18.104.22.168 Thermal Factors
One of the major problems in the use of transistor circuits is
the stabilization of operating conditions so that the circuit can give the
required performance over an adequate range of environmental conditions.
There are two principal thermal factors that affect the
stability of transistor circuits. The first factor is the reverse leakage
current of the collector base junction, the so-called I, and the second factor
is the variation of V with temperature. The leakage current increases rapidly
as the temperature of the transistor is increased. This effect limits the
conditions under which the transistor can provide effective operation (Figure
7.4-18). This current, in conjunction with the current gain of the transistor,
limits the minimum usable current through the common emitter amplifier,
thereby restricting the available range of operation.
Even though it is possible to use the transistor in the common
emitter circuit with very small values of currents, the nonlinearity of the
device when the base current has a reverse polarity is so pronounced that it
is not practical to attempt to do so.
The variation of the base-to-emitter voltage with temperature
for fixed values of base and emitter current is the second important thermal
property of a transistor requiring compensation. The voltage between base and
emitter affects the static operation of the transistor, and it also affects
the small signal operation. Because the static, or Q-point for the transistor
varies rapidly with temperature if the base voltage is fixed, it is necessary
to fix the Q-point in a way to assure that a full range of operating
conditions is available over the required range of operating temperature. The
static stability must be determined in terms of the practical circuit in use,
and the circuit must be designed to provide the required stability.
provides detailed design procedures for thermal stabilization of circuits, as
well as design procedures to prevent thermal