20EC401-EC-II -COURSE MATERIAL - Course material ECE.pdf

20EC401-ELECTRONIC
CIRCUITS-II
Course Material
II ECE/IV SEM
Regulation-2020
KONGUNADU COLLEGE OF ENGINEERING AND TECHNOLOGY
(AUTONOMOUS)
NAMAKKAL- TRICHY MAIN ROAD, THOTTIAM
DEPARTMENT OF ELECTRONICS AND COMMUNICATION
ENGINEERING

Feedback
➢ Consists of returning part of the output of a system to
the input.
➢ Negative Feedback: a portion of the output signal is
returned to the input in opposition to the original input
signal
➢ Positive Feedback: the feedback signal aids the original
input signal(same phase)

Feedback amplifier. Note that the signals are denoted as xi, xf, xo, and so on.
The signals can be either currents or voltages

A
A
Af
+
=
1 Negative feedback(Af<A)

A
A
Af
−
=
1
Positive feedback (Af>A)
Af--- closed loop gain
A---Open loop gain
A----loop gain

Positive feedback provides an easy way to obtain large gain.
It leads to poor gain stability , a slight shift in power supply
Or temp can change the magnitude of loop gain to unity &
cause the Amplifier to break into oscillation .

Comparison
Parameter +ve feedback -ve feedback
Phase shift b/w i/p-o/p 0 or 360(in phase) 180(out of phase)
Voltage gain Increases Decreases
Stability Decreases Increases
I/O voltage Increases Decreases
Applications Oscillator circuits Amplifier circuits

Types of Feedback
There are 4 basic types of feedback that have different effects:
➢Voltage series
➢Current series
➢ Voltage shunt
➢ Current shunt
The units of  are the inverse of the units of the amplifier gain
•For series-voltage feedback A=Av and
 is unit less
•For series-current feedback A=Gm and
 is in W
•For voltage shunt feedback A=Rm and
 is in Siemens
•For current shunt feedback A=Ai and
 is unit less

The four basic feedback topologies: (a) voltage-sampling series-mixing (series-shunt) topology; (b) current-
sampling shunt-mixing (shunt-series) topology; (c) current-sampling series-mixing (series-series) topology; (d)
voltage-sampling shunt-mixing (shunt-shunt) topology.

Voltage amplifier with voltage-series
feedback

Transconductance amplifier with current-
series feedback

Current amplifier with current-shunt
feedback

Transresistance amplifier with
voltage-shunt feedback

Effects of various types of feedback on gain

A
A
x
x
A
s
f
+
=
=
1
0

Av
Av
v
v
A
s
vf
+
=
=
1
0

Gm
Gm
x
x
G
s
mf
+
=
=
1
0

Rm
Rm
x
x
R
s
mf
+
=
=
1
0

Ai
Ai
x
x
A
s
if
+
=
=
1
0
Gain Stabilization
➢If we design the amplifier so that A >> 1, then the
closed loop gain Af is approximately 1/
➢Under this condition Af depends only on the stable
passive components (resistor or capacitors) used in the
feedback network, instead of depending on the open loop
gain A which in turn depends on active device parameters
(gm) which tend to be highly variable with operating point
and temperature

Summary (Effects on feedback)
S.No Parameters Voltage -series
amplifier
Current-series
amplifier
Voltage- shunt
amplifier
Current -shunt
amplifier
1 Gain
𝐴𝑉𝑓 =
𝐴𝑉
1 + 𝛽𝐴𝑉
𝐺𝑀𝑓 =
𝐺𝑀
1 + 𝛽𝐺𝑀
𝑅𝑀𝑓 =
𝑅𝑀
1 + 𝛽𝑅𝑀
𝐴𝐼𝑓 =
𝐴𝐼
1 + 𝛽𝐴𝐼
2 Stability Improves Improves Improves Improves
3 Frequency
response
Improves Improves Improves Improves
4 Distortion Reduces Reduces Reduces Reduces
5 Input
Resistance
Increases
𝑅𝑖𝑓 = 𝑅𝑖(1 + 𝛽𝐴𝑉)
Increases
𝑅𝑖𝑓 = 𝑅𝑖(1 + 𝛽𝐺𝑀)
Decreases
𝑅𝑖𝑓 =
𝑅𝑖
1 + 𝛽𝑅𝑚
Decreases
𝑅𝑖𝑓 =
𝑅𝑖
1 + 𝛽𝐴𝐼
6 Output
Resistance
Decreases
𝑅𝑖𝑓 =
𝑅𝑜
1 + 𝛽𝐴𝑉
Increases
𝑅𝑜𝑓 = 𝑅𝑜(1 + 𝛽𝐺𝑀)
Decreases
𝑅𝑜𝑓 =
𝑅𝑜
1 + 𝛽𝑅𝑚
Increases
𝑅𝑜𝑓 = 𝑅𝑜(1 + 𝛽𝐴𝑖)

Characteristics of negative feedback
amplifiers
1. Better stabilized voltage gain
2. Enhanced frequency response
3. Higher input impedance
4. Lower output impedance
5. Reduction in noise
6. Increase in linearity

Analysis of feedback amplifiers
Steps
1.Identify the type of feedback
2.Redraw the amplifier circuit without the effect of feedback .
3.Use a thevenin’s source at the input for series mixing and use
a Norton’s source at the input for shunt mixing.
4.After drawing the amplifier circuit without feedback determine
the ac parameters of the circuit using the h parameter model.
5.Determine the feedback ratio  = xf / xo from the original circuit.
6. Find he desensivity factor(D).
7.Knowing A,D,Ri,and Ro , Find Af, Rif , Rof.

Nyquist criterion
Criterion Of Nyquist:
The amplifier is unstable
if this curve encloses the
point –1+j0 and the
amplifier is stable if the
curve does not enclose
this point
Re (A)
Im(A)
-1+j0
A
f2 f
1
fn
1+ A
Gain and
phase margins
These are a measure
of the stability of a
circuit

Poles of feedback Amplifier
• Poles→ used to measure the amplifier stability and frequency
response.
• Stable→ pole lies in the LHS of complex plane.

• Gain Margin (GM) is defined as the value of | Aβ | in dB at the
frequency at which the phase angle of Aβ is 180°. If the gain
margin is negative, then the amplifier is stable. If the gain
margin is positive, then the amplifier is unstable.
• Phase Margin (PM) is defined as the angle of 180° minus the
magnitude of the angle of Aβ at which | Aβ | is unity (0 dB).

