A Brief History of Amplifiers
A full and detailed account of semiconductor amplifier design since its beginnings would be a book in itself, and a most fascinating volume it would be. This is not that book, but I still feel obliged to give a very brief account of how amplifier design has evolved in the last three or four decades.
Valve amplifiers, working in push–pull Class-A or AB1, and perforce transformer coupled to the load, were dominant until the early 1960s, when truly dependable transistors could be made at a reasonable price. Designs using germanium devices appeared first, but suffered severely from the vulnerability of germanium to even moderately high temperatures; the term thermal runaway was born. At first all silicon power transistors were NPN, and for a time most transistor amplifiers relied on input and output transformers for push–pull operation of the power output stage. These transformers were as always heavy, bulky, expensive, and nonlinear and added insult to injury as their LF and HF phase shifts severely limited the amount of negative feedback (NFB) that could be applied safely.
The advent of the transformerless Lin configuration,1 with what became known as a quasi-complementary output stage, disposed of a good many problems. Because modestly capable PNP driver transistors were available, the power output devices could both be NPN and still work in push–pull. It was realized that a transformer was not required for impedance matching between power transistors and 8-Ω loudspeakers.
Proper complementary power devices appeared in the late 1960s, and full complementary output stages soon proved to give less distortion than their quasi-complementary predecessors. At about the same time, DC-coupled amplifiers began to take over from capacitor-coupled designs, as the transistor differential pair became a more familiar circuit element.
A much fuller and generally excellent history of power amplifier technology is given in Sweeney and Mantz.2
Amplifier Architectures
This grandiose title simply refers to the large-scale structure of the amplifier, that is, the block diagram of the circuit one level below that representing it as a single white block- labeled power amplifier. Almost all solid-state amplifiers have a three-stage architecture as described here, although they vary in the detail of each stage.