Difference Between Amplifier and Comparator

1. The main difference between amplifiers and comparators is the closed-loop characteristics

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), but 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 and not self-excite. However, a large number of tests must be done to ensure that the product is stable under all working conditions! At this time, you need to carefully calculate the cost/risk.

2. The operational amplifier and the comparator are exactly the same. Simply speaking, the comparator is the open-loop application of the operational amplifier, but the comparator is designed for voltage threshold comparison, requiring the comparison threshold to be accurate, the output edge rise or fall time after comparison to be short, the output to comply with TTL/CMOS level/or OC, etc., and does not require the accuracy of the intermediate links. At the same time, the driving capabilities are also different. In general, when using an operational amplifier as a comparator, most of them cannot achieve full-scale output, or the edge time after comparison is too long, so it is better to use less operational amplifiers as comparators in the design.

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.

⑶: The output of the comparator (LM339) 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 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.