• Oscillators→ produce a continuous signal with constant amplitude and
fixed frequency
• An oscillator is a circuit that produces a repetitive signal from a dc voltage.
• The feedback type oscillator which rely on a positive feedback of the
output to maintain the oscillations.
OSCILLATOR

Barkhausen criterion
1.The magnitude of the loop gain A must be 1
2.The phase shift of the loop gain A  must be 0 or
360 or integer multiple of 2pi

Amplitude stabilisation
➢in both the oscillators above, the loop gain is set by component
values
➢in practice the gain of the active components is very variable
➢if the gain of the circuit is too high it will saturate
➢if the gain of the circuit is too low the oscillation will die
➢real circuits need some means of stabilising the magnitude of the
oscillation to cope with variability in the gain of the circuit
Mechanism of start of oscillation:
➢The starting voltage is provided by noise, which is produced due to
random motion of electrons in resistors used in the circuit.
➢The noise voltage contains almost all the sinusoidal frequencies.
This low amplitude noise voltage gets amplified and appears at the
output terminals.
➢The amplified noise drives the feedback network which is the phase
shift network. Because of this the feedback voltage is maximum at a
particular frequency, which in turn represents the frequency of
oscillation.

Circuit Description:
❖ R1, R2 → Provides voltage divider biasing for Hartley Oscillator
❖ RE-CE → Acts as bypass network
❖ RFC →Radio frequency coil which is used to isolate the
oscillator input from biasing
❖ Cc1 → Coupling capacitor 1, used to couple the input signal
between feedback network into oscillator input.
❖ Cc2 →Coupling capacitor 2, used to couple the output signal
between oscillator output to feedback network.
❖ L1-L2-C → Tuned / Tank circuit which is used to provide
continuous oscillation for the oscillator.

Working Principle:
❖ Input is applied at base terminal of transistor which produces
180o phase shift signals.
❖ Part of the oscillator output is applied across feedback
network which also produces 180o phase sift signal.
❖ Total phase shift around the oscillator becomes (180o + 180o
=360o)
❖ Hence the condition of oscillator is satisfied.

❖ R1, R2→ Provides voltage divider biasing for the circuit
❖ RFC → Radio frequency coil which is used to isolate the
❖ Cc1 → Coupling capacitor 1, used to couple the input
signal between feedback network into oscillator input.
❖ Cc2 → Coupling capacitor 2, used to couple the output
signal between oscillator output to feedback network.
❖ C1-C2-L → Tuned / Tank circuit which is used to provide

Working Principle:
❖ Input is applied at base terminal of transistor which
produces 180o phase shift signals.
❖ Part of the oscillator output is applied across feedback
network which also produces 180o phase sift signal.
❖ Total phase shift around the oscillator becomes (180o +
180o =360o)

❖ Cc1 → Coupling capacitor 1, used to couple the input signal
between feedback network into oscillator input.
❖ Cc2 → Coupling capacitor 2, used to couple the output signal
between oscillator output to feedback network.
❖ C1-C2-C3-L → Tuned / Tank circuit which is used to provide

Working Principle:
❖ Part of the oscillator output is applied across feedback network
which also produces 180o phase sift signal.
❖ Total phase shift around the oscillator becomes (180o + 180o
=360o)

❖ R1C1- R2C2- R3C3 → Tuned / Tank circuit which is used to
provide continuous oscillation for the oscillator.

Working Principle:
❖ Part of the oscillator output is applied across feedback network
where each RC (R1C1=60o R2C2= 60o R3C3 =60o ) network
produces 60o phase sift signal.
❖ Then the total phase shift across the feedback network is180o.
❖ Hence the total phase shift around the oscillator becomes (180o
+ 180o =360o)
❖ So the condition of oscillator is satisfied.

❖ R5,R6,R7,R8→ Provides voltage divider biasing for the circuit
❖ R1C1- R2C2- R3C3 → Tuned / Tank circuit which is used to
provide continuous oscillation for the oscillator.

Crystal Oscillator
Frequency of oscillation fr = 1/2LC
CM-Mounting capacitance
❖Piezoelectric effect→Due to applying mechanical
Pressure ,voltage is generated across the opposite
faces of crystal.
Equivalent circuit

• It is a modified circuit of colpitts oscillator, where
inductor is replaced by crystal.(Working Principle
same as a colpitts oscillator)
• Crystal behave like inductor and C1,C2 which form a
tank circuit.
• RFC used to provide isolation between a.c and d.c
operation.
• Frequency stability of crystal depends on supply
voltage,temperature,transistor.

• It is a modified circuit of Hartley oscillator, where
one inductor is replaced by crystal.(Working Principle
same as a Hartley oscillator)
• Crystal behave like inductor(L2) and L1,C which
form a tank circuit.
• Crystal decide the operating frequency of oscillator.
• It is used in trigger circuits,sawtooth generator and
timing circuits.

• Primary winding of transformer to the form a resonant
circuit.
• Feed back signal is taken from a secondary winding
and fed back to the Base of transistor.
• Transistor→180 Phase shift+Transformer→180 Phase shift.
• Total Phase shift=360 or 0
• Frequency of oscillation fr = 1/2LC
• Feedback factor  = M/L
• The Armstrong oscillator uses transformer coupling in
the feedback loop. For this reason some losses occur.

• Two CE amplifiers cascaded by Rb.
• When VCC is ON tuned circuit (LC) produce damped
oscillation at a resonant frequency.
• This signal fed to the input of Transistor-2, it amplify
and produce 180 Phase shift.
• The output T2 is fed back to the input of Transistor-1
via Rf.
• Transistor-1, it amplify and produce another 180
Phase shift,hence the total Phase shift=360 or 0

• In this circuit, inductive f/b from the collector of a
Transistor to base.
• When VCC is ON, L1-C tuned and f/b signal is taken
from L2 and f/b to base.
• Due to transistor and transformer collectively produce
total Phase shift=360 or 0
• Feedback factor  = M/L

Frequency Stability of Oscillator
• Due to temperature changes, the value of the tank circuit get
affected.
• Due to changes in temperature, the active device like BJT,FET
affected.
• Due to variation in power supply.
• Due to atmospheric conditions.
• Due to load resistance and stray capacitance.
• Frequency stability

➢ Frequency selectivity of resonant circuits allows a radio to be tuned to one of a set
of transmitting stations.
➢ Tuning is usually undertaken by varying the capacitance of an adjustable capacitor.
➢ Resonant circuits are also important for tuning and for transmitting signals.
Tuned amplifiers
➢ To amplify the selective range of frequencies , the resistive load , Rc is replaced by
a tuned circuit.
➢ The tuned circuit is capable of amplifying a signal over a narrow band of
frequencies centered at fr.
Tuned Circuits

Types Of tuned amplifiers
Single tuned amplifier
➢ one parallel tuned circuit is used as a load
➢ Limitation: Smaller Bandwidth , smaller gain bandwidth product, does
not provide flatten response.
Double tuned amplifier
➢ It provides high gain, high selectivity and required bandwidth.
➢ Used in IF in radio and TV receivers.
➢ It gives greater 3db bandwidth having steep sides and flat top . But
alignment of double tuned amplifier is difficult
Stagger tuned amplifier
➢ Two single tuned amplifier are connected in cascaded form.
➢ Resonant frequency are displaced.
➢ To have better flat , wideband charcteristics with a very sharp rejective,
narrow band characteristics.

