push-pull amplifier

push-pull amplifier

[′pu̇sh ¦pu̇l ′am·plə‚fī·ər]
A balanced amplifier employing two similar electron tubes or equivalent amplifying devices working in phase opposition.
McGraw-Hill Dictionary of Scientific & Technical Terms, 6E, Copyright © 2003 by The McGraw-Hill Companies, Inc.

Push-pull amplifier

An electronic circuit in which two transistors (or vacuum tubes) are used, one as a source of current and one as a sink, to amplify a signal. One device “pushes’’ current out into the load, while the other “pulls” current from it when necessary. A common example is the complementary-symmetry push-pull output stage widely used to drive loudspeakers (see illustration), where an npn transistor can source (push) current from a positive power supply into the load, or a pnp transistor can sink (pull) it into the negative power supply. The circuit functions as an amplifier in that the current levels at the output are larger than those at the input.

A so-called bias network in a complementary-symmetry push-pull output stage (see illustration) functions to maintain a constant voltage difference between the bases of the two transistors. It can be designed either by setting a bias current, and diode sizes or by replacing it with a different network for class B, class A, or the common compromise, class AB mode of operation. In class B operation, where the bases of the transistors might simply be shorted together, only one transistor is “on’’ at a time and each is on average “on” for only 50% of the time; when the output current is zero, no current at all flows in the circuit. In class A operation a large voltage is maintained between the bases so that both devices stay “on’’ at all times, although their currents vary so that the difference flows into the load; and even when the output is zero, a large quiescent current flows from the power supplies. Class B operation is much more efficient than class A, which wastes a large amount of power when the signal is small. However, class B suffers from zero-crossing distortion as the output current passes through zero, because there is generally a delay involved as the input swings far enough to turn one transistor entirely off and then turn the other on. In class AB operation, some intermediate quiescent current is chosen to compromise between power and distortion.

Class AB amplifiers are conventionally used as loudspeaker drivers in audio systems because they are efficient enough to be able to drive the required maximum output power, often on the order of 100 W, without dissipating excessive heat, but can be biased to have acceptable distortion. Audio signals tend to be near zero most of the time, so good performance near zero output current is critical, and that is where class A amplifiers waste power and class B amplifiers suffer zero-crossing distortion. A class AB push-pull amplifier is also conventionally used as the output stage of a commercial operational amplifier. See Operational amplifier, Power amplifier, Transistor

McGraw-Hill Concise Encyclopedia of Engineering. © 2002 by The McGraw-Hill Companies, Inc.
References in periodicals archive ?
The RFPP2870 is a GaN-based push-pull amplifier featuring high gain 28dB (minimum at 1003MHz) and excellent linearity at low distortion levels.
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Sirenza Microdevices has announced the availability of its CGA-6618 HBT push-pull amplifier, targeted at applications in new cable TV digital distribution networks.
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Typical output power of a single PA module is in the 100 to 200 W range and has been accomplished almost exclusively using push-pull amplifier modules, which are then combined into a larger assembly to form a 500 to 700 W PA.
The amplifier also features a unique amplifier design that, according to Thule, effectively eliminates switching distortion, the kind that occurs in push-pull amplifiers when then sine wave crosses from positive to negative.
Baluns, (1) which transform a balanced input transmission signal to unbalanced output signals, are widely used in many wireless applications, such as balanced push-pull amplifiers, frequency doublers, (2) antenna feed networks (3) and double-balanced mixers.
This InP technology has been employed to fabricate NPN-PNP push-pull amplifiers, which yield higher linearity and better efficiency than conventional amplifiers.