Audio Engineering

188 Chapter 7

stages. Moreover, developments in the design of such voltage amplifi er stages have
allowed continuing improvement in amplifi er performance.

The developments in solid-state linear circuit technology that have occurred over the past
30 years seem to have been inspired in about equal measure by the needs of linear integrated
circuits and by the demands of high-quality audio systems; engineers working in both of
these fi elds have watched each other’s progress and borrowed from each other’s designs.

In general, the requirements for voltage gain stages in both audio amplifi ers and
integrated-circuit operational amplifi ers are very similar. These are that they should be
linear, which implies that they are free from waveform distortion over the required output
signal range, have as high a stage gain as is practicable, have a wide AC bandwidth and a
low noise level, and are capable of an adequate output voltage swing.

The performance improvements that have been made over this period have been due
in part to the availability of new or improved types of semiconductor devices and in
part to a growing understanding of the techniques for the circuit optimization of device
performance. The interrelation of these aspects of circuit design is considered next.

7.10 Linearity

7.10.1 Bipolar Transistors

In the case of a normal bipolar (NPN or PNP) silicon junction transistor, for which the
chip cross section and circuit symbol are shown in Figure 7.17 , the major problem in

N

N Collector

Mounting substrate

P

Base Emitter

N

Collector

Mounting substrate Circuit symbols

P

P

Base Emitter

‘PNP’

‘NPN’ B

C

E

B

C

E

Figure 7.17 : Typical chip cross section of NPN and PNP silicon planar epitaxial transistors.