ECT 202: Analog Circuits Module V

MODULE V

Power amplifiers: Classification, Transformer coupled class A power amplifier, push pull class B and
class AB power amplifiers, complementary-symmetry class B and Class AB power amplifiers,
efficiency and distortion (no analysis required)

Regulated power supplies: Shunt voltage regulator, series voltage regulator, Short circuit protection
and fold back protection, Output current boosting.

POWER AMPLIFIERS

It is a large signal amplifier. It is the output stage in any amplifier system. The amplifier stage preceding
power amplifier stage is a small signal amplifier. This provides sufficient voltage level to the power
amplifier.

Power simplifier capable of handling large signal and deliver appreciable power to output
devices such as loud speakers, CRT, servo motor etc.

A transistor suitable for power amplification is called a power transistor. It has the following
three differences compared to an ordinary transistor.

(i) The base is made thicker to handle large currents. i.e., in power amplifiers, transistor with smaller β (=
IC/IB) are used.

(ii) The E-B pairs are heavily doped and hence it requires low input power.

(iii) The area of collector region of a power transistor is made considerably larger in order to dissipate the
heat developed in the transistor during operation. Metallic heat sinks are used so that heat is quickly
removed, thereby improving the heat dissipation.

The factors that limit the operation of power transistors are the collector dissipation and breakdown
voltages. Heat energy generated in the resistance of reverse biased collector base junction rises the junction
temperature. Maximum operating temperature Tj is about 1000C for Si- transistor. Higher temperature
destroys permanently the Si junction.
To dispose the heat generated in power transistors, heat sinks are used. Transistors are mounted
permanently on conducting metal plates. To ensure insulation, mica wafers are used between them. High
power transistors are often provided with radiating cooling fans of various shapes.
BC107 - Pd max = 300 mW
SL100 – Pd max = 500 mW

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ECT 202: Analog Circuits Module V

CLASSIFICATION OF POWER AMPLIFIERS

Classification according to mode of operation

According to collector current waveform, power amplifiers are classified as:

1. Class A
2. Class B
3. Class AB
4. Class C

In class A power amplifier, the transistor conducts for entire cycle of input signal. Output is exact replica
of input. Q point will be at the centre of DC load line. Since current flows during full cycle. Distortion is
very low. Class A operation has lowest efficiency. They are used as voltage amplifiers.
The conduction angle is 360°

In class B amplifier, conduction occurs from 0 to π (during one half cycle of input). Conduction angle is
180°. Q point is fixed near to cut-off region. Output is not a replica of input. Current is not continuous, and
hence distortion is high. As there is no supply of current during the non-conduction period, the efficiency
is high. They are usually used in the output stages of operational amplifiers.

In class AB amplifier, is an intermediate between class A and class B amplifiers. The conduction angle
is 180°-200°. Distortion is lesser than class B. Efficiency is better than class A but less that class B.

In class C amplifier, the Q point is selected beyond cut-off. Output current flows for less than one half
cycle of input signal. The result is periodically pulsating current waveform. Even though power conversion
efficiency is very high, the resulting waveform is distorted. Use of amplifier is restricted to radio frequency
amplification where tuned circuits are used as load.

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ECT 202: Analog Circuits Module V

Classification based on Use.

The amplifiers, according to use, may be classified as voltage, power, current or general purpose
amplifiers.

Voltage Amplifiers (small signal) Power Amplifiers (Large signal)

Produces a large increase in output power or an
1) Produces an increased voltage into a high appreciable amount of current flow into a
resistive load relatively low resistance load
Physical size of the transistor is large and is
2) Physical size of the transistor is small and is known as power transistors
known as low or medium-power transistor
Transistor having thick base to handle large
3) Transistor having thin base (β> 10) current. (β = 10)
Input resistance is generally very large as
4) Input resistance is required to be low as compared to its output resistance.
compared to its output resistance
Transformer coupling is preferred for impedance
5) RC coupling is preferred. matching.
Due to the harmonics, signal is likely to be
6) Distortion is not present. distorted.
Power dissipation capacity is more
7) Power dissipation capacity is less
Used as last stage in public address system and
8) Used as voltage amplifier other audio circuits.

