I think this is a push-pull system biased substantially into Class A. There are two base emitter junctions in the driver output transistor combinations and the output emitters will drop some voltage.
The value of R119 is 18 Ohms which is low - the the current flowing across this would be multiples of that in a Class AB design - well in excess of 100 mA under static conditions and much higher under dynamic conditions.
This would be by design as the crossover point would arise at a high power output level where the output transistor base charges are kept more firmly in place and it does require some force to remove them - hence the drivers are the same device types as the outputs.
As well as the symmetry you mention there is also the matter of generating output stage bias.
If the MJE devices were not present then R119 would have to increase in value and the discharge current for stored charges in the outputs would reduce the output power level transition point from Class A to Class AB.
I have used LTSpice for part of the amplifier. Following the schematic, the amplifier will work. However, the voltage at emitter of Q107 will clip at about -15 V (with a sinusoidal input) , but there is no clipping at the input to the darlingtons at the output. In other words, Q107 is operated at saturation most of the time.
I would like to hear if anyone can explain to me why they are using Q107 in this way.
I would like to hear if anyone can explain to me why they are using Q107 in this way.
Q106 and Q107 form a tandem constant current source - the current level in R111 should be around 10 m.A.
What does the simulation give for Ie for both of these devices and I R111.
What does the simulation give for Ie for both of these devices and I R111.
+1 knutn -- R112 and R113 are starving Q108 -- 7,3mA of the ~10mA the upper CCS provides is warming the resistors. Doesn't leave much room for the bias spreader to respond to the heat sink temperature.
You good folks are surely welcome to keep studying this design, but I'm running out of patience trying to find the *genius* in it. So many oversights and design shortcomings .. hard to give it a serious lookover while tripping over one unsound practice after another.
I'll check back in periodically, but I gotta bail for now.
Cheers
You good folks are surely welcome to keep studying this design, but I'm running out of patience trying to find the *genius* in it. So many oversights and design shortcomings .. hard to give it a serious lookover while tripping over one unsound practice after another.
I'll check back in periodically, but I gotta bail for now.
Cheers
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