Push–pull output: Difference between revisions

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Revision as of 06:27, 1 February 2021

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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_classes#Class_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|accessdateaccess-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.

The '''White Cathode Follower''' (Patent 2,358,428, Sep 1944 by E. L. C. White) is similar to the SEPP design above, but the signal input is to the ''top'' tube, acting as a cathode follower, but one where the bottom tube (in common cathode configuration) if fed (usually via a step-up transformer) from the current in the plate (anode) of the top device. It essentially reverses the roles of the two devices in SEPP. The bottom tube acts part way between a constant current sink and an equal partner in the push–pull workload. Again, the drive to each tube therefore might not be equal.