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The bistable has two
stable states.
At switch on, one
transistor is ON and the
other is OFF.
This is one stable
state.
An external pulse makes
the circuit change
state, with the ON
transistor now OFF, and
the OFF transistor now
ON.
This is the second
stable state.
OPERATION
Since the two
transistors are not
exactly the same, one ,
say Tr1, will start
conducting before the
other, Tr2.
As the current through
Tr1 increases, the
voltage across R1
increases and the
collector voltage of Tr1
falls.
This fall is coupled to
the base of Tr2 via R2,
causing the collector
current of Tr2 to fall
and its collector
voltage to rise.
This rise in voltage is
cross coupled to the
base of Tr1 increasing
its forward bias and
increasing the rise of
collector current.
Since the collector
current is already
rising, the effect is
CUMULATIVE and the
collector voltage of Tr1
falls rapidly and the
collector voltage of Tr1
rises just as quickly.
The circuit is now in
one of its stable states
with the collector
voltage of Tr1 low, and
that of Tr2 high.
D1 has a low voltage on
its cathode via R5 and a
high voltage on its
anode via R3, making it
forward biased.
D2 has a high voltage on
its cathode via R6 and a
low voltage on its anode
via R2, making it
reverse biased.
An external negative
pulse is steered to the
base of Tr1 since D1 is
forward biased, but
blocked from the base of
Tr2 by reverse biased
D2.
Tr1 is turned off and
Tr2 is turned off by the
cross coupling.
This happens very
quickly because of the
cumulative effect
mentioned earlier.
The circuit is now in
its second stable state
and waits for another
trigger pulse.
Since the collector
voltage of Tr2 changes
state for every trigger
pulse, there is one
pulse appearing at its
collector for every two
pulses in.
It can therefore be used
as a divide by two
circuit.
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