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With no signal in,Tr1
has no forward bias and
is cut off.
The collector voltage of
Tr1 is high, turning Tr2
on.
The emitter current of
Tr2 flowing through R2
produces 1 volt across
R2.
Since the base of Tr1 is
at zero volts, the
base/emitter junction of
Tr1 is reverse biased by
1 volt.
The input signal has to
exceed this voltage plus
0.6 volts (1+0.6 = 1.6
volts) to forward bias
Tr1.
INPUT GOES HIGH TO MAKE
OUTPUT HIGH
The input signal
increases from zero.
Once the input voltage
exceeds 1.6 volts,Tr1
begins to conduct.
Its collector voltage
starts to fall and the
base voltage of Tr2
falls.
The emitter current of
Tr2 through falls,
reducing the voltage
across it.
This further increases
the conduction of Tr1,
producing a cumulative
effect.
Tr1 comes on very
rapidly and Tr2 goes
off.
Tr2 collector voltage
goes high.
INPUT GOES LOW TO MAKE
OUTPUT LOW
When the input voltage
falls, it has to go
below 0.6 volts before
Tr1 collector current
starts to fall.
Again there is a
cumulative action which
rapidly turns Tr1 off
and Tr2 on.
Tr2 collector voltage
falls.
The difference in the
values of Tr1 base TURN
ON and TURN OFF voltages
is known as HYSTERESIS.
The Schmitt Trigger can
be used to clean up
noisy signals or to
speed up slow rise and
fall times of pulses.
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