Audio Engineering

294 Chapter 9

forward voltage drop, whereas the layout of Figure 9.11 will have two, giving a combined
quiescent voltage offset of the order of 1.3–1.5 V. Nevertheless, in commercial terms,
the popularity of power transistors, connected internally as a Darlington pair, mainly for
use in the output stages of audio amplifi ers, is great enough for a range of single chip
Darlington devices to be offered by the semiconductor manufacturers.

9.6 Thermal Dissipation Limits ..................................................................................

Unlike a thermionic valve, the active area of a BJT is very small, in the range of 0.5 mm
for a small signal device to 4 mm or more for a power transistor. Because the physical
area of the component is so small—this is a quite deliberate choice on the part of the
manufacturer because it reduces the individual component cost by allowing a very large
number of components to be fabricated on a single monocrystalline silicon slice—the
slice thickness must also be kept as small as possible—values of 0.15–0.5 mm are
typical—in order to assist the conduction of any heat evolved by the transistor action
away from the collector junction to the metallic header on which the device is mounted.

Whereas in a valve, in which the internal electrode structure is quite massive and
heat is lost by a combination of radiation and convection, the problem of overheating
is usually the unwanted release of gases trapped in its internal metalwork, the problem

0V 0V

Output

R 2

Q 1

R 1

Q 2

Input

Vcc

Figure 9.11 : Darlington pair.