A Class AB output stage is nowadays the most common type of output stage used for almost every "power" solid state amplifiers, from the tiny power output section of an integrated operational amplifier to the big power amplifiers used in open field concerts or in dancing clubs. There are many topologies employed in this stage but mostly can be grouped in two categories:
1) "Clean" emitter followers (EF) type which exhibit near unity voltage gain (really a bit less, comprised between 0.9 and 0.95, depending on value of emitter ballasting resistors and its ratio with the value of load connected at the output) which operated essentially as a pure current booster of a preceeding large signal voltage amplifier that constitute the "effective" amplifier, usually called the "VAS" (plainly: "Voltage Amplifier Stage").
2) "Assisted" emitter followers which include Sziklay compound stage, triplets and similars. This kind of output is generally set for near unity voltage gain but embed a chance of moderate to consistent voltage amplifying capability which sometimes is used by amplifiers designers. The most common type of this kind of compound stage is build with a driver device which act at the same time as emitter follower and as a driver for a successive current booster device (the *real* power device from the dissipation and current management standpoint) that is usually connected as common emitter stage (and that can provide, when required, the desired extra voltage gain as we told before).
The two kind of output stages come with own specific vantages and drawbacks; "clean" EF may be a little worse in linearity (as viewed by the preceeding VAS) but is usually more stable than of "assisted" EF which, in turn, trade their higher susceptibility to oscillate with better linearity (*).
3) There is a third "mixed" category of output stages, the so called "quasi complementary output stage". This was effectively the original topology of the first OTL solid state amplifier (the "H.C. Lin circuit" first proposed in 1955) and was for decades the best and cheapest compromise with the goal of having a "complementary" output stage when complementary pairs of *power* transistor were available with difficulty if at all. Although fallen a bit in disuse, this kind of stage may give near equivalent performance of full complementary output stage, using output devices of the same polarity, once provided that driver stages is equipped with the so called "Baxandall diode" (just a common diode in series with the emitter of the driver of the compounded NPN-PNP section of output stage), which, behaviouring as a non linear resistor, embed a non linear local NFB that equalize the raw voltage gain of compounded section more closely to that obtained from the "clean" section (drive and power device of the same polarity) and, in turn equalize the crossover poin of the two section of output stage, otherwise very harsh to match satisfactorily.
Theoretical efficiency of class AB output stage (with sinusoidal driving and resistive loading) is rated up to 78% but practical efficiency, due to some limits of real designs, usually is not more than of 60%. In the midpower range of an amplifier efficiency lay between 40 and 50%, that may be heavily worsened by non-resistive loading offered from the majority of loudspeaker systems.
Usually, in the most of application, output stages are really *Darlington* output stages, especially when powers above 4-5 Watt are required. "Darlington connection" is implicit in output stages of type (2) and (3) but not compulsory in type (1) which, for small power (or in output section of many op-amps), often employ a simple pair of relatively high gain (high HFE and often high FT) devices. This, although traded with limited power capabilities, get better crossover performances and better behaviouring at higher frequencies than those permitted by more complex output stages, where the cascading of devices limit the switching capabilities of the entire ouput section to, at the most, a 30-40 kHz with cleverly selected (and somewhat less rugged) power devices (**).
If high frequency performance are to be obtained with moderate to high power capabilities, then switching to other devices (mosfet) or other operation modes (class A) became unavoidable.
(*) With output stages employing bipolar devices (working in class A or AB, is not a concern here) "linearity" must be intended first of all as "current gain linearity" because the VAS "build" it's output voltage on a composed impedance which has, between its components, the so called "reflexed load" that is just not more than of the load impedance *multiplied* by the gain current of output stage.
High non linearity in output devices current gain coupled with reactive load may turn the entire output stage in a undue "varactor" generator of harmonics (and may even trigger some oscillation, especially at mid to high power level of working where, other than harshing the sound, output devices may be "pushed" out of their safe area - second breakdown is here a special issue to be take in account! - and drive them to blow. This is one of the main factors that "kill" bipolars in power amplifiers when connected to heavily reactive loudspeakers as may be some electrostatics).
(**) Don't let be fooled by the "amazing" FT performances of devices like 2SC3281-2SA1302 pair: their switching times are not much better than of others, officially "more slow", bipolar devices (are just halved or so); are these that define high frequency capabilities of class AB output stages, not FT, which, at most, may prove be effective only in non switching circuits (class A). The best plus, in audio power amplifier, of devices of this kind is first of all their inherent stable and soustained HFE (up to the high useful output current usually required in domestic amplifiers), which enables a somewhat more "relaxed" design of the driver section and surely get more chances to obtain a more linear power amplifier even with simple and straightforward topologies.
Some stuffs from the old version of this page:
1.
https://sound.westhost.com/project3a.htm
This is a fairly common amplifier.
2.
https://home.alphalink.com.au/~cambie/ETI466Web.htm
3.
https://users.ece.gatech.edu/~mleach/papers/lowtim/feb76feb77articles.pdf
Another popular amp.
