amplifier

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 same nature. The most common types of amplifiers are electronic and have transistors or electron tubes as their principal components. Electronic amplifiers are used in radio and television transmitters and receivers, audio and stereo systems, intercoms, and other consumer electronics devices. Amplifiers in their simplest form are built around a single transistor. In one type of single-transistor amplifier, known as a common-emitter circuit, a varying input voltage is fed to the base of the transistor, and the output appears at the transistor's collector; the ratio of the output voltage to the input voltage is called the voltage gain. For many purposes a single transistor does not provide sufficient gain, or amplification. In a cascade, or multistage, amplifier, the output of the first amplifying device (transistor) is fed as input to the second amplifying device, whose output is fed as input to the third, and so on until an adequate signal amplification has been achieved. In a device such as a radio receiver, several amplifiers boost a weak input signal until it is powerful enough to drive a speaker. Usually, multistage amplifiers are not made of discrete components, but are built as integrated circuits. Another less common group of electronic amplifiers use magnetic devices as their principal components. There are also many kinds of mechanical amplifiers, e.g., the power steering system of an automobile. See operational amplifier.

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amplifier

Device that responds to a small input signal (voltage, current, or power) and delivers a larger output signal with the same waveform features. Amplifiers are used in radio and television receivers, high-fidelity audio equipment, and computers. Amplification can be provided by electromechanical devices (e.g., transformers and generators) and vacuum tubes, but most electronic systems now employ solid-state microcircuits. One amplifier is usually insufficient, so its output is fed into a second, whose output is fed to a third, and so on, until the output level is satisfactory.

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amplifier

A device that accepts a small signal and outputs a larger signal that generally matches the waveform characteristics of the input. Amplifiers are available to boost electrical and optical signals. See amplifier classes, Class D amplifier and PWM.

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amplifier

1. an electronic device used to increase the strength of the signal fed into it

2. such a device used for the amplification of audio frequency signals in a radio, etc.

3. Photog an additional lens for altering the focal length of a camera lens

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amplifier [′am·plə‚fī·ər]

(engineering)

A device capable of increasing the magnitude or power level of a physical quantity, such as an electric current or a hydraulic mechanical force, that is varying with time, without distorting the wave shape of the quantity.

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Amplifier

A device capable of increasing the magnitude of a physical quantity. This article discusses a couple of basic electronic amplifiers whose operation depends on transistors. Some amplifiers are magnetic, while others may take the form of rotating electrical machinery. Forms of nonelectrical amplifiers are hydraulic actuators and levers which are amplifiers of mechanical forces. See Direct-current motor, Hydraulic actuator

The operation of an amplifier can be explained with a model (see illustration). A controlled voltage source of gain K generates an output voltage V_o = KV_i from an input voltage V_i. This input voltage is obtained from a source voltage V_S with source resistance R_S via voltage division with the amplifier's input impedance z_i. The load voltage V_L across the load impedance Z_L is obtained from V_o by voltage division with the amplifier's output impedance z_o. The input voltage and load voltage are given by Eqs. (1), where k_i and k_o, respectively, express the effects of the amplifier loading the source and of the load impedance loading the amplifier. The impedances z_i and z_o mostly consist of a resistor in parallel with a capacitor; often they may be assumed to be purely resistive: z_i = r_i and z_o = r_o. From Eq. (1), the amplifier's operation is given by Eq. (2).

(1) 

(2) 

Thus, the amplification is decreased from the ideal value K by the two load factors k_i and k_o. The reduction in gain is avoided if the two factors equal unity, that is, if z_i = ∞ and z_o = 0. Thus, in addition to the required gain K, a good amplifier has a very large input impedance z_i and a very small output impedance z_o. See Gain

Amplifier model with source, load, and input and output impedances

The operational amplifier (op amp) is a commonly used general-purpose amplifier. It is implemented as an integrated circuit on a semiconductor chip, and functions as a voltage amplifier whose essential characteristics at low frequencies are very high voltage amplification, very high input resistance, and very low output resistance. See Integrated circuits

The transconductance amplifier has become widely used. In contrast to operational amplifiers, which convert an input voltage to an output voltage, transconductors are voltage-to-current converters described by the transconductance parameter g_m, which satisfies Eq. (3).

(3) 

Thus, the output current I_out is proportional to the input voltage V_in. As do operational amplifiers, ideal transconductors have an infinite input resistance, but they also have an infinite output resistance so that the output is an ideal current source. One of the attractive properties of transconductance amplifiers is their wide bandwidth. Very simple transconductance circuits can be designed which maintain their nominal g_m values over bandwidths of several hundred megahertz, whereas operational amplifiers often have high gain only over a frequency range of less than 100 Hz, after which the gain falls off rapidly. Consequently, in high-frequency communications applications, circuits built with transconductance amplifiers generally give much better performance than those with operational amplifiers.

Typically, amplifiers increase the power or signal levels from low-power sources, such as microphones, strain gages, magnetic disks, or antennas. After the small signals have been amplified, the amplifier's output stage must deliver the amplified signal efficiently, with minimal loss, and with no distortion to a load, such as a loudspeaker.