push-pull amplifier

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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.

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

References in periodicals archive ?
The push-pull output stage is configured either active high or active low; the open-drain output stage is only configured active high.
All devices, with the exception of the AH1812, include an internal push-pull output structure, which enables a reduction in external component count and a simplification of PCB layouts.
For the OLS2 and OLS5 sensors, output is via a TTL compatible push-pull output.
Pete even supplies a technical translation, saying: "A superhet employs superhetrodyne tuning, push buttons being employed for station selection and a push-pull output stage uses two valves working alternately to give a high quality output.
LIS0 linear encoders, available with either a push-pull output or an RS-422 interface, provide a voltage range from 4.
LI50 linear encoders are available with either a push-pull output or an RS-422 interface and provide a voltage range from 4.
Users can set the device to any push-pull output voltage between 3.
2 V window comparator features a push-pull output and consumes 1.
75 to 30 VDC power and provide short-circuit proof, push-pull output, or 5 VDC operation with RS,422 functionality.
Moreover the push-pull output circuitry reduces power consumption to the absolute minimum when driving any capacitive or resistive load.