Audio Amplifi er Performance
Douglas Self10.1 A Brief History of Amplifi ers ...............................................................................
A full and detailed account of semiconductor amplifi er 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 amplifi er design has evolved in the
last three or four decades.
Valve amplifi ers, 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 fi rst, but suffered severely from the vulnerability of germanium to even
moderately high temperatures; the termthermal runaway was born. At fi rst all silicon
power transistors were NPN, and for a time most transistor amplifi ers 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 confi guration,^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
CHAPTER 10