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R1 and R2 bias the
transistor in class A so
that a steady dc
collector flows.
R4 is the emitter
stabilising resistor.
When collector current
flows it causes the
transistor to heat up.
This causes the base
current to increase
which in turn causes the
collector current to
rise.
This rise in collector
current causes the
temperature to increase
even more, and the base
and collector currents
continue increasing.
This behaviour is called
THERMAL RUNAWAY and will
destroy the transistor.
If we consider the
emitter/base junction as
a diode as shown in the
right hand diagram we
can see that the base
(anode) voltage is fixed
by R1 and R2.
If the collector current
tries to rise due to
heating, then the
voltage across R4 will
try to rise, making the
emitter (cathode) more
positive.
This would reduce the
voltage across the
junction (diode) making
it less forward biased
and reducing the base
current and hence the
collector current, which
was trying to rise.
Therefore the circuit
has been stabilised
against thermal runaway.
However, if an ac signal
is applied to the base,
the varying collector
current will cause a
varying voltage across
the emitter resistor.
This voltage will follow
the base voltage. This
means that the
base/emitter
(anode/cathode) voltage
will be constant instead
of the base varying with
respect to the emitter.
To fix this, the emitter
is joined to earth, as
far as the ac signal is
concerned, by the
emitter decoupling
capacitor.
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