Quasi AB driver A

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Hi all.
What do you think about this quasi AB with driver A? Anyone can explain how this output stage work?


  • Quasi AB driver A 360W.jpg
    Quasi AB driver A 360W.jpg
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Joined 2007
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What do you mean by 'Driver A' ?

The output stage is a conventional quasi stage. The upper two NPN's (Q7 and QF) form a standard driver + output. The lower PNP driver (Q8) and its associated NPN output device (also QF) form a quasi PNP output device and driver. Pulling the base of TR8 negative turns the transistor on. That in turn develops a voltage across R12 which turns on the lower QF. It behaves very much like two PNP devices.

Adding a so called "baxandall diode" is generally a very good idea.


In this case add 220R with a parallel diode to the base of the lower driver.
It makes the output biasing far more symmetric and lowers x/o distortion.

rgds, sreten.
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Joined 2007
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Thank Mooly.
I find it strange the R11 between emisor Q7 and Q8.

Looking more closely and I believe there is an error in the diagram and that it would not work as shown. It should be like this.


Connected as it was is valid if the lower output device were PNP and in this configuration it allows each of the drivers to actively pull down (or up) the other drivers base and so aid a faster turn off.
Very interesting their scheme
In the thread "Very simple quasi complimentary MOSFET amplifier"(Ranchu32) in the post 162, Ilimzn says "Now, becasue of the unusual nature of a BJT CFP/MOSFET combination, there is actually more that can be done to simplify the design. For one, the R-C-D parallel combo is not necessary. This would be used if the top was a BJT darlington, and it's used to make the crossover region more symmetrical, because for the CFP it's much narrower than that of the darlington. In the case of this amplifier, it's very difficult to do this since the devices are so different. However, it is possible to narrow the Vgs range the top MOSFET works in, by excluding the source ressitor. At the same time, the 'emitter' resistor of the CFP mentioned above should be increased, perhaps to 0.47ohms"
My question is if necessary diode Baxandall in his scheme and why has no capacitor in parallel with R12?
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