Sziklai pair power amplifier bias

** Class AB bipolar amps are by far the most common both in use and manufac ture. Millions of them are pouring out of China each year - both as individ ual products and incorporated inside items that include speakers.

The alternative is class D amplifiers that, aside from single chip designs, remain a small minority.

It is rare to see emitter resistors paralleled with diodes, as it creates i ssues with crossover distortion and the use of VI limiting. VI limiting rel ies on emitter resistors to accurately sense current in each BJT and this b ecomes impossible when there is a diode in parallel.

The problems of biasing class AB stages were solved long ago.

... Phil

AB sort of always has crossover distortion. Needs lots of feedback.

Good point; it takes an added current sense resistor to fix that glitch.

Another technique is to run the finals at zero quiescent current and bleed in some of the driver power to the output, to ensure continuity in the crossover region.

I like to compute (analog or with a uP) actual device power dissipation and measure heatsink temp, then run a model to simulate junction temperature. That is a much better way to protect (and push) the transistors than a current limit.

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John Larkin         Highland Technology, Inc 
picosecond timing   precision measurement  

jlarkin att highlandtechnology dott com 
http://www.highlandtechnology.com

accurately sense current in each BJT and this becomes impossible when there is a diode in parallel.

** One is needed for each device and will add to conduction losses at full output. Nobody does it. ** Then you have a class B output stage.

The best and most famous of which is the Quad "Current Dumping" topology w hich eliminates crossover distortion and biasing issues completely.

** VI limiting allows use of the available SOA of BJTs and often includes d elayed action so brief excursions outside the published DC curves are cater ed for - important for coping with reactive loads.

A thermal device that senses heatsink temp and takes appropriate action com pletes the job of protecting an output stage in case of long term overload or cooling failure.

Crown Audio ( of the USA ) are the only amp maker know who went down the pa th of having an analogue computation of SOA built into each power channel i n their "Microtech" & "Macrotech" series. Worked well enough but had no pra ctical advantages over a well implemented VI limiter.

.... Phil

facture. Millions of them are pouring out of China each year - both as indi vidual products and incorporated inside items that include speakers.

ns, remain a small minority.

s issues with crossover distortion

"Sort of" means "very little" if you do your biasing right. Feedback helps, but if you can live with a bias current of about 100mA through a quiescent output stage for an 8 ohm load (and that's not a lot) you can go through c rossover without getting any visible kink.

accurately sense current in each BJT and this becomes impossible when there is a diode in parallel.

Not the cleverest way of doing it.

And a lot more complicated. Good analog multipliers are expensive, but uPs are cheap as dirt. Putting an A/D converter in the right place to digitise something that can be translated into output power isn't trivial, but won't complicate manufacture a lot. Putting a thermistor (or whatever) on the ou tput heatsinks to measure their temperatures is dead expensive in mass-prod uced gear.

Measuring the Vbe of bipolar output devices avoids extra wiring, but you on ly know the base current at cross-over, which makes for an even more compli cated sampling scheme.

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Bill Sloman, Sydney