Class AB output stages - Common Collector vs Common Emitter

ubergeeknz

Member
2018-08-31 12:32 pm
I've been doing a fair bit of reading and searching but there's something which kind of has me at a bit of a loss. I'm hoping someone of the members here with a great deal more experience in these matters might be in a position to comment [emoji6]

In most older (pre 1980) designs that I've seen, mostly British or British based designs, the output stage is either a totem pole arrangement or in the later designs, some combination of complementary szilikai/Darlington arrangement, but where the outputs are complementary they tend to be common emitter with the collectors driving the load.

But at some point, it seems consensus shifted towards common collector complementary output stages in class AB amps.

Can anyone explain the rationale behind both arrangements? And why the preferred arrangement seems to have changed over at some point in the late 70s/early 80s?
 
Output configurations

Speed/bandwidth. Power transistor are slow, or at least historically they were, and still are relatively slow. In order to make a stable amplifier with a predictable gain, you need feedback and in order for that feedback to be stable, the amplifier has to be "compensated", ie the slowest stage has to dominate the total phase lag so that the gain falls to zero dB (unity) before the total phase lag reaches 180 degrees and makes the negative feedback into positive feedback. Common collector / emitter follower stages are driven by a low impedance voltage source that provides more current at higher frequencies and therefore can drive the power transistor on and off faster than a current source and with less phase lag at any given frequency. Speed translates into less distortion because the feedback controls the output at higher frequencies, ie more closed loop gain at that frequency. Also, a follower has a local feedback and low impedance so the amplifier output buffering is faster and less dependant on the global feedback.
Note that a Szilikai pair requires a Zobel network to stabilize the local feedback, and a follower does not.
Note that older amplifiers used "quasi-complimentary" outputs (one side using a Szilikai pair) because power PNP transistors were rare and expensive.
 
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ubergeeknz

Member
2018-08-31 12:32 pm
Ok, after a couple of readings, that makes sense to me.

So I wonder then, what caused designers to prefer common emitter in the first place? Was it done in order to squeeze the last bit of voltage gain from the outputs? Or some other limitation of early devices?
 

ubergeeknz

Member
2018-08-31 12:32 pm
I see, so it was most likely a case of, it works well enough and requires fewer parts, leave it there ;) ... Makes enough sense :)

An interestng case in point however -- my next repair and resto project is a Hitachi HA-610 from 1975 and it boasts 0.006% THD - although it does have two complementary pairs of output transistors in TO-3, which seem to have fairly impressive specifications for the time, they've used FETs and no shortage of other silicon in this thing to achieve good performance, no expense spared it would seem, but then the outputs are common emitter szilikai pairs.

I wonder if they couldn't have achieved lower distortion by using an emitter follower setup. It seems like they spared no expense otherwise, so it leaves one to wonder if that was simply the "state of the art" as far as the designers were concerned, and that was that? Or some other reason...
 
There will always be exceptions, people who do things differently. A good example are circlotron amps where two floating power supplies are required per channel and the outputs are neither grounded emitter nor grounded collector. The only power supply that is grounded is a third supply used by the drivers. I used to operate an old shake table driven by a 30KW circlotron made by UDCO. The outputs were an array of NPNs in stud packages mounted in water tubes, and the whole thing occupied three 6 foot racks. That was the first time I encountered a circlotron amp.
 
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Sziklais / CFPs give you higher output voltage, especially on lower supplies (think +/-25-30 V). Vceo handling / SOA of power transistors was long problematic (think 3055 and the like), so lower supplies = better. This is why you will also see 1970s' high-power amplifier designs with stacked transistors.

True, but hardly worth the extra volt swing unless the supply is 12V (auto stereo). CFP with gain can fix the lack of voltage swing from a poor VAS but brings rail sticking problems as well as slower speed and the need for a Zobel network. If the VAS has boosted supply voltages or is boot-strapped on both sides then an EF can get as close to the rail as a CFP.