Amplifier

Amplifier, device for increasing the amplitude, or power, of an electric signal. It is used to amplify the weak electric current drawn from the antenna of a radio-receiving set, the weak output of a photoelectric cell (electric eye), the diminished current in a long-distance telephone circuit, the electrical signal representing sound in a public address system, and for many other purposes. One commonly used amplification device is the transistor. Other forms of amplifying devices are various types of thermionic vacuum tubes such as the triode, pentode, klystron, and magnetron.

Typically, small variations in the input voltage produce corresponding, but much larger, variations in output voltage. The ratio of these voltage changes is called the gain, or amplification factor. When the gain exceeds a certain amount, the output signal no longer matches the input signal; it is distorted. This condition is alleviated by operating the amplifier at less than the maximum gain. When greater amplification is required than is possible with one stage of amplification (that is, one transistor or one vacuum tube and its associated circuits), a multi-stage amplifier is used. The output of one stage is used as the input of another. Amplification in photoelectric circuits may be increased by the use of highly sensitive phototubes known as photomultipliers.

Transistors have largely replaced vacuum tubes in most common devices. These solid-state semiconductive elements exhibit a high gain, operate without distortion over a wide frequency range, and can be made extremely small. Using integrated-circuit techniques, thousands of transistor amplifiers can be placed on very small wafers of silicon.

The pulse response of an amplifier determines its ability to reproduce a square-wave input pulse (a type of regular electric signal) quickly and accurately; square-wave inputs are fed to an amplifier for timing or counting purposes. Pulse response is important in digital-computer circuits, pulse-code modulation, and radar and nuclear instrumentation, that is, wherever high-frequency square-wave pulses are processed. In a radiation counter, for example, the frequency with which radiation particles impinge upon a sensitive element such as a semiconductor junction diode is a measure of the particle intensity or concentration. The resulting diode output may be a series of pulses, which are then amplified and fed to a transducer so that they may be recorded.

Any output produced by an amplifier when the input is zero is called noise. This will also be present when the amplifier is being fed an input signal and will be a form of distortion of the output.

Amplifiers that produce very little noise are critical to communications satellites. Microwave (extremely high frequency) electromagnetic signals are amplified by maser devices (microwave amplification by the stimulated emission of radiation). Instead of amplifying electric current, the maser directly amplifies electromagnetic signals.

Amplifiers are frequently classified by the type of electrical elements in the circuit. Inductance-coupled amplifiers are connected chiefly by coils and transformers; capacitance-coupled by capacitors; and impedance-coupled by resistors. Direct-coupled amplifiers are connected directly, without such electrical components, and are used for alternating currents of very low frequency such as those that occur in many analogue computers. The other types are used for a wide range of frequencies. Audio-frequency amplifiers operate at 0 to about 100,000 cycles per second (hertz), or 100 kilohertz (kHz). Intermediate-range amplifiers deal with frequencies from 400 kHz up to 5 million Hz, and so on.