operational amplifier

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operational amplifier,

device that accepts a varying input signal and produces an output signal that varies in the same way as the input but has a larger amplitude. The input signal may be a current, a voltage, a mechanical motion, or any other signal; the output signal is usually of the
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 whose output voltage is proportional to the negative of its input voltage and that boosts the amplitude of an input signal many times, i.e., has a very high gain. It is usually connected so that part of the output is fed back to the input. Operational amplifiers were originally developed to be used in synthesizing mathematical operations in analog computerscomputer,
device capable of performing a series of arithmetic or logical operations. A computer is distinguished from a calculating machine, such as an electronic calculator, by being able to store a computer program (so that it can repeat its operations and make logical
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, hence their name. Because of recent advances in semiconductor technology, they have become available as integrated circuits. They are widely used when a closely controlled amount of gain or some form of signal processing is necessary in an electronic system.

operational amplifier

[‚äp·ə′rā·shən·əl ′am·plə‚fī·ər]
An amplifier having high direct-current stability and high immunity to oscillation, generally achieved by using a large amount of negative feedback; used to perform analog-computer functions such as summing and integrating.

Operational amplifier

A voltage amplifier that amplifies the differential voltage between a pair of input nodes. For an ideal operational amplifier (also called an op amp), the amplification or gain is infinite.

Most existing operational amplifiers are produced on a single semiconductor substrate as an integrated circuit. These integrated circuits are used as building blocks in a wide variety of applications. See Integrated circuits

Although an operational amplifier is actually a differential-input voltage amplifier with a very high gain, it is almost never used directly as an open-loop voltage amplifier in linear applications for several reasons. First, the gain variation from one operational amplifier to another is quite high and may vary by ±50% or more from the value specified by the manufacturer. Second, other nonidealities such as the offset voltage make it impractical to stabilize the dc operating point. Finally, performance characteristics such as linearity and bandwidth of the open-loop operational amplifier are poor. In linear applications, the operational amplifier is almost always used in a feedback mode.

A block diagram of a classical feedback circuit is shown in illus. a. The transfer characteristic, often termed the feedback gain Aƒ of this circuit, is given by Eq. (1). In the limiting case, as (1)  A becomes very large, the feedback gain is approximated by Eq. (2). (2)  See Feedback circuit

An operational amplifier is often used for the amplifier designated A in this block diagram. Since Af in the limiting case is independent of A, the exact gain characteristics of the operational amplifier become unimportant provided the gain is large. Although linear applications of the operational amplifier extend well beyond the simple feedback block diagram of illus. a, the applications invariably involve circuit structures with feedback that make the characteristics of the circuit nearly independent of the exact characteristics of the operational amplifier. Such circuits are often termed active circuits.

The commonly used operational amplifier symbol is shown in illus. b. In this circuit, the output voltage is related to the gain A of

the operational amplifier by Eq. (3), where A is very large and the input currents I+ and I- are nearly zero. See Amplifier, Circuit (electronics)