or OC, etc. They do not require accuracy in the intermediate links, and their driving capabilities are also different. In general, when op amps are used as comparators, most cannot achieve full-scale output, or the edge time after comparison is too long. Therefore, it is better to use fewer op amps as comparators in designs.

The difference between op amp and comparator:

Although comparators and op amps have the same symbols on the circuit diagram, there are very big differences between these two devices and they are generally not interchangeable. The differences are as follows:

1. The comparator has a fast flipping speed, which is about the order of ns, while the op amp flipping speed is generally on the order of us (except for special high-speed op amps).

2. The op amp can be connected to the negative feedback circuit, but the comparator cannot use negative feedback. Although the comparator also has two input terminals, the in-phase and inverting input terminals, it does not have a phase compensation circuit inside. Therefore, if negative feedback is connected, the circuit cannot work stably. There is no internal phase compensation circuit, which is also the main reason why the comparator is much faster than the op amp.

3. The output stage of the op amp generally adopts a push-pull circuit and a bipolar output. However, the output stage of most comparators is an open collector structure, so a pull-up resistor is required, and the unipolar output is easy to connect to digital circuits.

(3) The output of the comparator (LM339 and LM393) is an open collector (OC) structure, which requires a pull-up resistor to be able to output current. The output stage of the op amp is a push-pull structure, which has symmetrical current sourcing and sinking capabilities. In addition, in order to speed up the response speed, the comparator has very few intermediate stages and no internal frequency compensation. The op amp has added a compensation circuit to meet the needs of working in the linear region. Therefore, the comparator (LM339 and LM393) is not suitable for use as an op amp. The op amp is mainly used in the feedback circuit, sampling and amplification of overcurrent protection, etc. in the switching power supply.

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