Push–pull output: Difference between revisions

Revision as of 18:26, 21 August 2021

Revision as of 18:03, 10 September 2021

Line 1:

{{Other uses of|push–pull|Push–pull (disambiguation){{!}}Push–pull}}

{{RefimproveMore citations needed|date=November 2017}}

[[File:Pushpull (English).png|right|thumb|A Class B push–pull output driver using a pair of complementary PNP and NPN [[bipolar junction transistor]]s configured as [[emitter follower]]s]]

Line 8:

Push–pull outputs are present in [[Transistor-transistor logic|TTL]] and [[CMOS]] digital [[logic circuit]]s and in some types of [[electronic amplifier|amplifiers]], and are usually realized by a complementary pair of [[transistor]]s, one dissipating or ''sinking'' current from the load to ground or a negative power supply, and the other supplying or ''sourcing'' current to the load from a positive power supply.

A push–pull amplifier is more efficient than a single-ended [[Power_amplifier_classesPower amplifier classes#Class_AClass A|"class-A"]] amplifier. The output power that can be achieved is higher than the continuous dissipation rating of either transistor or tube used alone and increases the power available for a given supply voltage. Symmetrical construction of the two sides of the amplifier means that even-order harmonics are cancelled, which can reduce distortion.<ref>Joe Carr, ''RF Components and Circuits'', Newnes, page 84</ref> DC current is cancelled in the output, allowing a smaller output transformer to be used than in a single-ended amplifier. However, the push–pull amplifier requires a phase-splitting component that adds complexity and cost to the system; use of center-tapped [[transformer]]s for input and output is a common technique but adds weight and restricts performance. If the two parts of the amplifier do not have identical characteristics, distortion can be introduced as the two halves of the input waveform are amplified unequally. [[Crossover distortion]] can be created near the zero point of each cycle as one device is cut off and the other device enters its active region.

[[File:Tube push pull poweramplifier.PNG|thumb|alt=Schematic diagram of vacuum-tube amplifier| A vacuum tube amplifier often used a center-tapped output transformer to combine the outputs of tubes connected in push–pull.]]

Line 40:

In analog push–pull power amplifiers the two output devices operate in [[antiphase]] (i.e. 180° apart). The two antiphase outputs are connected to the load in a way that causes the signal outputs to be added, but distortion components due to non-linearity in the output devices to be subtracted from each other; if the non-linearity of both output devices is similar, distortion is much reduced. Symmetrical push–pull circuits must cancel even order harmonics, like f2, f4, f6 and therefore promote odd order harmonics, like (f1), f3, f5 when driven into the nonlinear range.

A push–pull amplifier produces less [[distortion]] than a single-ended one. This allows a [[Power_amplifier_classesPower amplifier classes#Class_AClass A|class-A]] or [[Power_amplifier_classesPower amplifier classes#Class_ABClass AB|AB]] push–pull amplifier to have less distortion for the same power as the same devices used in single-ended configuration. [[Power_amplifier_classesPower amplifier classes#Class_ABClass AB|Class AB]] and [[Power_amplifier_classesPower amplifier classes#Class_BClass B|class B]] dissipate less power for the same output than class A; distortion can be kept low by [[negative feedback]] and by biassing the output stage to reduce crossover distortion.

A class-B push–pull amplifier is more efficient than a class-A power amplifier because each output device amplifies only half the output waveform and is cut off during the opposite half. It can be shown that the theoretical full power efficiency (AC power in load compared to DC power consumed) of a push–pull stage is approximately 78.5%. This compares with a class-A amplifier which has efficiency of 25% if directly driving the load and no more than 50% for a transformer coupled output.<ref name=Yunik73>Maurice Yunik ''Design of Modern Transistor Circuits'', Prentice-Hall 1973 {{ISBN|0-13-201285-5}} pp. 340-353</ref> A push–pull amplifier draws little power with zero signal, compared to a class-A amplifier that draws constant power. Power dissipation in the output devices is roughly one-fifth of the output power rating of the amplifier.<ref name=Yunik73/> A class-A amplifier, by contrast, must use a device capable of dissipating several times the output power.

Line 50:

=== Push–pull transistor output stages ===

[[File:Aura VA 100 Evolution 2 (4061759992) - closeup of output stage.jpg|thumb|upright=1.5|Typical transistor output stage of one channel of a 65 watt stereo amplifier from 1993. The 2 MOSFET push-pull output transistors (''FET2, FET4'') are bolted to the black [[heat sink]]. They are driven by transistors ''Q2, Q5, Q6,'' and ''Q7'']]

{{refimprovemore citations needed section|date=November 2012}}

Categories include:

Line 69:

==== Square-law push–pull ====

The output devices, usually [[MOSFET]]s or [[vacuum tube]]s, are configured so that their [[Power-law#square-law|square-law]] transfer characteristics (that generate second-harmonic [[distortion]] if used in a single-ended circuit) cancel distortion to a large extent. That is, as one transistor's gate-source voltage increases, the drive to the other device is reduced by the same amount and the drain (or plate) current change in the second device approximately corrects for the non-linearity in the increase of the first.<ref>{{cite journal | author=Ian Hegglun | title=Practical Square-law Class-A Amplifier Design | journal=Linear Audio - Volume |volume=1}}</ref>

=== Push–pull tube (valve) output stages ===

Line 75:

[[Vacuum tube]]s (valves) are not available in complementary types (as are pnp/npn transistors), so the tube push–pull amplifier has a pair of identical output tubes or groups of tubes with the [[control grid]]s driven in antiphase. These tubes drive current through the two halves of the primary winding of a center-tapped output transformer. Signal currents add, while the distortion signals due to the non-linear [[Current–voltage characteristic|characteristic curve]]s of the tubes subtract. These amplifiers were first designed long before the development of solid-state electronic devices; they are still in use by both [[audiophile]]s and musicians who consider them to sound better.

Vacuum tube push–pull amplifiers usually use an output transformer, although [[Output transformerless|Output-transformerless (OTL)]] tube stages exist (such as the SEPP/SRPP and the White Cathode Follower below).{{citation needed|date=December 2012}} The phase-splitter stage is usually another vacuum tube but a transformer with a center-tapped secondary winding was occasionally used in some designs. Because these are essentially square-law devices, the comments regarding [[Distortion#Cancellation of even-order harmonic distortion|distortion cancellation]] mentioned [[Push–pull output#Square-law push–pull|above]] apply to most push–pull tube designs when operated in [[Power_amplifier_classesPower amplifier classes#Class_AClass A|class A]] (i.e. neither device is driven to its non-conducting state).

A '''Single Ended Push–Pull''' ('''SEPP''', '''SRPP''' or '''mu-follower'''<ref>{{cite web|title=SRPP Decoded|url=http://www.tubecad.com/may2000/|website=The Tube CAD Journal|access-date=7 November 2016}}</ref>) output stage, originally called the '''Series-Balanced amplifier''' (US patent 2,310,342, Feb 1943). is similar to a totem-pole arrangement for transistors in that two devices are in series between the power supply rails, but the input drive goes ''only to one of the devices,'' the bottom one of the pair; hence the (seemingly contradictory) Single-Ended description. The output is taken from the cathode of the top (not directly driven) device, which acts part way between a constant current source and a cathode follower but receiving some drive from the plate (anode) circuit of the bottom device. The drive to each tube therefore might not be equal, but the circuit tends to keep the current through the bottom device somewhat constant throughout the signal, increasing the power gain and reducing distortion compared with a true single-tube single-ended output stage.