7.4.4 Fundamental Design
Probably the first and prime step in the establishment of
reliability criteria is the establishment of the boundaries which represent
the limitations on the controlled characteristics for the component or device
in question. Some of the limitations are commonly known: breakdown voltage,
power dissipation limitation, current density limitations, and similar
factors. Many, however, are either poorly known, or possibly not known at all.
Often it is these factor which cause difficulties in circuits.
If one examines the behavior of components in systems, one finds
that there normally is a region of operation in which failures are rare or
unlikely, but when operating conditions reach a possibly undefinable level,
the probability of failure rises substantially. Conversely, with any given
configuration, improvements in reliability as a result of redesign may be easy
to obtain to a certain level of improvement, and then become progressively
more difficult to obtain.
Improvement of reliability in terms of these criteria generally
makes more sense than either attempting to attain an excessively high value
for all components or being satisfied with an excessively small value based on
the poor reliability of the few components. Limiting the collector supply
voltage to the minimum provides a very economical way of improving the
reliability of a given circuit.
The optimization of the reliability of a system on a
circuit-by-circuit basis might appear to be an excessively time consuming and
difficult problem. Actually, however, such need not be the case, since it is
entirely practical to test at the design state (on paper) the effects of
voltage reduction on circuit performance. Since it is necessary to limit
voltage gain for reasons of circuit stability, proceeding in this manner might
lead to an occasional additional amplifier circuit but it should at the same
time lead to substantially reduced power consumption and substantially reduced
cooling problems. Both of these are important criteria for reliability.
The following paragraphs discuss some fundamental design
limitations which are important to designers of military electronic