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

9/18/2015EC1261 ALICE / Sr.lect-EEE Monostable multivibrator

9/18/2015EC1261 ALICE / Sr.lect-EEE Monostable Multivibrator  Q1 and Q2 are identical NPN transistors.The two collector resistances are equal. 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 Q1, it starts conducting because of which VC1 decreases this causes decrease base voltage of Q2. decreases this causes decrease base voltage of Q2.

9/18/2015EC1261 ALICE / Sr.lect-EEE  The drop I 1 R C 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 Q1 given 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, the capacitor starts charging through VCC the capacitor starts charging through VCC and makes ON Q1 and makes ON Q1

9/18/2015EC1261 ALICE / Sr.lect-EEE Waveforms of a monostable multivibrator (triggered). Waveforms of a monostable multivibrator (triggered).

9/18/2015EC1261 ALICE / Sr.lect-EEE Astable Multivibrator.

9/18/2015EC1261 ALICE / Sr.lect-EEE Astable Multivibrator  Q1 and Q2 are identical NPN transistors.The two collector resistances are equal. 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 C 2 starts charging and at a state become equal to VCC. to VCC.

9/18/2015EC1261 ALICE / Sr.lect-EEE  C1 which was already charged Vcc starts discharging,because of this Q1 starts conducting. conducting.  As C2 discharges base VB2 is greater therefore Q2 ON and Q1 OFF.

9/18/2015EC1261 ALICE / Sr.lect-EEE Bistable Multivibrator

9/18/2015EC1261 ALICE / Sr.lect-EEE Bistable Multivibrator ► Positive pulse is applied at the reset pin causes Q2 to conduct as collector voltage causes Q2 to conduct as collector voltage of Q2 falls,it cuts OFF Q1. of Q2 falls,it cuts OFF Q1.  When positive pulse is applied at the set pin it causes Q1 ON and Q2 OFF. pin it causes Q1 ON and Q2 OFF.

9/18/2015EC1261 ALICE / Sr.lect-EEE Bistable Multivibrator waveform

9/18/2015EC1261 ALICE / Sr.lect-EEE

9/18/2015EC1261 ALICE / Sr.lect-EEE 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 it voltage increases to VBE1 + VE  This input voltage is called UTP –upper threshold point. threshold point.

9/18/2015EC1261 ALICE / Sr.lect-EEE ► When Q1 is conducting Ic1 causes a drop VE across RE. 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. point.

9/18/2015EC1261 ALICE / Sr.lect-EEE ► The output voltage has two stable states it is called voltage comparator to provide it is called voltage comparator to provide a change in output whenever input exceeds a particular desired value. a change in output whenever input exceeds a particular desired value.

9/18/2015EC1261 ALICE / Sr.lect-EEE UJT Relaxation Oscillator

9/18/2015EC1261 ALICE / Sr.lect-EEE 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. a device like UJT is called UJT Relaxation Oscillator.  R1 and R2 are biasing resistors CT and RT are the capacitance and resistance decide are the capacitance and resistance decide the oscillating rate. the oscillating rate.

9/18/2015EC1261 ALICE / Sr.lect-EEE ► When the capacitor voltage VC exceeds VP the UJT fires VP= ηVBB+VD. the UJT fires VP= ηVBB+VD.  The capacitor discharges through R1,this produces pulse. produces pulse.  When capacitor voltage falls below VV the UJT gets turned OFF. UJT gets turned OFF.

9/18/2015EC1261 ALICE / Sr.lect-EEE Pulses of UJT relaxation oscillator

9/18/2015EC1261 ALICE / Sr.lect-EEE clippers The circuit which is used to clip certain portion of the waveform without distorting the remaining part of the waveform are The circuit which is used to clip certain portion of the waveform without distorting the remaining part of the waveform are called clipper circuits. called clipper circuits. TYPES OF CLIPPERS. TYPES OF CLIPPERS. 1.Positive clippers 1.Positive clippers 2. Negative clippers 2. Negative clippers

9/18/2015EC1261 ALICE / Sr.lect-EEE 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.

9/18/2015EC1261 ALICE / Sr.lect-EEE ► 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.

9/18/2015EC1261 ALICE / Sr.lect-EEE Series clipper Diodes “clip” a portion of the AC wave. 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. 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.

9/18/2015EC1261 ALICE / Sr.lect-EEE Series clipper

9/18/2015EC1261 ALICE / Sr.lect-EEE Series clipper with dc supply ► ► By adding a DC source to the circuit, the voltage required to forward bias the diode can be changed.

9/18/2015EC1261 ALICE / Sr.lect-EEE 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.

9/18/2015EC1261 ALICE / Sr.lect-EEE Parallel Clipper

9/18/2015EC1261 ALICE / Sr.lect-EEE 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.

9/18/2015EC1261 ALICE / Sr.lect-EEE Clampers

9/18/2015EC1261 ALICE / Sr.lect-EEE 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.

9/18/2015EC1261 ALICE / Sr.lect-EEE A diode in conjunction with a capacitor can be used to “clamp” an AC signal to a specific DC level.

9/18/2015EC1261 ALICE / Sr.lect-EEE

9/18/2015EC1261 ALICE / Sr.lect-EEE ► ► 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.