9/18/2015 EC1261 ALICE / Sr.lect-EEE UNIT –IV PULSE CIRCUITS.

04/19/2304/19/23 EC1261 ALICE / Sr.lect-EEEEC1261 ALICE / Sr.lect-EEE

UNIT –IV UNIT –IV

PULSE CIRCUITS PULSE CIRCUITS

04/19/23 EC1261 ALICE / Sr.lect-EEE

Monostable multivibrator Monostable multivibrator


Monostable MultivibratorMonostable Multivibrator

Q1 and Q2 are identical NPN transistors.TheQ1 and Q2 are identical NPN transistors.The two two

collector resistances are equal.collector resistances are equal. C1 is the speed –up capacitor used to reduce C1 is the speed –up capacitor used to reduce

the the transistion time.transistion time. When a positive trigger pulse is given to base of When a positive trigger pulse is given to base of

Q1, it starts conducting because of which Q1, it starts conducting because of which VC1VC1

decreases this causes decrease base voltage of decreases this causes decrease base voltage of Q2 .Q2 .


The drop IThe drop I11RRCC decreases the forward bias of decreases the forward bias of Q2 and the current I2 decreases therefore Q2 and the current I2 decreases therefore the collector drop of Q2 increases which is the collector drop of Q2 increases which is given to base of Q1 because of which Q1given to base of Q1 because of which Q1 is drawn to saturation and Q2 to cutoff.is drawn to saturation and Q2 to cutoff.

This state is called as Quasi stable state ,This state is called as Quasi stable state , the capacitor starts charging through VCCthe capacitor starts charging through VCC and makes ON Q1and makes ON Q1


Waveforms of a monostable Waveforms of a monostable multivibrator (triggered). multivibrator (triggered).


Astable Multivibrator. Astable Multivibrator.


Astable MultivibratorAstable Multivibrator

Q1 and Q2 are identical NPN transistors.TheQ1 and Q2 are identical NPN transistors.The two two

collector resistances are equal.collector resistances are equal. The capacitive coupling is used between the The capacitive coupling is used between the

stages .stages . Assume that Q1 ON and Q2 OFF . The capacitor Assume that Q1 ON and Q2 OFF . The capacitor

C 2 starts charging and at a state become C 2 starts charging and at a state become equal equal

to VCC.to VCC.


C1 which was already charged Vcc C1 which was already charged Vcc starts starts discharging ,because of this discharging ,because of this Q1 starts Q1 starts

conducting.conducting. As C2 discharges base VB2 is greater As C2 discharges base VB2 is greater

therefore Q2 ON and Q1 OFF.therefore Q2 ON and Q1 OFF.


Bistable MultivibratorBistable Multivibrator


Bistable MultivibratorBistable Multivibrator

► Positive pulse is applied at the reset pin Positive pulse is applied at the reset pin

causes Q2 to conduct as collector causes Q2 to conduct as collector voltage voltage

of Q2 falls ,it cuts OFF Q1.of Q2 falls ,it cuts OFF Q1.

When positive pulse is applied at the set When positive pulse is applied at the set

pin it causes Q1 ON and Q2 OFF. pin it causes Q1 ON and Q2 OFF.


Bistable Multivibrator Bistable Multivibrator waveformwaveform



Schmitt Trigger CircuitSchmitt Trigger Circuit

► The emitter of the two transistors are The emitter of the two transistors are connected to each other and grounded connected to each other and grounded through resistance REthrough resistance RE

► An input voltage VI is given Q1 is ON An input voltage VI is given Q1 is ON when when

it voltage increases to VBE1 + VE it voltage increases to VBE1 + VE This input voltage is called UTP –upperThis input voltage is called UTP –upper

threshold point.threshold point.


► When Q1 is conducting Ic1 causes a drop When Q1 is conducting Ic1 causes a drop

VE across RE .VE across RE . When Input voltage starts decreasing When Input voltage starts decreasing

and reaches VBE1+VE then Q1 stops and reaches VBE1+VE then Q1 stops conducting and Q2 is ON.conducting and Q2 is ON.

This point is called LTP – lower threshold This point is called LTP – lower threshold

point.point.


► The output voltage has two stable The output voltage has two stable statesstates

it is called voltage comparator to it is called voltage comparator to provideprovide

a change in output whenever input a change in output whenever input exceeds a particular desired value.exceeds a particular desired value.


UJT Relaxation OscillatorUJT Relaxation Oscillator


UJT Relaxation OscillatorUJT Relaxation Oscillator

► The pulse required to drive a circuit can The pulse required to drive a circuit can be obtained from a single stage oscillator be obtained from a single stage oscillator using using

a device like UJT is called UJT Relaxation a device like UJT is called UJT Relaxation Oscillator.Oscillator.

