# UNIT –IV PULSE CIRCUITS

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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 . 4/17/2017

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 4/17/2017

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. 4/17/2017

As C2 discharges base VB2 is greater therefore Q2 ON and Q1 OFF.
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. 4/17/2017

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. 4/17/2017

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. 4/17/2017

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. 4/17/2017

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. 4/17/2017

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. 4/17/2017

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. 4/17/2017

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 4/17/2017

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. 4/17/2017

2 general of clippers: a) Series clippers b) Parallel 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. 4/17/2017

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. 4/17/2017

Series clipper 4/17/2017

Series clipper with dc supply
By adding a DC source to the circuit, the voltage required to forward bias the diode can be changed. 4/17/2017

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. 4/17/2017

Parallel Clipper 4/17/2017

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. 4/17/2017

Clampers 4/17/2017

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. 4/17/2017

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. 4/17/2017