UNIT –IV
PULSE CIRCUITS
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Monostable multivibrator
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Monostable Multivibrator
 Q1 and Q2 are identical NPN transistors.The
two
collector resistances are equal.
 C1 is the speed –up capacitor used to reduce the
transistion time.
 When a positive trigger pulse is given to base of
Q1, it starts conducting because of which VC1
decreases this causes decrease base voltage of
Q2 .
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
The drop I1RC decreases the forward bias of
Q2 and the current I2 decreases therefore
the collector drop of Q2 increases which is
given to base of Q1 because of which Q1
is drawn to saturation and Q2 to cutoff.

This state is called as Quasi stable state ,
the capacitor starts charging through VCC
and makes ON Q1
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Waveforms of a monostable
multivibrator (triggered).
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Astable Multivibrator.
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Astable Multivibrator
 Q1 and Q2 are identical NPN transistors.The
two
collector resistances are equal.
 The capacitive coupling is used between the
stages .
 Assume that Q1 ON and Q2 OFF . The capacitor
C 2 starts charging and at a state become equal
to VCC.
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C1 which was already charged Vcc starts
discharging ,because of this Q1 starts
conducting.
 As C2 discharges base VB2 is greater
therefore Q2 ON and Q1 OFF.

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Bistable Multivibrator
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Bistable Multivibrator
►
Positive pulse is applied at the reset pin
causes Q2 to conduct as collector voltage
of Q2 falls ,it cuts OFF Q1.

When positive pulse is applied at the set
pin it causes Q1 ON and Q2 OFF.
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Bistable Multivibrator waveform
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Schmitt Trigger Circuit
The emitter of the two transistors are
connected to each other and grounded
through resistance RE
► An input voltage VI is given Q1 is ON when
it voltage increases to VBE1 + VE
 This input voltage is called UTP –upper
threshold point.
►
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When Q1 is conducting Ic1 causes a drop
VE across RE .
 When Input voltage starts decreasing and
reaches VBE1+VE then Q1 stops
conducting and Q2 is ON.
 This point is called LTP – lower threshold
point.
►
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►
The output voltage has two stable states
it is called voltage comparator to provide
a change in output whenever input
exceeds a particular desired value.
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UJT Relaxation Oscillator
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UJT Relaxation Oscillator
The pulse required to drive a circuit can be
obtained from a single stage oscillator using
a device like UJT is called UJT Relaxation
Oscillator.
 R1 and R2 are biasing resistors CT and RT
are the capacitance and resistance decide
the oscillating rate.
►
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►


When the capacitor voltage VC exceeds VP
the UJT fires VP= ηVBB+VD.
The capacitor discharges through R1,this
produces pulse .
When capacitor voltage falls below VV the
UJT gets turned OFF.
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Pulses of UJT relaxation oscillator
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clippers
The circuit which is used to clip certain
portion of the waveform without distorting
the remaining part of the waveform are
called clipper circuits.
TYPES OF CLIPPERS.
1.Positive clippers
2. Negative clippers
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Clippers
► Clippers
or diode limiting is a diode network that
have the ability to “clip”(cut short/crop) off a
portion on the input signal without distorting the
remaining part of the alternating waveform.
► Clippers are used to eliminate amplitude noise or
to fabricate new waveforms from an existing
signal.
► Simplest form of diode clipper- one resistor and a
diode
► Depending on the orientation of the diode, the
positive or negative region of the applied signal is
clipped off.
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►2
general of clippers:
a) Series clippers
b) Parallel clippers
► Series Clippers
 The series configuration is defined as one where
the diode is in series with the load.
 A half-wave rectifier is the simplest form of
diode -clipper-one resistor and diode.
► Parallel Clippers
 The parallel configuration has the diode in a
branch parallel to the load.
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Series clipper
•
Diodes “clip” a portion of the AC wave.
•
The diode “clips” any voltage that does not
put it in forward bias. That would be a
reverse biasing polarity and a voltage less
than 0.7V for a silicon diode.
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Series clipper
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Series clipper with dc supply
► By
adding a DC source to the circuit, the
voltage required to forward bias the diode
can be changed.
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Parallel Clipper
► By
taking the output across the diode, the
output is now the voltage when the diode is
not conducting.
►
A DC source can also be added to change
the diode’s required forward bias voltage.
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Parallel Clipper
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Clampers
►A
clamper is a network constructed of a diode,
resistor, and a capacitor that shifts a waveform to
a different dc level without changing the
appearance of the applied signal.
► Clamping networks have a capacitor connected
directly from input to output with a resistive
element in parallel with the output signal. The
diode is also in parallel with the output signal but
may or may not have a series dc supply as an
added element.
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Clampers
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Element of the clamper circuit
► Magnitude of R and C must be appropriate
to ensure г=RC where the time constant is
large enough and capacitor may not
discharge during the time interval while
diode is not conducting.
► We will assume that all practical purposes
the diode will fully charge or discharge in 5
time constant.
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A diode in conjunction
with a capacitor can be
used to “clamp” an AC
signal to a specific DC
level.
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► The
input signal can be any type of
waveform:
 - sine, square, triangle wave, etc.
► You
can adjust the DC camping level with a
DC source.
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