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6.10. CLASS B AUDIO AMPLIFIER
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input signal range where no change in speaker output voltage will occur. Here is where intricate
biasing techniques are usually introduced to the circuit to reduce this 1.4 volt "gap" in transistor
input signal response. Usually, something like this is done:
+V
-V
8
speaker
Input
signal
1.4 volts
+
-
The two series-connected diodes will drop approximately 1.4 volts, equivalent to the combined
V
be
forward voltage drops of the two transistors, resulting in a scenario where each transistor is just
on the verge of turning on when the input signal is zero volts, eliminating the 1.4 volt "dead" signal
zone that existed before.
Unfortunately, though, this solution is not perfect: as the transistors heat up from conducting
power to the load, their V
be
forward voltage drops will decrease from 0.7 volts to something less,
such as 0.6 volts or 0.5 volts. The diodes, which are not subject to the same heating effect because
they do not conduct any substantial current, will not experience the same change in forward voltage
drop. Thus, the diodes will continue to provide the same 1.4 volt bias voltage even though the
transistors require less bias voltage due to heating. The result will be that the circuit drifts into
class AB operation, where both transistors will be in a state of conduction part of the time. This,
of course, will result in more heat dissipation through the transistors, exacerbating the problem of
forward voltage drop change.
A common solution to this problem is the insertion of temperature-compensation "feedback"
resistors in the emitter legs of the push-pull transistor circuit:
Summary :
Usually, something like this is done: +V -V 8 speaker Input signal 1.4 volts + - The two series-connected diodes will drop approximately 1.4 volts, equivalent to the combined V be forward voltage drops of the two transistors, resulting in a scenario where each transistor is just on the verge of turning on when the input signal is zero volts, eliminating the 1.4 volt "dead" signal zone that existed before.
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oltage,olts,transistors,signal,forward,input,same,diodes,change,olt,drop,transistor,circuit