Working Principle:
❖ Input is applied at base terminal of transistor and output is
taken across collector terminal.
❖ The components r-L-C together forms a tuned circuit for
continuous oscillation of the output.
❖ It is also used to select the desired range of frequency.
❖ The resistor RL act as a load resistance

CAPACITIVE COUPLED SINGLE TUNED
AMPLIFIER

Working Principle:
❖ It is also called as single tuned multistage amplifier.
❖ Each stage consists of only one tuned circuit.
❖ Each stage is tuned by same set of frequency or same desired
signal.

Analysis
• Both tuned circuits are tuned to the same frequency.
• Double tuned ckt provide a BW of several percent of the resonant
frequency.
• C1-L1 and C2-L2 are tank circuit components respectively at primary and
secondary. Frequency response in Double Tuned amplifier depends on the
Magnetic Coupling of L1 and L2 .

C𝐚𝐥𝐜𝐮𝐥𝐚𝐭𝐢𝐨𝐧 𝐨𝐟 𝐁𝐚𝐧𝐝𝐰𝐢𝐝𝐭𝐡

Instability of tuned circuits
• In RF tuned Amplifiers,at very high frequencies, the junction
capacitance of a transistor(B-C) could introduce sufficient
feedback from output to input to cause unwanted oscillations
to take place in an amplifier.
• Neutralization is used to cancel the oscillations by feeding back
a portion of the output that has the opposite phase but same
amplitude as the unwanted feedback.
• Also it oscillate enough energy is feedback from the C-B in the
correct phase to overcome the circuit loss.
Stabilization techniques:
1. Hazeltine neutralization
2. Neutrodyne neutralization
3. Neutralization using coil
Stability of Tuned Amplifier

Circuit Description
• R1-R2→provides voltage divider biasing for the circuit.
• RE-CE→forms the bypass network
• VCC→Provides proper biasing for the circuit.
• Output is taken across betwenn L1 and L2
• CN→ Variable capacitance connected from bottom coil to
Base of Tr.
Working Principle
• CN→Feeds a signal of equal magnitude but opposite
polarity from the bottom coil to base of Tr.
• The Neutralizing capacitor CN,can be adjusted correctly to
cancel the signal through Cbc

Advantages
• Due to the presence of CN ,the internal capacitance in the
transistor can be eliminated
Drawbacks
• Input capacitance can’t be eliminated fully because CN is
connected to the Vcc via primary of transformer hence
some leakage current occurs

Circuit Description
• Output is taken across betwenn L1 and L2
• CN→ Variable capacitance connected between Base of
Transistor and lower end of the base coil of next stage.
Working Principle
• CN→Feeds a signal of equal magnitude but opposite
polarity from the bottom coil to base of Tr.
• The Neutralizing capacitor doesn’t have the supply voltage
across it.
• Hence Extra leakage current doesnot occurs across CN

Circuit Description
• Output is taken across transformer.
Working Principle
• Bottom coil L provide minimum coupling to other
windings.
• If the windings are properly polraised,the voltage across L
have proper phase to cancel the signal coupled through
Base-Collector ,Cbc capacitance.

Advantages
• Neutralization capacitance completely eliminate the
internal capacitance leakage effect.
Drawbacks
• Because of more number of transformers, circuit becomes
complex.
• Some leakage occurs because of improper coupling
between the output transformer windings.

Unit-4
WAVE SHAPING AND MULTIVIBRATOR
CIRCUITS

Attenuators
• It is a device used to reduce the amplitude of a signal wave form.
• It can be designed by resistors
1)Simple attenuator
• Potential divider R1-R2 which is a together forms attenuator
• The input gets multiplied by the ratio (R2/R1+R2)
• But in practice there exists a shunt capacitanceC2 across R2
• It is necessary to ensure that there is no distortion due to
attenuator, in presence of stray capacitance C2

• Generally R1 and R2 are very large to keep the input
impedance high, to reduce the loading effect.
• Hence R is also large, due to this. the time constant RC2 of
the circuit is large which is totally unacceptable.
• Due to large time constant, time is also large, Which
causes a distortion. The high frequency components get
attenuated.
• Hence attenuation no anger remains independent of the
frequency.

Compensated Attenuator
• The compensation is provided in the actual attenuator circuit
by shunting R1 by capacitor C1
• The circuit can be redrawn such that the two resistors
and two capacitors act as the four arms of a bridge.
• The bridge will be balanced when,R1C1=R2C2
• Under the balanced condition no current can flow through
the branch joining the terminals X and Y.
• Hence for calculating output, the branch X-Y can be omitted,
under balanced bridge condition. This output is equal to
aVi and is independent of frequency.

Attenuator in bridge form
Compensated Attenuator

Step response
• To find out the output waveform, when step voltage is
applied to the compensated attenuator.
• The step input has applied at t = 0. So the input changes
from O to A instantaneously at t = o.
• Now the voltage across C1 and C2 must change abruptly.
• But we know that voltage cannot change instantly.
• From this we can conclude that an infinite exists at t = 0
for an infinitesimal time. The current is impulsive as act as
short circuit. Due to such infinite current a finite charge

Attenuator in CRO
• The use of attenuator in CRO probe is very common.
• Basically CR0 has limitation with respect to the amplitude of the signal to be
displayed.
• The attenuator probe reduces the level of the signal, so that the signal can be
perfectly displayed on the CRO.
• When the point at which signal exists is at some distance from the CRO an it
appears at a high impedance level then the shielded cable is used to connect
the signal to the oscilloscope.
• The shielding is used to isolate the input lead terminals from the stray fields,
which generally exist with power line.
• CRO consists of metal shield is of few inches while the shielded cable is few
feet long.
• This probe assembly uses the shielded cable but stills keep the capacitance to
the probe assembly is about 10 to 20 pF.
• The attenuation factor used practically with the help of such a probe is 10 or
20.

