Op amps have so much gain that engineers deliberately throw most away
A typical op amp chip multiplies the voltage difference between its two inputs by 100,000 or more. That is far too much to use directly; a few millionths of a volt would slam the output to its limit. So designers feed part of the output back to tame it, and in exchange get a circuit that behaves precisely.
An operational amplifier is a small electronic amplifier with two inputs, one marked plus and one minus, and usually a single output. It amplifies only the difference between the two input voltages. The name recalls its original job: performing mathematical operations such as adding and integrating inside analogue computers. Today op amps sit in consumer gadgets, factory equipment and lab instruments. Ordinary ones cost a few cents, while specialised versions can exceed 100 dollars.
The raw gain, called open-loop gain, is not only huge but poorly controlled in manufacturing, varying from one chip to the next. Used bare, an op amp simply acts as a comparator: if the plus input is higher, the output swings fully positive; if lower, fully negative. The fix is negative feedback, routing a fraction of the output back to the minus input. Once that loop is in place, the circuit's gain depends almost entirely on the external resistors chosen, not on the quirks of the chip or its temperature.
A classic example is the non-inverting amplifier. Two resistors split the output voltage and feed a portion back. The op amp adjusts its output until the minus input matches the incoming signal, so the gain becomes one plus the ratio of the two resistors. With equal resistors, a 1 volt input produces a 2 volt output. Engineers often reason with two rules of thumb: in a feedback circuit the output does whatever it takes to make the input difference zero, and practically no current flows into the inputs.
Real chips fall short of the ideal. Their gain is large but finite, a little current leaks into the inputs, and imperfections create an input offset voltage, a small error that is amplified along with the signal. Low output impedance helps drive heavy loads but costs idle power. Good designs account for these limits rather than pretending they vanish.
Source: Operational amplifier
