Why are op amps used so frequently in analog electronics?

Op amps are pretty close to being ideal differential amplifiers. So the real question is, what's so great about amplifiers? There are (at least!) three answers.

First, the obvious -- amplifiers let you change the amplitude of a signal. If you have a small signal (say, from a transducer), an amplifier lets you raise its voltage to a useful level. Amplifiers can also reduce the amplitude of a signal, which could be useful to fit it into the range of an ADC, for example.

Amplifiers can also buffer a signal. They present a high impedance on the input side and a low impedance on the output side. This allows a weak source signal to be delivered to a heavy load.

Finally, negative feedback allows amplifiers to filter a signal. So-called active filters (which use amplifiers) are much more flexible and powerful than passive filters (which use only resistors, capacitors, and inductors). I should also mention oscillators, which are made using amplifiers with filtered positive feedback.

Amplitude control, buffering, and filtering are three of the most common things you can do to analog signals. More generally, amplifiers can be used to implement many kinds of transfer functions, which are the basic mathematical descriptions of signal processing tasks. Thus, amplifiers are all over the place.

Why op amps in particular? As I said, op amps are essentially high-quality amplifiers. Their key characteristics are:

  • Very high differential gain (sometimes as high as 1,000,000!)
  • Very high input impedance (teraohms at low-frequency for FET-input op amps)
  • Very high common-mode rejection ratio (typically >1000)

These characteristics mean that the behavior of the amplifier is almost entirely determined by the feedback circuit. Feedback is done with passive components like resistors, which are much better-behaved than transistors. Try simulating a simple common emitter amplifier across voltage and temperature -- it's not great.

With modern improvements in integrated circuits, op amps are cheap, high-performance, and readily available. Unless you need extreme performance (high power, very high frequency) there's not much reason to go with discrete transistor amplifiers anymore.

An op amp is three 5 basic tools in one (if not more).

  • First a comparison device, like an if else statement (if a > b, output = a, else b).

  • Second a buffer (in = 1, out = 1, refreshed).

  • Third an amplifier, like a multiplier (in = 1, out = 10).

  • Fourth, a phase shift/delay (in = x, out = x + 1).

  • Fifth, an inverter (in = x, out = 1/x).

They tend to be very versatile, and able to adapt to many circuits as needed.

Fundamentally, as a signal gets processed through analog discrete elements, its amplitude—its voltage—drops. An op amp can buffer and boost an analog signal, ensuring it is readable or useful at the end.

Incidentally, a for loop would be a counter. A decade counter works like a for (i = 0, i < 10, i++) loop.

Some of the key benefits of an op-amp are

high input impedance: Due to its high input impedance, an op-amp doesn't unduly load the preceding circuit. An op-amp itself might have input impedance in the 10's or 100's of gigohms. An op-amp feedback circuit will likely have a lower input impedance, but the high input impedance of the op-amp allows this to be entirely set by the other components.

low output impedance: Due to its low output impedance, an op-amp circuit can generally drive another op-amp circuit (or an ADC or ...) without the load affecting its behavior.

high gain: The high gain of the op-amp allows it to be used in a negative feedback circuit such that the behavior of the circuit is dominated by the feedback elements rather than by the op-amp. This means

  1. Often only a few precision components are required in the feedback circuit to achieve precision performance from the overall circuit.

  2. Since the behavior of the circuit is controlled by the feedback circuit, the op-amp can be used with numerous different feedback elements to achieve different functions like amplification, differentiation, integration, logarithmic amplification, etc. (This may be the key reason that op-amps have such "pervasive application").