Classification based on deriving output

Transistor amplifiers, according to the method of deriving output, are classified as single-ended,
double-ended or (push-pull) and complementary symmetry push-pull power amplifiers.

Single-ended power amplifier uses single transistor and derives output power w.r.t one end
permanently grounded.

Double ended or push-pull amplifier uses two transistors in a single stage. It consists of two loops
in which the transistor collector currents flow in opposite directions but add in the load.

Complementary symmetry push-pull amplifier uses two transistors having complementary

symmetry (one N-P-N and another P-N-P). They have symmetry as they are made with the same
material and technology and are of same maximum ratings.

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ECT 202: Analog Circuits Module V

CLASS A POWER AMPLIFIERS

1) SERIES-FED CLASS A AMPLIFIER

The term series is derived from the fact that the load RC is
connected in series with transistor output. This circuit is rarely
used, because of its poor collector efficiency.

The signal handled by this circuit has large voltage and

transistor used is a power transistor capable of operating in the
range of few watts.

When an input ac signal is applied to the

amplifier, the output will vary from its
dc bias operating voltage and current.

As the input signal is made larger, the

output will vary around the established
dc bias point until either the current or
the voltage reaches a limiting condition.
For the current this limiting condition is
either zero current at the low end or
VCC / RC at the high end of its swing. For
the collector–emitter voltage, the limit
is either 0 V or the supply voltage, VCC.

The efficiency of an amplifier represents the amount of ac power delivered (transferred) from the dc
source. The efficiency of the amplifier is calculated using

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ECT 202: Analog Circuits Module V

For the class A series-fed amplifier, the maximum efficiency can be determined using the maximum
voltage and current swings.

For the voltage swing it is

For the current swing it is

The ac power delivered to the load (RC) may be expressed using

𝑉𝑟𝑚𝑠 𝐼𝑟𝑚𝑠
Efficiency ɳ =
𝑉𝐶𝐶 𝐼𝐶𝑄

𝑉 𝐼
( 𝑚 )( 𝑚 )
√2 √2
=
𝑉𝐶𝐶 𝐼𝐶𝑄

𝑉𝐶𝐸(𝑝−𝑝) 𝐼𝐶(𝑝−𝑝)
( )( )
2√2 2√2
=
𝑉𝐶𝐶 𝐼𝐶𝑄

𝑉𝐶𝐸(𝑝−𝑝) 𝐼𝐶(𝑝−𝑝)
=
8 𝑉𝐶𝐶 𝐼𝐶𝑄

Using the maximum voltage swing

𝑉𝐶𝐶
𝑉𝐶𝐶1
𝑅𝐶
ɳ= 𝑉
= = 25%
8 𝑉𝐶𝐶 𝐶𝐶 4
2𝑅𝐶

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ECT 202: Analog Circuits Module V

2. TRANSFORMER-COUPLED CLASS A AMPLIFIER

Series fed Class A power amplifier has following limitations:

• Low conversion efficiency

• Since resistive load is used impedance matching is difficult
• DC power dissipation loss

To overcome these limitations, transformer coupled class A amplifier is used. It is also called as single
ended power amplifier, since single transistor is used.

In series fed class A power amplifier, the

quiescent current flows through collector
resistive load and causes large wastage
of DC power. So this doesn’t contribute
to useful ac output power. This can be
avoided by using a suitable transformer
for coupling the load to the amplifier.

This arrangement enhances the

conversion efficiency by a factor of 2 by
eliminating the DC power dissipation in
the load.

Since primary of transformer has

negligible dc resistance, there is
negligible dc power lost at this point.
The Ac power is however coupled
magnetically across the transformer into
the load.

