Some of the problems associated with the original design can be solved in the variation shown here. The usual bias and temperature compensation transistor is used as part of the class-A sub-amplifier in the output stage. This is consequently still in effect a triple, so again care is needed to ensure stability. The class-B half now only uses two transistors, so stability should be improved here. The disadvantage is that the driver stage must provide more current to the output stage than if the original triples are used, so a good, low distortion driver stage with plenty of local feedback is a good idea. The current source shown as 10mA may need to be much larger than this in high power amplifiers designed to drive low impedance loads. A big advantage is that the two 100 ohm feedback resistors provide only the base current for the compensation transistor, so voltage drop across these is a much smaller source of error.
Two versions are shown here, the second is just changed a little so that two identical power devices can be used instead of a complementary pair. In this case both are PNP, but of course all the polarities can be reversed to use two NPNs if required. There is a small problem that the base-emitter voltage of the compensation transistor may be too large, so that even reducing the bias adjusting resistor VR1 to zero will give excessive quiescent current, so selection of suitable devices is needed to avoid this. Generally it is found that high current-power devices have lower base-emitter voltages for a given collector current, so Q1 may need to be at least a medium power device. The value of VR1 needs to be small, around 10 or 20 ohms, but an alternative is to put a variable resistor between the base and emitter of the compensation transistor instead of in series with the emitter. The value then must be large, maybe 10 kohms or more. Q1 and Q2 should be in good thermal contact to give adequate temperature compensation.
If the class-A triple has stability problems this circuit has the option to include a capacitor, perhaps with a series resistor, from the base to collector of the temperature compensation transistor
The need to divide one of the 1 ohm output resistors is a small disadvantage, but the only alternative seems to be adding a potential divider at the emitter of Q1, which may add more problems of its own, and is best avoided.
M.J.Renardson. 2:29 PM 22/12/2002