1)RC Integrator
•An RC integrator is a circuit that approximates the mathematical process of
integration.
• Integration is a summing process, and a basic integrator can produce an
output that is a running sum of the input under certain Conditions
•A basic RC integrator circuit is simply a capacitor in series with a resistor
and the source. The output is taken across the capacitor.
VS
R
C
Vout

The RC Integrator
When a pulse generator is connected to the input of an RC integrator,
the capacitor will charge and discharge in response to the pulses.
When the input
pulse goes
HIGH, the
source acts as a
battery in
series with a
switch.
Switch closes
The output is an
exponentially
rising curve. Only
the first part of
this looks like
true
mathematical
integration.
R
C

The RC Integrator
When the pulse generator goes low, the internal impedance of the
generator makes it look like a closed switch has replaced the battery.
Battery is
replaced with
a switch that
closes.
The output is an
exponentially
falling curve.
Again, only the
first part of this
looks like true
mathematical
integration.
R
C

2) RC Differentiator
Differentiation is a process that finds the rate of change, and a basic
differentiator can produce an output that is the rate of change of the
input under certain conditions.
A basic RC differentiator circuit is simply a resistor in series with a
capacitor and the source. The output is taken across the resistor.
VS
R
C
Vout

When the input pulse goes HIGH, The capacitor looks like a short to
the rising edge because voltage across C cannot change
instantaneously.
During this first
instant, the
output follows
the input.
The RC Differentiator
0
VC = 0

capacitor charges at the constant input and output voltage decays.
The voltage across C
is the traditional
charging waveform.
The output falls
exponentially as
the pulse levels
off.

The falling edge is a rapid change, so it is passed to the output because
the capacitor voltage cannot change instantaneously.
The voltage across C
at the instant the
generator turns off
does not change;
then it decays.
After dropping to
a negative value,
the output
voltage rises
exponentially as
the capacitor
discharges.

When the pulse generator output rises, a voltage immediately appears
across the inductor in accordance with Lenz’s law. The instantaneous
current is zero, so the resistor voltage is initially zero.
The output is
initially zero
because there is
no current.
VS
R
L
+ −
The induced
voltage across L
opposes the
initial rise of
the pulse.
0 V
3) RL Integrator

Inductor voltage decays exponentially and current rises. As a result, the
voltage across the resistor rises exponentially.
The output
voltage rises as
current builds in
the circuit.
VS
R
L
+ −
The induced
voltage across L
decays.
The RL Integrator

When the pulse falls, a reverse voltage is induced across L opposing the
change. The inductor voltage initially is a negative voltage that is equal
and opposite to the generator; then it exponentially rises.
The output voltage
decays as the
magnetic field
around L collapses.
VS
R
L
+
−
The induced
voltage across L
initially
opposes the
change in the
source voltage.
Note that these waveforms
were the same in the RC
integrator.
The RL Integrator

4)RL Differentiator
VS L
R
Vout
Differentiation is a process that finds the rate of change, and a basic
differentiator can produce an output that is the rate of change of the
input under certain conditions.
A basic RL differentiator circuit is simply a resistor in series with a
Inductor and the source. The output is taken across the resistor.

When the input pulse goes HIGH, initially, no current in R.
Current is
initially zero, so
VR= 0.
During this first
instant, the
inductor develops
a voltage equal
and opposite to
the source
voltage.
The RL Differentiator
VR = 0
VS
L
R
+
−

After the initial edge has passed, current builds in the circuit.
Eventually, the current reaches a steady state value given by Ohm’s
law.
The voltage across R
rises as current
increases.
The output falls
exponentially as
the pulse levels
off.
VS
L
R
+
−

Next, the falling edge of the pulse causes a (negative) voltage to be
induced across the inductor that opposes the change. The current decays
as the magnetic field collapses.
The voltage across R
decays as current
decreases.
The output
drops initially
and then rises
exponentially.
VS
L
R
+
−

Wave shaping circuits
➢ Linear wave shaping :Process by which the shape of a non sinusoidal signal
is changed by passing the signal through the network consisting of linear
elements
➢ Diodes can be used in wave shaping circuits.
➢ Either
limit or clip signal portion--- clipper
shift the dc voltage level of the signal --- clampers
➢ Types of non sinusoidal input
step, pulse ,square, Ramp input

Diode Clippers
A clipper (or limiter) is a circuit used to eliminate some
portion (or portions) of a waveform.
– A series clipper is in series with its load.
– A shunt clipper is in parallel with its load.

Negative Shunt Clipper Operation

A Positive Shunt Clipper
S
L
L
in
L
R
R
R
V
V
+
=
F
L V
V =
When the diode is
conducting:
When the diode is not
conducting: S
L
L
in
L
R
R
R
V
V
+
=

Diode :- Clamper
Positive Clamper
During the -ve half cycle of
the input signal, the diode
conducts and acts like a short
circuit. (output voltage Vo 
Vm)
The capacitor is charged to
the peak value of input
voltage Vm. and it behaves like
a battery.
During the positive half of the
input signal, the diode does
not conduct and acts as an
open circuit. Hence the output
voltage Vo Vm+ Vm This gives
a positively clamped voltage.
Vo Vm+ Vm = 2 Vm

Diode :- Clamper
Positive Clamper
Vo=Vi+Vm
Vo=Vm for Vi=0v
Vo=2Vm for Vi=Vm
Vo=o for Vi=-Vm
Vm→capacitor voltage

Diode :- Clamper
Negative Clamper
During the positive half
cycle the diode conducts
and acts like a short circuit.
The capacitor charges to
peak value of input voltage
Vm.
During this interval the
output Vo which is taken
across the short circuit will
be zero During the negative
half cycle, the diode is open.
The output voltage.

Diode :- Clamper
Negative Clamper
Vo=Vi-Vm
Vo=-Vm for Vi=0v
Vo=0 for Vi=Vm
Vo=-2Vm for Vi=-Vm
Vm→capacitor voltage

Diode :- Clamper
Biased Clamper

Diode :- Clamper
During the negative half cycle of the input signal the
diode is forward biased and acts like a short circuit.
The capacitor charges to Vi + Vs . Applying the KVL to
the input side
During the positive half cycle of the input signal, the
diode is reverse biased and it acts as an open circuit.
Hence Vs has no effect on Vo. Applying KVL around
the outside loop.

Multivibrators
• It is a two stage amplifier,operating in two
modes.
• The output of Ist stage fedback to input of 2nd
stage and the output of 2ndstage fedback to
input of Ist stage.
• If One stage is saturation, then next stage will
be cut-off vice versa..

Types
1) Bi-stable Multivibrator→Two stable states. External trigger
pulse required to change one state to other..

2) Mono-stable Multivibrator→only one
stable state and other state is unstable or
quasi state .If external supply applied,ckt
goes to Normal to quasi(OFF-ON) and after
some time automatically return to normal
state.

3) Astable Multivibrator→Both states are quasi or
unstable. Without any external trigger change the
state automatically.