R1,R2,RE provides bias stabilization. CE prevents negative feedback in the emitter circuit. The input
capacitor couples AC signal voltage to the base of the transistor but blocks any DC from previous
stages. Step-down transformer is provided to couple the high impedance collector circuits to low
impedance load.

Impedance Matching

The power transferred from the power amplifier to the load (say a loudspeaker) will be maximum only if
the amplifier output impedance equals the load impedance R L. This is in accordance with the maximum
power transfer theorem. Hence for transfer of maximum power from amplifier to the output device
matching of amplifier output impedance with the impedance of output device is necessary. This is
accomplished by using a step-down transformer of suitable turn ratio.

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ECT 202: Analog Circuits Module V

The transformer impedance matching is

shown separately in figure, where 𝑅𝐿′ is the
resistance looking into the primary of the
transformer and is given by

Thus the ratio of the transformer input and an output resistance varies directly as the square of the
transformer turn ratio.

Where a is ratio of primary to secondary turns of step-down transformer , RL is the resistance of load
connected across the transformer secondary and 𝑅𝐿′ is effective resistance looking into the transformer
primary.

For a step down transformer, the secondary voltage is less than the primary and high voltage side is
always high impedance side. Hence 𝑅𝐿′ is always higher than RL, for a step down transformer. Thus by
using a step down transformer of proper turns ratio, we can match a low RL with high impedance of
the transistor.

Circuit Operation& Efficiency

Since a transformer is used to couple the load, dc current does not flow through the load thereby
avoiding the wastage of dc power in the load. So entire dc voltage drop is across the transistor.

𝑉𝐶𝐸 = 𝑉𝐶𝐶 ; 𝑉𝐶𝐸(𝑝−𝑝) = 2𝑉𝐶𝐶

2𝑉𝐶𝐶 𝑉𝐶𝐶
𝐼𝐶(𝑝−𝑝) = 𝐼𝐶𝑄 =
𝑅𝐿′ 𝑅𝐿′

2𝑉𝐶𝐶
2𝑉𝐶𝐶
𝑉𝐶𝐸(𝑝−𝑝) 𝐼𝐶(𝑝−𝑝) 𝑅′𝐿 1
ɳ= = 𝑉 = = 50%
8 𝑉𝐶𝐶 𝐼𝐶𝑄 8 𝑉𝐶𝐶 𝐶𝐶 2
𝑅′𝐿

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ECT 202: Analog Circuits Module V

Drawbacks:

• Harmonic distortion is high

• To reduce the distortion we may have to design the amplifier with a load, then the ac power
output reduces.
• The dc current flowing through the primary causes core saturation and magnetic distortion.

CLASS B POWER AMPLIFIER

A power amplifier in which collector current flows only for one half of input signal or for 180° of input
signal is called a class B power amplifier.

In class B amplifier, the transistor is biased at cut off (base is shorted to emitter) ie., there is no power
dissipation when there is no input signal. This gives a class B amplifier, more efficiency than class A
amplifier.

V0 corresponds to the +ve half cycle of the input signal only and therefore the output signal is highly
distorted.

PUSHPULL AMPLIFIER

Push-pull amplifier is a type of electronic circuit that uses a pair of active devices that alternately
supply current to or absorb current from a connected load. This kind of amplifier can enhance the load
capacity and switching speed.

These amplifiers are called Push-pull amplifier because here one of the transistor pushes the current in
one direction where as the other transistor pulls current in another direction. In the push-pull amplifier,
one transistor works during the positive half cycle of the signal while the other works during the
negative half cycle.

CLASS B PUSH-PULL POWER AMPLIFIER

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ECT 202: Analog Circuits Module V

Circuitry for a class B push-pull amplifier is shown in figure. The bases of the two CE- connected
identical transistors Q1 and Q2 has been connected to the opposite ends of the secondary of the input
transformer Tr1. The emitter terminals of the transistors are connected to the centre tap of the
secondary of Tr1 which is grounded.

