SRPP Deconstructed

   "One method of reducing distortion is to replace the single tube in the amplifier by two tubes operating in push-pull. It will be recalled that in push-pull operation the even-harmonic distortion currents produced by the two tubes are in opposite directions. An exact cancellation of these even-harmonic components requires that the two tubes have identical characteristics and be driven with signals which are identical except for opposite polarity.
   In push-pull operation, the output signal will be available as a symmetrical signal which appears between two terminals, neither one of which is grounded. These terminals might be the two plates of the amplifier tubes. The coaxial cable clearly requires a single-ended signal, that is, a signal which appears between terminals, one of which is ground. It is possible to change a symmetrical signal to a single-ended signal through the use of a transformer but the ability of a transformer to transmit a signal without frequency distortion is quite limited.

     The SRPP is a controversial circuit. Even its name is not set in stone, as it is called the SRPP, the SEPP, the mu follower, the mu amplifier, the cascoded cathode follower, and the totem-pole amplifier. Interestingly, the name given in the 1940 patent (US 2,310,342) for the circuit is seldom used: a "balanced  direct and alternating current amplifier."
     At the center of the controversy is two very different interpretations of how the circuit works. This is a bit paradoxical, as the circuit was seen as being quite simple and as having only one accepted explanation of functioning. (In fact, when the circuit appears in Samuel Seely's
Electronic Circuits, it shows up at the end of a chapter in the quiz section. The circuit is seen as being so obvious that it is left to the first year electrical engineering student to derive a formula for the circuit's gain as a home work assignment.) In general, the circuit only appeared in mid-century engineering books when the subject was amplifiers whose DC operating points are immune to heater and B+ voltage drift. And seldom was the circuit described as being particularly linear or special. The exception is in Millman & Taub's Pulse and Digital Circuits,McGraw-Hill (1956); in section 3-17, The Totem Pole Amplifier, we read:

   An amplifier that approximates push-pull operation and provides a single-ended signal without a transformer is shown in Fig. 3-30. The amplifier is indicated driving a 75-ohm coaxial cable terminated at both ends. The circuit enjoys a wide popularity in color television systems where linearity of operation is extremely important. It has acquired the name totem-pole amplifier."

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