Operational Amplifiers

The term operational amplifier or "op-amp" refers to a class of high-gain DC coupled amplifiers with two inputs and a single output. The modern integrated circuit version is typified by the famous 741 op-amp. Some of the general characteristics of the IC version are:

High gain, on the order of a million 高增益，约百万倍 High input impedance, low output impedance 高输入阻抗，低输出阻抗 Used with split supply, usually +/- 15V 用于分压供电，通常为±15V Used with feedback, with gain determined by the feedback network. 与反馈配合使用，增益由反馈网络决定

Their characteristics often approach that of the ideal op-amp and can be understood with the help of the golden rules.

The 741 Op-amp

The most common and most famous op-amp is the mA741C or just 741, which is packaged in an 8-pin mini-DIP. The integrated circuit contains 20 transistors and 11 resistors. Introduced by Fairchild in 1968, the 741 and subsequent IC op-amps including FET-input op-amps have become the standard tool for achieving amplification and a host of other tasks. Though it has some practical limitations, the 741 is an electronic bargain at less than a dollar.

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.

Real vs Ideal Op-amp

Readily available, inexpensive IC op-amps have characteristics which are reasonable approximations of an ideal op-amp (data from Simpson):

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