Supply Vcc is connected to the center tap of the primary of output transformer Tr2. This is done to
have balanced circuits. Zero bias is required for cut-off.

Circuit Operation

The input signal is fed through a centre tapped transformer in which the voltage appears in equal
magnitude and phase opposition across the secondary winding. The transistors Q1 and Q2 are driven
by these two signals.

During positive half cycle of input signal, Q1 will become forward biased and Q2 reverse biased.
Hence IC1 flows in transformer Tr2. Similarly during negative half cycle, Q2 become forward biased
and Q1 reverse biased. Hence IC2 flows in transformer Tr2.

Thus either of these transistors conducts to obtain an undistorted output across RL. The collector current
IC1 and IC2 flows in opposite direction across the primary of Tr2 and hence a bidirectional output is
obtained across RL. Joining of outputs of two transistors (i.e., Q1 and Q2) is never perfect and hence
the output is distorted. Since RC is zero, the quiescent voltage across the collector is VCC.

When no signal is present, both transistors remain off, and thus no current is drawn from dc supply
source VCC. Hence there is no dissipation of power with zero signal.

The load resistance (usually a loudspeaker) is connected across the secondary of the output
transformer. The turn-ratio N1: N2 of the transformer is chosen so that load resistance RL is matched
with the output impedance of the transformer. Under matched conditions, maximum power is delivered
to the load by the amplifier.

Conversion Efficiency of Class B Amplifier

The power provided to the load RL coupled to the amplifier is drawn from the dc power supply VCC
and is considered an input dc power.

The input dc power is given as

Where Idc is the average or direct current taken from the dc supply VCC. In class B operation, the current
drawn from a single power supply is a full-wave rectified signal, thus

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ECT 202: Analog Circuits Module V

AC power output,

Advantages

• It gives higher operating efficiency (theoretical 78.5%). It is primarily due to the fact that no
power is drawn by the circuit under zero signal condition.
• The use of push-pull system in the class-B amplifier eliminates even order harmonics in the
a.c. output signal.
• Because of the absence of even harmonics, the circuit gives more output, per device, for a given
amount of distortion.

Drawbacks

• Harmonic distortion is higher, self-bias cannot be used, and the supply voltages must have good
regulation.

Applications

• Widely employed for audio work in portable record-players, as stereo amplifiers and in high
fidelity radio receivers.

Cross Over Distortion

If the value of dc biased voltage is 0 both transistors are not ON and input signal voltage should be
larger than VBE before the transistor turns ON.

Due to this there is a time interval during the positive and negative half cycle when there is no transistor
operating as shown below.

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ECT 202: Analog Circuits Module V

The cross over distortion can be eliminated if the transistors are allowed to conduct during zero
crossing intervals. The resulting configuration is class AB operation. In class AB, a small stand by
current flows at zero excitation.

COMPLEMENTARY SYMMETRY PUSH-PULL CLASS B POWER AMPLIFIER

(TRANSFORMER-LESS)

Disadvantages of Push-pull amplifiers:

• Use of transformer at input and output makes circuits bulky.

• They require 2 out of phase signal, which makes the driver circuit complex

To overcome this complementary symmetry push-pull amplifiers were developed.

This arrangement uses two transistors having complementary symmetry (one N-P-N and another P-N-
P). The circuit is shown in figure.

During positive half cycle of input signal, base emitter voltage of Q1 becomes positive.
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ECT 202: Analog Circuits Module V

Transistor Q1 conducts and collector current flows from Vcc through RL as shown in figure.

During negative half cycle of input signal, transistor Q2 conducts and collector current flows from Vcc
through RL as shown in figure. Thus we get complete amplified waveform of input signal across RL.

Limitations:

• Two power supplies and two perfectly matched transistors are required.

These drawbacks are overcome by using below circuit.

Here transistors are configured as emitter follower which gives output signal of amplitude almost equal
to input signal level.