R1 and R2 are biasing resistors CT and RT R1 and R2 are biasing resistors CT and RT

are the capacitance and resistance are the capacitance and resistance decide decide

the oscillating rate.the oscillating rate.


► When the capacitor voltage VC exceeds When the capacitor voltage VC exceeds VP VP

the UJT fires VP= the UJT fires VP= ηηVBBVBB+VD.+VD. The capacitor discharges through R1,thisThe capacitor discharges through R1,this

produces pulse .produces pulse . When capacitor voltage falls below VV When capacitor voltage falls below VV

the the

UJT gets turned OFF.UJT gets turned OFF.


Pulses of UJT relaxation Pulses of UJT relaxation oscillatoroscillator


clippersclippers

The circuit which is used to clip certain The circuit which is used to clip certain portion of the waveform without portion of the waveform without distorting the remaining part of the distorting the remaining part of the waveform are waveform are

called clipper circuits.called clipper circuits.

TYPES OF CLIPPERS.TYPES OF CLIPPERS.

1.Positive clippers1.Positive clippers

2. Negative clippers2. Negative clippers


ClippersClippers

► Clippers or diode limiting is a diode network that Clippers or diode limiting is a diode network that have the ability to “clip”(cut short/crop) off a have the ability to “clip”(cut short/crop) off a portion on the input signal without distorting the portion on the input signal without distorting the remaining part of the alternating waveform.remaining part of the alternating waveform.

► Clippers are used to eliminate amplitude noise or Clippers are used to eliminate amplitude noise or to fabricate new waveforms from an existing to fabricate new waveforms from an existing signal.signal.

► Simplest form of diode clipper- one resistor and a Simplest form of diode clipper- one resistor and a diodediode

► Depending on the orientation of the diode, the Depending on the orientation of the diode, the positive or negative region of the applied signal is positive or negative region of the applied signal is clipped off.clipped off.


► 2 general of clippers:2 general of clippers:a) Series clippersa) Series clippersb) Parallel clippersb) Parallel clippers

► Series ClippersSeries Clippers The series configuration is defined as one The series configuration is defined as one

where the diode is in series with the load.where the diode is in series with the load. A half-wave rectifier is the simplest form A half-wave rectifier is the simplest form

of diode -clipper-one resistor and diode.of diode -clipper-one resistor and diode.► Parallel ClippersParallel Clippers

The parallel configuration has the diode in The parallel configuration has the diode in a branch parallel to the load.a branch parallel to the load.


Series clipper Series clipper

• Diodes “clip” a portion of the AC wave.Diodes “clip” a portion of the AC wave.

• The diode “clips” any voltage that The diode “clips” any voltage that does not put it in forward bias. That does not put it in forward bias. That would be a reverse biasing polarity would be a reverse biasing polarity and a voltage less than 0.7V for a and a voltage less than 0.7V for a silicon diode.silicon diode.


Series clipperSeries clipper


Series clipper with dc supplySeries clipper with dc supply

►By adding a DC source to the circuit, the voltage required to forward bias the diode can be changed.


Parallel ClipperParallel 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.


Parallel ClipperParallel Clipper


ClampersClampers

► A clamper is a network constructed of a diode, A clamper is a network constructed of a diode, resistor, and a capacitor that shifts a resistor, and a capacitor that shifts a waveform to a different dc level without waveform to a different dc level without changing the appearance of the applied signal.changing the appearance of the applied signal.

► Clamping networks have a capacitor Clamping networks have a capacitor connected directly from input to output with a connected directly from input to output with a resistive element in parallel with the output resistive element in parallel with the output signal. The diode is also in parallel with the signal. The diode is also in parallel with the output signal but may or may not have a output signal but may or may not have a series dc supply as an added element.series dc supply as an added element.


ClampersClampers


Element of the clamper circuitElement of the clamper circuit►Magnitude of R and C must be Magnitude of R and C must be

appropriate to ensure appropriate to ensure гг=RC where the =RC where the time constant is large enough and time constant is large enough and capacitor may not discharge during the capacitor may not discharge during the time interval while diode is not time interval while diode is not conducting.conducting.

►We will assume that all practical We will assume that all practical purposes the diode will fully charge or purposes the diode will fully charge or discharge in 5 time constant.discharge in 5 time constant.


A diode in conjunction with a capacitor can be used to “clamp” an AC signal to a specific DC level.



►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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9/18/2015 EC1261 ALICE / Sr.lect-EEE UNIT –IV PULSE CIRCUITS.

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