Op amps and comparators are similar in appearance and schematic symbols, and it is difficult to distinguish between them if the logo on the top is polished. In addition, many circuits will have applications that use op amps as comparators, so many beginners often confuse op amps and comparators, so this article shares the differences between them.
Figure 1 Schematic diagram of the op amp pins
Operational amplifiers differ from comparators
The function of op amps and comparators is stated as follows:
An operational amplifier is an analog chip used for signal conditioning. For example, amplifying, filtering, summing, etc. of the signal.
A comparator is a circuit that compares an analog voltage signal to a reference voltage.
Op amps are used in negative feedback systems, while comparators are typically used in open-loop systems, and positive feedback can also be introduced to design hysteresis comparators.
a) The reaction speed of the op amp is related to the slew rate, and when a step signal is input, the op amp is established at a certain speed, while the comparator flips immediately.
b) For some high-voltage op amps, as well as op amps for Bipolar (transistor) processes, there will be bidirectional clamp diodes between the two input pins (see Figure 2). The comparator does not have a bidirectional clamped diode, and a larger input differential mode signal has less effect on the comparator. Therefore, to use the op amp as a comparator, it is necessary to determine whether it is necessary to add a current limiting resistor so as not to burn out the op amp.
c) Op amps generally measure their flip speed by bandwidth and slew rate, while comparators use T PHL, T PLH, T fall, T rise, to represent speed. A simple way to calculate comparator speed is:
d) The output of the comparator can characterize the potential of the two inputs. The outputs of most comparators on the market are now compatible with TTL and CMOS levels, and the output of the comparator is always one of the positive and negative power rails. However, the output of the op amp is an analog signal, and the current low-voltage CMOS-type op amp can generally achieve rail-to-rail output.
e) When the op amp output is overloaded, it requires a relatively long recovery time, while the comparator's overload recovery time is very short.
f) Some op amps have input common-mode voltage ranges that do not include a positive rail; many comparator inputs are supported to the positive rail.
g) The op amp can be used as a comparator (in some low-speed flip situations), while the comparator cannot be used as a op amp.
Figure 2 The internal input protection circuit of the op amp
In summary, op amps differ from comparators in that they are in different operating regions, and op amps generally operate in linear regions (closed loops). The input voltage of the op amp in the linear region is proportional to the output voltage, and the amplification function can be used.
The comparator operates in a nonlinear region (open loop), i.e. the input and output of the comparator are no longer proportional to the output in this region, and there is no amplification effect. At this stage its output has only two states, the "high" and "low" states, which are the concepts of "1" and "0" if expressed numerically. In the general general amplifier, there will be two regions of linear and nonlinear, while in the comparator there is only one nonlinear area, which can be summarized simply, the amplifier can be used as a comparator in scenarios where the speed requirements are not high (note the common mode input voltage range), but the comparator cannot be used as an amplifier.
The internal circuit structure of the two is different, and the output stage of the op amp is generally a push-pull circuit architecture. The output stage of some comparators is an open-drain or open-source architecture, which requires a pull-up resistor or pull-down resistor to match the voltage of the post-stage digital circuit.
Also included is a comparison of the open-drain output and the push-pull output structure:
Table 1 Open-drain output versus push-pull output