In the circuit Q1 conducts during positive half cycle and capacitor charges through RL from Vcc.
During negative half cycle Q2 conducts and emitter current is supplied from charge stored in the
capacitor.

Thus the value of capacitor is sufficiently large to act as dc source.

CLASS AB PUSH-PULL AMPLIFIER

Intermediate class between class A and class B, where transistor is biased between class A and class
B operation. So output current flows for more than half cycle.

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ECT 202: Analog Circuits Module V

Circuit uses two identical transistors. Emitter terminals of both transistors are connected together.
The input signal is applied to transistors through a center -tapped transformer which provides opposite
polarity signals to the two transistors. The collector terminals of both transistors are connected to end
terminals of center tapped primary of output transformer. The power supply Vcc is connected between
the emitter terminals and centre tap of primary of output transformer.

Here, the value of biasing resistors R1 and R2 are so selected that the transistors are biased just at the
cut-in voltage (Vr = 0.7 V). The voltage drop across R2 is adjusted to be approximately equal to Vr

This reduces the time for which both transistors are simultaneously OFF (the time for the input signal
is between -0.7 V and + 0.7 V) and so the cross over distortion gets reduced.

The load resistor RL is connected across the secondary of the output transformer. The turns ratio of
output transformer is chosen so as to match the load with output impedance of amplifier and
therefore transfer maximum power.
The quiescent current of the two transistors, which are equal in magnitude, flow in opposite
directions.

Class AB configuration has reduced efficiency and wastes a reasonable amount of power during zero
input condition. Used as AF power amplifiers.

COMPLEMENTARY SYMMETRY CLASS AB PUSH-PULL AMPLIFIER

Circuit is similar to complementary symmetry class B amplifier. Only difference is that the transistors
are biased just above cut-off.

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ECT 202: Analog Circuits Module V

REGULATED POWER SUPPLY

The block diagram of regulated power supply is given.

Power transformer (step-down) : 220V,50Hz AC signal is converted to AC of required level and it is

given to a rectifier.

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ECT 202: Analog Circuits Module V

Rectifier: Converts AC input to pulsating DC. This passes through the filter.

Filter: removes ripple components in the rectified output.

Regulator: used to give a constant DC output irrespective of load variations , input variations and
temperature variations

VOLTAGE REGULATORS

Maintain DC output constant irrespective of variation in load current, input voltage and temperature
variation

𝜕𝑉0 𝜕𝑉0 𝜕𝑉0

∆𝑉0 = ∆𝑉𝑖 + ∆𝐼𝐿 + ∆𝑇 = 𝑆𝑣 ∆𝑉𝑖 + 𝑅0 ∆𝐼𝐿 + 𝑆𝑇 ∆𝑇
𝜕𝑉𝑖 𝜕𝑉𝐿 𝜕𝑇
𝜕𝑉 ∆𝑉
Where 𝑆𝑣 = 𝜕𝑉0 = ∆𝑉0 |∆𝐼𝐿 = 0, ∆𝑇 = 0 → 𝑖𝑛𝑝𝑢𝑡 𝑟𝑒𝑔𝑢𝑙𝑎𝑡𝑖𝑜𝑛 𝑓𝑎𝑐𝑡𝑜𝑟
𝑖 𝑖

𝜕𝑉0 ∆𝑉0
𝑅0 = = |∆𝑉𝑖 = 0, ∆𝑇 = 0 → 𝑜𝑢𝑡𝑝𝑢𝑡 𝑟𝑒𝑠𝑖𝑠𝑡𝑎𝑛𝑐𝑒
𝜕𝐼𝐿 ∆𝐼𝐿

𝜕𝑉0 ∆𝑉0
𝑆𝑇 = = |∆𝐼𝐿 = 0, ∆𝑉𝑖 = 0 → 𝑡𝑒𝑚𝑝𝑒𝑎𝑟𝑎𝑡𝑢𝑟𝑒 𝑐𝑜𝑒𝑓𝑓𝑖𝑐𝑖𝑒𝑛𝑡
𝜕𝑇 ∆𝑇
For better regulator the performance measures 𝑆𝑣 , 𝑅0 &𝑆𝑇 should have very low values.

