The Ideal Op-amp

The IC Op-amp comes so close to ideal performance that it is useful to state the characteristics of an ideal amplifier without regard to what is inside the package.

These characteristics lead to the golden rules for op-amps. They allow you to logically deduce the operation of any op-amp circuit.

The Op-amp Golden Rules

From Horowitz & Hill:

For an op-amp with external feedback

Comments on 1: The voltage gain of a real op-amp is so high that a fraction of a millivolt input will swing the output over its full range. Practically, that amounts to rule 1.

Comments on 2: The input current is so low (0.08 microamps for the 741, picoamps for an FET-input op-amp) that rule 2 is practically correct.

The 741 Op-amp Circuit

The standard 741 Op-amp circuit contains 20 transistors and 11 resistors. It starts with a differential input stage with a current mirror load. This is followed by an npn voltage amplification stage with an active output. A pnp emitter follower drives a push-pull emitter follower output stage. The output stage includes current limiting circuitry.