|
It is sometimes
difficult to understand
the behaviour of a
circuit when the
transistors are drawn
upside down.
In diagram A,as the base
voltage goes towards the
+ rail (goes more
positive), the collector
voltage goes away from
the + rail (goes less
positive).
In diagram B,as the base
voltage goes towards the
- rail (goes more
negative), the collector
voltage goes away from
the - rail (goes less
negative).
In diagram C,as the base
voltage goes away from -
rail (goes more
positive), the collector
voltage goes towards the
- rail (goes less
positive).
In diagram D,as the base
voltage goes away from +
rail (goes more
negative), the collector
voltage goes towards the
+ rail (goes less
negative).
TR2 and TR3 are
complementary. They have
the same characteristics
but one is NPN and the
other PNP.
The NPN has + on its
collector and less
positive on its emitter.
The PNP Has + on its
emitter and zero on its
collector.
Therefore both have
correct polarity
voltages.
The voltage at the
junction of R3 and R4 is
half the supply voltage.
If R2 is replaced by a
wire link, and R1
selected to give half
the supply volts at TR1
and TR2 bases then they
would both be biased in
class B, and both would
be non conducting.
Look at the waveforms on
the bases of the output
pair.
During the first half
cycle the signal is
going less positive.
This is reversing
biasing the NPN
transistor and
increasing forward bias
on the PNP transistor.
This means that the NPN
remains cut off and the
PNP conducts during this
first half cycle.
In the next half cycle
the signal on the bases
is now increasing in a
more positive direction.
During this half cycle
the NPN conducts while
the PNP is cutoff.
So the NPN is off and
the PNP is on during the
first half cycle, and
current flows through
the loudspeaker as C1
charges.
The NPN is on and the
PNP off during the
second half cycle, and
current flows the other
way through the
loudspeaker, as C1
discharges.
R2 is added to give a
small forward bias to
both transistors thereby
avoiding crossover
distortion. (see the
page on push-pull power
amplifiers).
|