Classification of voltage regulators

They can be classified as linear and switching regulators.

• Linear voltage regulator

Active control element is continuously ON whose impedance is varied to achieve
required regulation.
• Switching voltage regulator
Active control element turned ON & OFF depending on variations in output to achieve
required regulations.

They can be classified as series and shunt regulators.

• Series voltage regulator

➢ Variable impedance device is connected in series with load and impedance is adjusted
to variation of output voltage
➢ Complex circuit & high cost
➢ Used for variable load applications
➢ Suitable for high voltage and low current application
➢ Additional circuits is used to protect against short circuit or overload.
➢ High efficiency and less power dissipation
• Shunt regulator

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ECT 202: Analog Circuits Module V

➢ Control element is in shunt with load. Depending on the variation in input current,
current through control element gets varied to maintain constant current flow through
load.
➢ Simple circuit and low cost
➢ Used for fixed load and fixed voltage applications
➢ Suitable for low voltage and high current applications.
➢ No need of additional protection circuit
➢ Low efficiency and high-power dissipation

SHUNT VOLTAGE REGULATOR

ZENER SHUNT REGULATOR

Here the Zener diode is connected in

shunt with load. The break down
property of Zener diode is utilized.

When breakdown occurs voltage across

the device remains almost constant with
rapid rise in current.

When input voltage increases, total current I increase.

𝐼 = 𝐼𝑍 + 𝐼𝐿

When input voltage increases, Zener undergoes break down and there is an increase in current through
it so 𝐼𝐿 is made to remain constant so output voltage 𝑉0 = 𝐼𝐿 𝑅𝐿 also remains constant.

Design:

When input voltage is minimum and load current is maximum, the current through the Zener should
be sufficient to operate it in its breakdown voltage.

𝑉𝑖(𝑚𝑖𝑛)−𝑉0
𝑅𝑆𝑚𝑎𝑥 =
𝐼𝑍(𝑚𝑖𝑛)+𝐼𝐿𝑚𝑎𝑥
Similarly when input voltage is maximum and load current is minimum, the current through Zener
should not exceed its maximum power dissipation capacity

𝑉𝑖(𝑚𝑎𝑥) − 𝑉0
𝑅𝑆𝑚𝑖𝑛 =
𝐼𝑍(𝑚𝑎𝑥)+𝐼𝐿𝑚𝑖𝑛

Then 𝑅𝑆𝑚𝑖𝑛 < 𝑅𝑆 < 𝑅𝑆𝑚𝑎𝑥

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ECT 202: Analog Circuits Module V

Line Regulation & Load Regulation

Line Regulation is the ability of power supply to maintain a constant output voltage even when input
voltage changes, with the output current drawn from the power supply remaining constant.

∆𝑽𝟎
𝐋𝐢𝐧𝐞 𝐑𝐞𝐠𝐮𝐥𝐚𝐭𝐢𝐨𝐧 = ∗ 𝟏𝟎𝟎%
∆𝑽𝒊

It is desirable for a power supply to maintain stable output regardless of changes in the input voltage.
The line regulation is important when the input voltage is unstable or unregulated and this would result
in significant variations in the output voltage.

A low line regulation is always preferred.

Load Regulation is the capability to maintain a constant voltage or current level on the output channel
of a power supply despite changes in the supply’s load.

