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Operational Amplifiers

The operational amplifier (op-amp) was designed to perform mathematical operations. Although now superseded by the digital computer, op-amps are a common feature of modern analog electronics.

The op-amp is constructed from several transistor stages, which commonly include a differential-input stage, an intermediate-gain stage and a push-pull output stage. The differential amplifier consists of a matched pair of bipolar transistors or FETs. The push-pull amplifier transmits a large current to the load and hence has a small output impedance.

The op-amp is a linear amplifier with . The DC open-loop voltage gain of a typical op-amp is to . The gain is so large that most often feedback is used to obtain a specific transfer function and control the stability.

Cheap IC versions of operational amplifiers are readily available, making their use popular in any analog circuit. The cheap models operate from DC to about 20 kHz, while the high-performance models operate up to 50 MHz. A popular device is the 741 op-amp which drops off 6 dB/octave above 5 Hz. Op-amps are usually available as an IC in an 8-pin dual, in-line package (DIP). Some op-amp ICs have more than one op-amp on the same chip.

Before proceeding we define a few terms:

linear amplifier
- the output is directly proportional to the amplitude of input signal.
open-loop gain, A
- the voltage gain without feedback ( ).
closed-loop gain, G
- the voltage gain with negative feedback (approximation to ).
negative feedback
- the output is connected to the inverting input forming a feedback loop (usually through a feedback resistor ).

Doug Gingrich
Tue Jul 13 16:55:15 EDT 1999