complimentary symmetry amplifier

[Kevin Weddle]

Often misinterpreted as a pushpull, this amplifier gives the highest voltage gain possible whereas a pushpull is just a current amplifier with a voltage gain of one.

http://www.radioelectronicschool.com/reading/reading26.pdf

I believe this is the basis for opamp gain. There is also a pushpull at the output of the opamp for current gain. Any comments?

[audioguru]

Hi Kevin,
Your article shows a variation of a vacuum tube amplifier called, "Push-Pull". It uses driver and output transformers and was used in transistor radios in the 60's.

Audio amps and opamps that use bipolar transistors don't have any voltage gain in their Complementary-Symmetry emitter-followers outputs, instead their voltage gain comes from using extremely high impedance current sources as the collector loads for transistors, followed by high impedance loads.

The complementary-symmetry amp in your article might have some voltage gain but suffers from extreme thermal runaway. It would be very difficult to set the bias current because the transistors are connected as a short across the supplies. If you set the idle current to a certain amount, they would heat-up, which would make them conduct more, which would cause them to heat more, which would cause them to, you know, release their majic smoke!  :o  ;D 

[Miles Prower]

I believe this is the basis for opamp gain. There is also a pushpull at the output of the opamp for current gain. Any comments?

Well, no, it isn't. Every op-amp has three basic subsystems: a differential amp as the input stage. This provides a moderate amount of gain. The next stage is the high gain interstage. This provides the lion's share of the open loop gain. Frequently constructed as a BJT stage with constant current loading, as a constant current source has a theoretical infinite impedance. The output is some variation on a complementary symmetry unity gain amplifier, as this provides the desireable low output impedance.

The design for a complementary symmetry amp in that PDF you linked is fuxxored. I fixed it. In the correct version, The two R_E resistors are usually chosen to be small, compared to the load resistance, and serve to set the no-signal idling current. The diodes provide the necessary bias to establish that no-signal idling current. The R_B resistors determine the maximum base current. This circuit won't have a very high input impedance, and so is often driven with either an op-amp or some other driver that can operate into a low impedance, such as a constant current driven voltage amp, or sometimes a bootstrapping arrangement can be used.