1)Fixed Bias Bi-stable Multivibrator
0v
Vcc
off
saturation

1)Fixed Bias -Bi-stable Multivibrator
0v
Vcc
saturation
off
Trigger pulse Trigger pulse

Circuit is turned on.
➢ One of the two transistors will conduct harder and
thereby reach saturation first. (Assume Q2)
➢ The 0V at the collector of Q2 is coupled to the base of
Q1which drives Q1 into cutoff.
➢The +VCC at the collector of Q1 is coupled to the base of
Q2 holding Q2 in saturation.
➢ An input trigger pulse is applied to the bases of both Q1
and Q2 simultaneously. Since Q2 is already in saturation,
there is no effect on Q2.
Circuit Operation

➢The trigger pulse turns on Q1 and drives the transistor
into saturation.
➢ The 0V on the collector of Q1 is coupled to the base of
Q2 driving Q2 into cutoff.
➢The -VCC on the collector of Q2 is coupled to the base of
Q1 holding Q1 in saturation.
➢ This process will continue as long as there are trigger
pulses applied to the circuit.
➢ The output frequency of the waveforms will be
determined by the frequency of the input trigger pulses.
➢Both the transistor are never ON or OFF simultaneously.

Output Waveforms
Output
Voltage
V
w
=V
C1
-V
C2

Triggering methods for Bistable multivibrators
• To Achieve transition from one state to another state in Bi-
stable multivibrator triggering is required.
• Such a triggering signal may be a step or pulse signal.
Types
• Symmetrical triggering→ only one input signal used
• Un-Symmetrical triggering→2 set of signal can be used.(one
for set and another for reset).

• A1 and A2 are NPN transistors.
• apply +ve step signal for (OFF→ON),Because apply vol need
above cut-off Tr.
• apply -ve step signal for (ON→OFF) ),Because apply vol need
below cut-off Tr.

a.1)Un-Symmetrical triggering using unilateral device

• Triggering is achieved by using diode.
• Diode is used to change the state of transistor because of polarity
of pulse.
• Step1:If Q1 is ON→drop across Rc which keeps Diode-reverse
biased.Hence Diode cannot tx –ve pulse,until its amplitude is larger
than Rc.
• Step2:If Q1 is off→points P and Q have equal potential and drop
across diode is 0 vol.Hence it cannot tx-ve pulse.
• Step3:If –ve pulse is applied→Diode act as a short ckt and tx the –
ve pulse to base of Q2.(ON→OFF).Hence transition occurs at
leading edge of –ve pulse.

b) Symmetrical triggering using diodes
R
Q
P
Vcc
0v

• Assume Q2-ON,Q1-OFF,hence Rc of Q2 is large but D2 reverse
biased.Due to Q1-OFF,Rc of Q1 is zero.
Hence points P and Q are equal potential.
❖ When –ve pulse applied at T,Q goes to –ve so D1 gets forward biased,
which tx –ve pulse to point P of collector Q1.
❖ Then this –ve pulse passed to base of Q2 via R1-C1,Which turns Q1-ON
and Q2-OFF.Hence Bi-stable transition occurs.

b.1)Symmetrical triggering without-
using diodes

2)Mono-Stable Multivibrator
• One is stable state and other is unstable.
Types
1)Collector coupled Monostable multivibrator
2)Emitter coupled Monostable multivibrator

1)Collector coupled Monostable Multivibrator
+
OFF ON

➢ The value of R2 and –VBB are selected to make Q1 is OFF,and Q2 gets
ON.This is called normal stable state.
➢ When +ve trigger pulse applied to base of Q1 through C2,hence Q1 starts
conducting.
➢ Due to this VC1 decreases and is coupled to base of Q2 so the base of Q2
decreases,this decrease forward bias of Q2 and hence IC2 decreases.
➢ In this time VC2 is applied base of Q1 through R1,hence Q1 is quickly driven
in saturation and Q2 gets to cut-off. This is called Quasi stable state.
➢ The multivibrator will remain in this Quasi state(T),until another gate
“triggering” pulse is received.
➢ Only one trigger pulse is required to generate a complete cycle of output.

• ii) Quasi stable-state
At t=0,Q2→OFF and Q1→ON
I1 RC=VCC-VCE(sat)

2) Emitter coupled Monostable Multivibrator
OFF ON
E
E

• Working same as Collector coupled, width of pulse T controlled.
• Emitter terminals both transistors are coupled together Emitter.
• Initially Q1-OFF,Q2-ON(saturation)→Normal state
• When +ve trigger pulse is applied at base of Q1,which drives Q1 in
to conduction,due to this Vol drop across Rc1,which makes Q2-
OFF.→Quasi state
• The capacitor C now gets charged through Q1.
• Due to C-charges,Q2 charging gradually.
• Due to the regenerative feedback,Q2 goes to ON and Q1-OFF.
• Gate width T=ln{(Vcc-VBN2+I1 Rc1)/(Vcc-VEN1-v2)}

Triggering of Mono-stable Multivibrator
• For npn transistor→-ve trigger pulse applied.
• Pnp→+ve pulse applied

• Initially Q1→OFF,Q2→ON.
• -ve trigger pulse applied through C1 & D
• Diode conducts & passes to the base of Q2.
• Hence its decreases Q2 current so Q2 moves from
saturation to cut-off.
• At the same time Q1 starts conduction. This is called Quasi
state.
• When Q1 in conduction,C1 gets charge (Vcc).
• Once Q1 moves to cut-off,C1 discharges towards base of Q2
gets ON.
• The circuit return back to normal state.

3)Astable Multivibrator
• No trigger pulse is required.
• Both states are quasi or unstable.
Types:
• Collector Coupled Astable Multivibrator
• Emitter coupled Astable multivibrator

3.a)Collector Coupled Astable
Multivibrator
Without any trigger signal, Transistor states can be switched periodically.
OFF ON

• Transistors Q1 and Q2 are identical transistors.
• Both collector resistances(Rc) are equal.
• C1,C2 are coupling capacitors.
• Case1→Q2 is ON and Q1 is OFF,The capacitor C2 starts charging
towards Vcc through the path Rc,C2 and ON Q2.
• At the same time C1 starts discharging towards base of Q1.
• Now Q1 starts conducting, but Q2 goes to off mode.
• At the same time C2 discharging through base of Q2.
• Then Q2 is ON then Q1 become OFF.

Waveforms of Astable Multivibrator
• When Q2→ON;Q1→OFF
VB1=VBE(sat)
VC1=VCE(sat)
VC2=VCC
• When Q1→ON;Q2→OFF
VB2=VBE(sat)
VC2=VCE(sat)
VC1=VCC

2)Emitter coupled Astable multivibrator

Schmitt trigger
❖Used for wave shaping circuit.
❖Used to generate square wave from a sine wave I/p.
❖Trigger is not pulse transform but slowly varying ac
Signal.
❖Switches at two trigger level : upper & lower trigger
Level.