As load varies, load current changes thus output voltage changes. This change is independent of input
signal. Load regulation is defined as percentage change in output voltage for a given change in load
current or load resistance. Load regulation is also expressed as a percentage change in output voltage
from no load to full load.
(𝑽𝑵𝑳 − 𝑽𝑭𝑳 )
𝒍𝒐𝒂𝒅 𝒓𝒆𝒈𝒖𝒍𝒂𝒕𝒊𝒐𝒏 = × 𝟏𝟎𝟎%
𝑽𝑭𝑳
TRANSISTOR SHUNT REGULATOR

It is an improvement over Zener

regulator in that it improves
regulation and increases the
maximum load current capacity.

When input voltage increases, the

output voltage also increases

𝑉0 = 𝑉𝑍 + 𝑉𝑅𝐵

Since 𝑉𝑍 remains constant 𝑉𝑅𝐵

increases or base emitter voltage of transistor 𝑉𝐵𝐸 increases. This increases base current and collector
current.

The increase in total current is divided between the transistor and the Zener, which maintains a constant
load current through the load. Thus output voltage is regulated.

Here the increased current gets two path to flow, thereby maintaining load current constant.

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ECT 202: Analog Circuits Module V

Design Equations:

𝑉𝑖(𝑚𝑖𝑛)−𝑉0
𝑅𝑆𝑚𝑎𝑥 =
𝐼𝑍(𝑚𝑖𝑛) + 𝐼𝐿𝑚𝑎𝑥 + 𝐼𝐶
𝑉𝑖(𝑚𝑎𝑥) − 𝑉0
𝑅𝑆𝑚𝑖𝑛 =
𝐼𝑍(𝑚𝑎𝑥) + 𝐼𝐿𝑚𝑖𝑛 + 𝐼𝐶

𝑅𝑆𝑚𝑖𝑛 < 𝑅𝑆 < 𝑅𝑆𝑚𝑎𝑥

𝑉0 − 𝑉𝑍
𝑅𝐵 =
𝐼𝑍 − 𝐼𝐵
SERIES REGULATORS

In series regulators, the regulating device is connected between load and unregulated power supply.
As input voltage or current varies, the series impedance varies & the voltage drop changes so as to
maintain constant voltage drop across the load.

1. EMITTER FOLLOWER REGULATOR

Here transistor is used as a series-pass element

whose impedance is varied according to the
output variation or load variation.

𝑉0 = 𝑉𝑍 + 𝑉𝐸𝐵 = 𝑉𝑍 − 𝑉𝐵𝐸
When input voltage increases, output voltage
increases. Increase in 𝑉0 decreases 𝑉𝐵𝐸 since 𝑉𝑍 is almost constant. The decrease in base voltage
of the transistor gets reflected at the output since its an emitter follower. Hence the output reduces
proportionally.

Design of 𝑅𝐵

𝑉𝑖 − 𝑉𝑍
𝑅𝐵 =
𝐼𝑍 + 𝐼𝐵

2.TRANSISTOR SERIES VOLTAGE REGULATOR

This is an improvement over emitter follower regulator.

The circuit consists of an error amplifier Q2, a series pass transistor Q1, a regulating
element and a potential divider network.
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ECT 202: Analog Circuits Module V

When input voltage, output voltage V0 increases, then VR2 increases. Then VBE2 increases, which mean
base current of Q2 increases, increasing its collector current.

When IC2 increases, base current of Q1 decreases, then voltage at base of Q1 reduces. As a result Q1
conducts less and current flowing to output also reduces, which causes reduction in output voltage.

Expression for output voltage

𝑉0 = 𝑉𝑅1 + 𝑉𝑅2

𝑉𝑅2 = 𝑉𝐵𝐸2 + 𝑉𝑍
𝑅1
then 𝑉0 = 𝑉𝑅1 + 𝑉𝐵𝐸2 + 𝑉𝑍 = 𝑉0 + 𝑉𝐵𝐸2 + 𝑉𝑍
𝑅1 +𝑅2

𝑅1
then 𝑉0 (1 − ) = 𝑉𝐵𝐸2 + 𝑉𝑍
𝑅1 +𝑅2

(𝑉𝐵𝐸2 +𝑉𝑍 )(𝑅1 +𝑅2 ) 𝑅

𝑉0 = = (𝑉𝐵𝐸2 + 𝑉𝑍 ) (1 + 𝑅1)
𝑅2 2

Thus by choosing R1 and R2, we can regulate output voltage at desired value.

