Originally Posted by brucest
Here's a good general reference:
Figure 1. is a typical complementary Output stage.
All of the upper half transistors are NPN - the little emitter arrow is pointing out and current from the positive supply flows into the transistor's collector and out the emitter.
All of the lower half transistors are PNP - the little emitter arrow is pointing in and current from the positive source which is the upper half of the output stage into the transistor's emitter and out the collector.
Figure 7. is a typical Quasi-complementary output stage.
The goal is to actually do the same thing, and the upper half works about the same.
All of the lower half transistors are a mixture of PNP and NPN transistors tricked up so that they work in a way that is similar to Figure 1. This method was used because in the early days of SS, PNP output transistors were like hen's teeth. The asymmetries lead to somewhat poorer measured performance, but the circuit could be perfected well enough that they sounded the same, anyway.
And is it safe to assume that such circuity components have improved over the years and gotten cheaper? Examples?
Yes. All of the transistors have become far less costly for a given power level. I can remember when fairly crappy fragile output transistors cost over $30 each. In similar quanties a far better part may be now less than a dollar. The SS devices have also become faster, better able to withstand heat, have more gain and are better able to handle reactive loads and shorts. The actual circuitry has not changed all that much. In some low end AVRs the output stages are implemented as single chip or as a hybrid integrated circuit, but the performance can still be more than good enough.Edited by arnyk - 2/6/13 at 5:44pm