Circuit Operation
• When Q1-OFF,it act as a open ckt.
• When Vi is applied to ckt,Q2 starts conducting and gets
saturated.
• VB2=I2R2 and Vc2 or Vo=Vcc-Ic2R2
• When Vi is increasing and to make Q1-ON,and Q1 is in active
region when Vi=VBE+VE.This input voltage level is called upper
threshold point.
• As Q1-ON,vol drop across Rc1,the base volt of Q2 reduces and
goes to cut-off,when Q1-goes to saturation.
• Vc1=Vcc-Ic1Rc1

• If Vc1 increases, this controls base volt of Q2 which also
increase,hence Q2 becomes again ON is called Lower Threshold
Point.
• Hysteresis loop→No changes in ouput volt,by any changes in
input.
• Width of Hysteresis=UTP-LTP

Input/Output waveforms
When a positive-going input passes the upper trigger
point (UTP) voltage.
When a negative-going input passes the lower trigger
point (LTP) voltage.

Transistor Switching Times
• When Pulse is applied to the transistor, at +ve rising
edge, transistor is switched ON while at the falling
edge it is switched OFF.
• But transistor cannot switched ON instantly,it takes
some time to start the change.

Waveform Time Measurements
• Delay time (td)
• Rise time (tr)
• Storage time (ts)
• Fall time (tf)
+VE
edge
-VE
edge
Pulse width
td tr ts tf
tON toff
ton=td+tr
toff=ts+tf

• Delay time (td)→The time required for Ic to increase to 10%
of its final value.
• Rise time (tr)→ The time required for Ic to rise from 10%
to 90% of its maximum value.
• Storage time (ts)→The time between IB is switched OFF
and Ic falling to 90% of its final max value.
• Fall time (tf)→ The time required for Ic fall from 90% to
10% of its maximum value.

Improving Switching Times
• Speed-up capacitor – A component used to reduce
delay time and storage time.
max
20
1
f
R
C
B
S 

UJT-Relaxation oscillator(Saw tooth generator)
•Used to generate the sawtooth waveform
•UJT→ON-OFF controlled by capacitor.
•Capacitor→charging→UJT OFF
•Capacitor→Discharging→UJT ON
Operations
When capacitor Vc<Vp,then capacitor gets
Charging at the time UJT goes to off mode.
When capacitor Vc>Vp,then capacitor gets
Dis-Charging at the time UJT goes to on mode.

UNIT V- BLOCKING OSCILLATORS
AND TIME BASE GENERATORS

Large signal tuned amplifier-Class C
amplifiers
➢ Class C amplifiers can be used as tuned RF amplifiers where
the undesired harmonic frequencies can be filtered out.
➢ A class C amplifier is more efficient than either a class A or
class B amplifier; its efficiency approaches 100%.

Power Amplifiers
• Used to amplify power or
current.
• Used in radio, TV receivers,tape
players,CRT
Types
• Class A→ Select Q-point at
the midpoint of the load line
• Class B →Select Q-point
exactly in the X- axis
• Class C →Select Q-point
below the X-axis

1)Class A amplifier-Directly Coupled
– Q 1 is a power transistor.
– The value of RB is selected in order to
place a Q-point in the centre of d.c
load line.
– Vcc and RB→provides Base-bias.
– IB=
– Vce=
– Pdc=VCC.ICQ
– Pac=Vrms.Irms=I2rms RL
– Efficiency η =(Pdc/Pac)*100
– Max η =25%
Advantages:
– Simple to design, less no of
components required.
Drawbacks:
– Power dissipation is more, poor
efficiency, output impedance is high.

1)Class A amplifier-Directly Coupled
– Q 1 is a power transistor.
– The value of RB is selected in order to
place a Q-point in the centre of d.c
load line.
– Vcc and RB→provides Base-bias.
– IB=(VCC-VBE)/RB
– Vce=Vcc-IcRL.
– Pdc=VCC.ICQ
– Pac=Vrms.Irms=I2rms RL
– Max η =25%
Advantages:
– Simple to design, less no of
components required.
Drawbacks:
– Power dissipation is more, poor
efficiency, output impedance is high.

2)Transformer Coupled Class A amplifier
• It eliminate impedance matching problem.
• Loudspeaker connected to the secondary of output
transformer.
• Loudspeaker act as a load.
• Turns ratio→ n=N2/N1.
• Voltage or current ratio→ n=V2/V1 or I1/I2
• Load or output impedance RL→ V2/I2.
• Reflected impedance R’L=V1/I1= RL/n2
• Collector current ICQ=βIBQ= β(Vcc-VBE)/RB
– Pdc=VCC.ICQ
– Pac=Vrms.Irms=I2rms R’L
– Max η =50%
Advantages:
– High efficiency compare to direct coupling.
Drawbacks:
– Circuit size is large, complicated ckt design, poor
frequency response.

3)Class B Push-Pull amplifier
• Drive transformer used in input side.
• Output transformer deliver the output signal
to loud speaker.
• During +ve half cycle of input signal→Q1
transistor goes to ON.
• During -ve half cycle of input signal→
Q2transistor goes to ON.
• Q1 and Q2 will be out of phase 1800
• Idc=Im/π
• Pdc=Vcc .Idc
• Pac=Vrms.Irms
– Max η =78.5%
– Power Dissipation Pd=Pdc-Pac

Waveform
Advantages:
– High efficiency, no power
dissipation. No harmonics.
Drawbacks:
– Circuit size is large, due to
transformer, poor
frequency response.

4) Complementary Symmetry Class B amplifier
• Amplifier circuit is designed by NPN-PNP
transistor.
• It is a transformer less circuit.
• Q1 is NPN type,Q2 is PNP type.
• During +ve half cycle of input signal→Q1
transistor goes to ON.
• During -ve half cycle of input
signal→Q2transistor goes to ON.
• It is used in CC configuration for impedance
matching purpose.
Advantages:
– Efficiency is high,frequency response is
good.
Drawbacks:
– Circuit need 2 supply voltages.
– Output is distorted to cross-over
distortion.

Working Principle
➢ Q-Point and the input signal are selected such that the output
signal is obtained less than half cycle of full input cycle.
➢ Due to selection of Q-Point,Transistor remains active for less
than half cycle and goes to cut-off the other half cycle.
➢ The input coupling capacitor, base resistor, and base-emitter
junction form a negative clamper.
➢ Because of the clamping action, only the positive peaks of the
input signal drive the transistor, Q1, into conduction.
➢ Parallel tuned circuit act as a filters the harmonics and
produce a sine wave output continuously.