SHORT CIRCUIT PROTECTION

This is a protection mechanism

used in series voltage regulators.

When short circuit occurs at

output, load current increases
which causes damage to transistor
Q1. To protect this, additional
diodes are provided at base of Q1
and a resistor Rsc connected to
emitter terminal.

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ECT 202: Analog Circuits Module V

As long as load current is with its normal value, Rsc value is selected such that drop across Rsc is less
than drop across diode. When this drop is less than 0.7V diodes cannot conduct and current works
under normal condition without interference of diode.

Under short circuit or over load condition, load current increases, drop across Rsc increases, drop
across diode increases so diodes starts conducting.

In circuit, IR3 = IB1 + IC2

Once diode starts conducting, the increased current IR3 flows mainly through diodes, thereby reducing
the base current IB1. Thus the power transistor Q1 gets protected from short circuit.

CURRENT LIMITING CIRCUIT

This is also a short circuit protection technique. Here instead of diodes, transistor is used.

If the load resistance RL is reduced or load terminals are shorted accidently, a very large load current
will flow. It may destroy the pass transistor Q1, diode or some other component. To avoid this situation
this protection circuit is used.

With normal load current Q3 remains off because the voltage drop across resistor R4 is small (less
than 0.7V). When load current increases, IR4 increases, transistor Q3 gets turned ON. The increased
current through R3 will be flowing mainly through Q3, which reduces current to base of Q1 thus
protecting it.

FOLD BACK PROTECTION

The above two circuits have some limitation:

• These is large power dissipation across pass transistor.

The protection circuit consists of a transistor, voltage divider network and a resistor R4.

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ECT 202: Analog Circuits Module V

When load current is within the normal

value, the limiting circuit has no effect on
the operation of circuit.

When over current flows, emitter current of

Q1 increases, VR4 increases which is
connected to emitter of Q3, so Q3 gets
turned on. As a result, the increased current
IR3 flows mainly into Q3, so base current of
Q1 reduces and IC1 also reduces thereby
protects the pass transistor, Q1.

Let the voltage at point A be VA and current

flowing through R4 is IL

VA = ILR4 + V0 --- (1)

Neglecting current through base of Q3, current through voltage divider network can be written as

𝑉𝐴
𝐼= --- (2)
𝑅5 +𝑅6

𝑉𝐴 𝑅6 (𝐼𝐿 𝑅4 +𝑉0 )𝑅6

Base voltage at Q3 : 𝑉𝐵3 = = = 𝑘 (𝐼𝐿 𝑅4 + 𝑉0 ) ---(3)
𝑅5 +𝑅6 𝑅5 +𝑅6

𝑅6
Where 𝑘 =
𝑅5 +𝑅6

The emitter voltage of Q3 is V0. Then the B-E voltage of Q3 is

𝑉𝐵𝐸3 = 𝑉𝐵3 − 𝑉𝐸3 = 𝑘(𝐼𝐿 𝑅4 + 𝑉0 ) − 𝑉0 ----(4)

𝑉𝐵𝐸3 −𝑉0 (𝑘−1)
Then 𝑘 𝐼𝐿 𝑅4 = 𝑉𝐵𝐸3 − 𝑉0 (𝑘 − 1) or 𝐼𝐿 = ---(5)
𝑘𝑅4

When output terminals are shorted the output voltage is zero.

𝑉𝐵𝐸3
𝐼𝐿 = 𝐼𝑠𝑐 = --- (6)
𝑘𝑅4

Then (5) becomes

𝑉0 (𝑘−1)
𝐼𝐿 = 𝐼𝑠𝑐 −
𝑘𝑅4

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