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The astable has two
unstable states, being
unable to rest in a
fixed state.
When you first switch
on, one transistor is on
(conducting) and the
other is off (non
conducting).
They stay in this
unstable state for a
time, determined by a CR
time constant.
Then the transistors
exchange states, the one
that was off coming on,
and the one that was on
going off.
They stay in this new
unstable state for a
time, again determined
by a CR time constant,
before reverting to the
original state.
This process is repeated
continuously.
OPERATION
The characteristics of
the two transistors are
not exactly the same.
When the circuit is
first switched on, the
current through one
transistor, say Tr1,
will increase faster
than the current through
Tr2.
Due to the rise of
current through R1, the
voltage across it will
increase, causing the
collector voltage of Tr1
to fall.
This fall in voltage is
coupled to the base of
Tr2.
This causes the
collector current of Tr2
to fall, and its
collector voltage to
rise, due to less
voltage being dropped
across R4.
This rise in collector
voltage is cross coupled
to the base of Tr1,
increasing the forward
bias of Tr1 and
increasing its collector
current.
Since the collector
current was already
rising, its rise is
aided by this rising
forward bias.
The effect is CUMULATIVE
and Tr1 becomes rapidly
fully on and Tr2
completely off.
The collector voltage of
Tr1 is now low, and that
of Tr2 is high.
C1 now begins to charge
from the supply rail,
via R2.
As the voltage on the
right hand side of C1
starts to rise, Tr2
starts to conduct.
Again we have the
cumulative effect and
Tr2 rapidly comes on and
Tr1 goes off.
The collector voltage of
Tr1 is now high and that
of Tr2 low.
It is now the turn of C2
to charge from the
supply via R3.
As the voltage on the
left hand side of C2
begins to rise, the base
voltage of Tr1
increases, turning it on
and turning Tr2 off.
The whole process
repeats continuously .
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