Input and Output Signals
21 December 2021 166

Output characteristics

Frequency Response
❖Frequency of oscillation fr = 1/2LC
❖Power Gain G=Pout/Pin
❖Output power Pout=V2rms/RL
❖Vrms=VPP/2√2
❖Pout=V 2PP/8RL
❖Quality Factor Q=Wr L/R
❖Efficiency= Pout/Pdc=78.5%-100%
❖Bandwidth=f2 –f1=fr /Q

Advantages
• High efficiency.
• Excellent in RF applications.
• Lowest physical size for a given power output.
Drawbacks
• Lowest linearity
• Not suitable in audio applications
• Creates a lot of RF interference
• It is difficult to obtain ideal inductors and coupling transformers

Applications of Class-c Amplifier
• Mixer in radio receivers.(Mixer is used to convert input signal
in to a low frequency or high frequency signal).
• In AM Transmitter, it is used to produce high modulating
signal.
• Frequency Conversion circuits

Voltage Regulator
• It is a electronic device used to keeps the output voltage constant inspite of changes in load
current or input voltage.
• The input a.c voltage (230v,50Hz) is applied to the transformer .
• The transformer steps down the a.c voltage to the level required for d.c output.
• The rectifiers circuits convert a.c voltage to pulsating d.d voltage.
• The filter circuits used to remove the ripple presents in the filter output and tries to make
smooth output signal.
• This voltage is called un regulated d.c voltage, applied to the regulator circuit which produces
the constant output voltage.
• The output of a regulator is called d.c supply, to which the load can be connected.

Power supply performance analysis
Load regulation
• It is the change in the regulated output voltage ,when the load current is changed from min
to max.
Line regulation
• It is the change in the regulated output voltage ,for a specified range of line voltage typically
230v.
Output Resistance
• It is the ratio of change in output voltage to change in load current.

Voltage stability factor
• It is the ratio of change in output voltage to change in input current.
Temperature stability factor
• It is the ratio of change in output voltage to change in temperature.
• Ripple Rejection factor
• It defines how effectively the regulator circuits rejects the ripples and attenuates it
from input to output.

Types of voltage regulator
1. Shunt voltage regulator
• Transistor shunt regulator
2. Series voltage regulator
• Emitter follower Series voltage regulator
• Transistorized series feedback type regulator
• Transistor Series regulator with Darlington pair

1) Shunt voltage regulator
• Used to produce constant d.c output voltage which is regulated.
• Control element is connected in shunt with the load.
• Sampling circuit provides a feedback signal to the comparator circuit, if there is any change in the
load.
• Comparator circuit compares the feedback signal with the reference voltage and generates a
control signal.
• Only part of the load current required to be diverted ,passes through the control element.
• Control element is a low current, high voltage component.

1.1) Transistor shunt regulator
• Transistor act as a control element.
• The zener voltage VZ,along with the transistor base-emitter voltage VBE,decides the voltage
Vo.(Vo=VZ+ VBE)
• If the load resistance decreases, there by reducing the load voltage, then due to constant
VZ,the biasing voltage of the transistor Q reduces.
• Thus the transistor conduction reduces, shunting the less collector current. hence load
current increases, thereby maintaining the load voltage constant.

2)Series voltage regulator
• Unregulated d.c voltage is applied to the input of control element.
• Control element→controls the amount of the input voltage, that gets to the output.
• Sampling circuit→ provides the necessary feedback signal.
• Comparator circuit→ compares the feedback signal with the reference voltage and generates
a control signal.
• If the load voltage tries to increase, the comparator generates a control signal based on the
feedback information.
• This control signal causes the control element to decrease the amount of the output voltage.
Thus the output voltage is maintained constant.

2.1)Emitter follower Series voltage regulator
• Transistor Q1 acts a series control element for the circuit.
• Zener diode D→provide the reference voltage.
• Zener diode is reverse biased so that it works in the breakdown region.
• Output voltage across the collector-emitter is the difference between Vin and Vo.
• Emitter current is equal to the load current. So transistor Q1 is in series with the load.
• Thus the transistor passes the load current and hence it is called series pass transistor.
• If the output voltage decreases, as VZ is constant, thus the VBE increases. Hence Q1 conducts
more, thereby raising the output voltage ,maintaining the output constant.
• Power dissipation is given by PD=Vin Isc
• Voltage stability factor Sv=RZ/(R+RZ)
• Output resistance RO=(RZ+hie) Z/(1+hfe)

2.2)Transistorized series feedback type regulator
• This is the improved version of the emitter follower series voltage regulator.
• It uses an additional transistor to amplify the change in output before being applied to the
control transistor.
• Transistor Q2 which amplifies the change in the output voltage, which controls the transistor
Q1.
• Emitter of Q2 is always at a constant reference voltage due to the zener diode.
• Base of Q2 is supplied with the potential divider to the circuit.
• It is compared with the reference voltage provided by the zener diode which is Vz.
• The current through the resistance R3,splits into two currents IB1 and IC2.
• When the circuit output voltage decreases due to the increased load current, there is an
error in the output voltage.
• The part of the output is applied to base of Q2 through potential divider. Hence VBE and IC2
decreases. Because of IC2 current through R3 also decreases.
• The current through R1and R2 I=VO/(R1+R2)

Output voltage is given by
→2
→1
→1
→2
→1
→5
1
→4
→3

2.3)Transistor Series regulator with Darlington pair
• In this circuit, pair of transistor is used as a series pass.
• They are connected in such a manner that the emitter of Q3 is connected to the base of Q1.
• Pair of transistor is used to increase hfe to reduce the Ro and also stability factor also decreases
by this arrangement.

3)Switching Regulators
• Used to convert unregulated d.c input to
regulated d.c output
• It uses transistor Q1 as a switch.
• Oscillator generates a triangular waveform ,
is applied to the non-inverting terminal of
comparator.
• The output of the comparator is high when
the triangular voltage waveform is above
the level of error amplifier output.
• Due to this Q1 remains in cut-off state.
• The output is fed back to the inverting input
of error amplifier. It is compared with the
ref voltage.
• The difference is amplified and given to the
comparator inverting terminal.
• The output of the comparator is a required
pulse waveform.(Vo=δVin)

Types of switching Regulators
1. Step-down switching regulator
2. Step-up switching regulator
3. Voltage inverter type switching regulator

3.1)Stepdown switching regulator(BUCK)
• It is also called as buck type switching regulator.
• When Q1 is ON, the capacitor charges through it
and when Q1 is off, the capacitor discharges
through the load resistance.
• When ON time is more compared to OFF time,
the capacitor charges more, increasing the output
voltage.
• If Vo increases→ voltage across R3 increases,
hence the input of error amplifier is decreases.
This produces small pulse width, ton lower in
Q1,which decreases Capacitor, produces reduced
output voltage, so increased o/p voltage gets
compensated.
• Output voltage Vout=δVin where δ=ton/T
• When OFF time is more compared to ON time,
the capacitor discharges more, reducing the
output voltage.
• If Vo decreases→ voltage across R3 decreases,
hence the input of error amplifier is more. This
produces high pulse width, ton higher in Q1,which
increases Capacitor, produces more output
voltage, so decreased o/p voltage gets
compensated.

