218 Chapter 7
to signal and earth line paths, avoidance of excessive HF bandwidth at the input stages)
that are applicable to these.
7.17 General Design Considerations .............................................................................
During the past three decades, a range of circuit design techniques has evolved to allow
the construction of highly linear gain stages based on bipolar transistors whose input
characteristics are, in themselves, very nonlinear. These techniques have also allowed
substantial improvements in possible stage gain and have led to greatly improved
performance from linear, but low gain, fi eld-effect devices.
These techniques are used in both discrete component designs and in their monolithic
integrated circuit equivalents, although, in general, the circuit designs employed in linear
ICs are considerably more complex than those used in discrete component layouts.
This is partly dictated by economic considerations, partly by the requirements of
reliability, and partly because of the nature of IC design.
The fi rst two of these factors arise because both the manufacturing costs and the
probability of failure in a discrete component design are directly proportional to the
number of components used, so the fewer the better, whereas in an IC, both the reliability
and the expense of manufacture are affected only minimally by the number of circuit
elements employed.
In the manufacture of ICs, as indicated earlier, some of the components that must be
employed are much worse than their discrete design equivalents. This has led the IC
designer to employ fairly elaborate circuit structures, either to avoid the need to use a
poor-quality component in a critical position or to compensate for its shortcomings.
Nevertheless, the ingenuity of the designers and the competitive pressures of the market-
place have resulted in systems having a very high performance, usually limited only
by their inability to accept differential supply line potentials in excess of 36 V unless
nonstandard diffusion processes are employed.
For circuitry requiring higher output or input voltage swings than allowed by small signal
ICs, the discrete component circuit layout is, at the moment, unchallenged. However, as
every designer knows, it is a diffi cult matter to translate a design that is satisfactory at a
low working voltage design into an equally good higher voltage system.