3.2)Stepup switching regulator(Boost)
• It is also called as boost type switching
regulator.
• When Q1 is ON→goes to
saturation(VCE),the voltage across the
inductor starts decreasing
exponentially from its maximum
value.(Vin-VCE(sat))
• When Q1 is OFF→goes to cut-off,the
voltage across the iductor starts
collapses and its polarity gets reverse
biased.
• When VL attend after exponential
decrease when Q1 is ON, now gets
reversed.

3.3)Voltage inverter type switching regulator(Buck-
Boost)
• It produces an output voltage having
polarity opposite to that of the input
voltage.
• When Q1 is ON→goes to ON
(VCE=0.3V),due to this inductor voltage
suddenly rises to (Vin-VCE(sat)) and
magnetic field also expands. Hence D1
gets reverse biased.
• When Q1 is OFF→goes to OFF, due to
this inductor voltage reduces, hence D1
gets forward biased, so capacitor
charges which producing output voltage
of opposite polarity to that Vin.hence
the regulator is called voltage inverter
type.
• The repetitive on-off action of Q1
produces a repetitive charging and
discharging of the capacitor.

Overvoltage Protection
• Used to protect the circuit from the overvoltage using zener diodes.
• If there is a voltage surge, then the zener diode conducts the voltage across R1 to
trigger SCR ON.
• When SCR conducts, the drop across SCR is very small.
• The SCR current rating should exceed the maximum surge current excepted and
the circuit should be provided with the short circuit protection.

Reverse voltage protection
• Node A is +ve and D1 becomes
forward biased and Diode D2 is
reverse biased under normal
condition. Relay coil carries
relay current and closes the
relay contacts.
• If a negative spikes occurs at
the input, reverse polarity exist
then node A becomes
negative.
• Due to this diode D1 becomes
reverse biased and it opens the
circuit of relay coil.hence relay
current becomes zero and
relay contact becomes open.

Thermal shutdown
• Due to self heating it is possible to damage
the circuits permanently.
• It prevents the junction temperature to rise
above a safe limiting value(1750).
• Transistor Q1 and Q2 form a Darlington pair
which act as a series pass element.
• Diode DZ is biased into reverse biased
breakdown region with the help of current
source IQ.
• The voltage VZ produces a bias voltage for Q4
and Q5.
• Transistor Q5 is off during normal thermal
conditions.
• If temperature increases→VZ increases, hence
Q5 is turned ON, which reduces the Q1 ‘s
power dissipation.
• This continues till temperature drop below
the safe value.

Switch mode power supply(SMPS)
• It is used for A.C to D.C conversion.
• It provides the constant d.c voltage, under variable load and variable input conditions.
• The system is highly reliable, efficient, noiseless and compact because the switching is done at
very high rate ( 10 to 100 KHz )
• This receives 230V AC and translates it into different DC levels such as +5V, -5V, +12V, -12V.
• It Uses feedback mechanism
Chopper
controlle
r

• Transformer
• Inverts AC voltage up to down to the required output level.
• Rectifier
• AC output from transformer is rectified.
• The rectifier circuit can be configured as a voltage doublers by the addition of a
switch operated either manually or automatically.
• If the output transformer is very small with few windings at a frequency of 10 or
100kHz.
• The frequency is usually chosen to be above 20 kHz, to make it inaudible to
humans.
• If an input range switch is used, the rectifier stage is configured to operate as a
voltage doublers when operating on the low voltage (~120 VAC) range and as a
straight rectifier when operating on the high voltage (~240 VAC) range.
• It produces an unregulated DC voltage which is then sent to a large filter
capacitor/inductor.

⚫ Filter
⚫ filter consisting of inductors and capacitors, the rectified output is smoothed.
• Then it is sent to the output of the power supply.
• Feedback Network
• A feedback circuit monitors the output voltage and compares it with a reference
voltage, which is set manually or electronically to the desired output.
• If there is an error in the output voltage, the feedback circuit compensates by
adjusting the timing with which the MOSFETs are switched on and off
• A sample of this output is sent back as feedback signal for regulation.
– Switcher
– Based on the feed back signal, it controls the high frequency switching pulse width
and frequency to stabilize the output
• Chopper controller
• It may contain an isolation mechanism (such as opto-couplers) to isolate it from
the DC output.
• Switching supplies in computers, TVs and VCRs have these opto-couplers to tightly
control the output voltage

Power supply performance and testing
• For high quality and reliable operation, it is necessary to verify the power supply
performance by conducting the tests.
• Required Test Equipments
• Accurate voltmeter
• Accurate ammeter and wattmeter.
• Accurate multi-meter
• D.C power supply which is capable of supplying voltage and current for the test.
• Oscilloscope with BW of 500MHZ.
• Network analyzer.

Testing Procedure and Fault Detection
Step(i) First switch ON
• Apply 1V is applied to check hard short circuit and 10V for excessive current condition.
• If excessive current is detected, the power is switched off and testing is done.
Step(ii) Inrush Current Test
• This test is conducted if the power supply must be very low. while conducting this test all
capacitors are discharged and maximum specified load is applied.
Step(iii) Transient Recovery Time Test
• It is a dynamic measurement of the time required by the output voltage of a power supply to
settle within a predefined settling band following the transients induced by a load current.
• If there is abrupt change in the load ,the transients are induced in the output.

• Step(iv) Static Load Regulation Test
• It indicates the ability of a power supply under test
to remain within specified output limits for a
predefined load change.
• In this test the load current is changed from zero
to current rating of the power supply and the
change in the output voltage is measured.
• Step(v) Line Regulation Test
• In this test, the output voltage of the power supply
is measured with constant load and varying input
voltage to cover the entire range.
• Step(vi) Efficiency Test
• The ratio of the total output power to total input
power.
• Multiple readings are taken by varying the load
and curves are plotted.
• Step(vii) Overvolatge shutdown
• This test gives the ability of the power supply to
correctly respond to excess voltage
• Step(viii) Leakge current
• It is a.c to d.c current flowing between input and
output of an isolated power supply

• Step(ix) Periodic and Random Deviation (PARD)Test
• Use to measure presence of ripple and noise in a signal.
• Measurements are taken at low and high frequencies of a.c input.
• Proper shielding and impedance matching is required
• Step(x) Power factor
• It is the ratio of true power to apparent power consumed by the power supply
• Step(xi) Short circuit output current
• Steady state current of the power supply of the voltage at turn on.

20EC401-EC-II -COURSE MATERIAL - Course material ECE.pdf

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