Unit V

POWER AMPLIFIERS
AND DC CONVERTERS
Introduction
• A power amplifier (or) large signal amplifier, develops relatively large
output voltages across low impedance loads. Audio amplifiers are typical
large signal amplifiers which supply ac output power to low impedance
speakers..
• An important function of the output stage is to provide the amplifier with
a low output resistance so that it can deliver the output signal to the load
without loss of gain.
• Since the output stage is the final stage of the amplifier, it usually deals
with relatively large signals.
• The most challenging requirement in the design of the output stage is that
it deliver the required amount of power to the load in an efficient manner.
CLASSIFICATION OF OUTPUT STAGES
• Output stages are classified according to the collector current
waveform that results when an input signal is applied.
Class A Amplifier:
• The class A stage is biased at a current I greater than the amplitude of
signal current.
• Thus the transistor in a class A stage conducts for the entire cycle of
the input signal; that is the conduction angle is 360°.
 Contd..
• Generally amplifiers perform at their best if they fall in the linear class,
such as class A.
• Their output is normally distortion free. However their performance
suffers when operated in the large signal region.
• As long as one operates in the linear region where V c and VI are linearly
related, one
• obtains high output power but poor power efficiency (30%).
Class B Power amplifier:
• A Transistor in class B stage conducts for only half the cycle of the input sine wave
resulting in a conduction angle of 1800.
Class AB Power amplifier:
• An intermediate class between A and B appropriately named class AB, involves biasing
the transistor at a non-zero dc current much smaller than the peak current of the sine
wave signal.
• As a result, the transistor conducts for an interval slightly greater than half a cycle
• The resulting conduction angle is greater than 180° but much less than 360º.
• The class AB stage has another transistor that conducts for an interval slightly greater
than
• that of the negative half cycle and the currents from the two transistors are combined in
the load.
• It follows that, during the intervals near the zero crossings of the input sinusoid, both
transistors conduct.
• Class B and class C amplifiers have power efficiencies over 78% but are also limited by
linear mode operation.
Class C Amplifier:
• The transistor conducts for an interval shorter than that of a half cycle
that is conduction angle is 180º.
• The result is the periodically pulsating current waveform
• To obtain a sinusoidal output voltage, this current is passed through a
parallel LC tuned circuit, tuned to the frequency of the input sinusoid.
• The tuned circuit acts as a band pass filter and provides an output
voltage proportional to the amplitude of fundamental component in the
Fourier series representation of current waveform.
• Class A, AB, B and C amplifiers are employed as output stages of op-
amp's and audio power amplifiers.
• In audio power amplifiers, class C amplifiers are usually employed for
radio frequency (RF) power amplification (required example in mobile
phones and radio and TV transmitters).
Class A Class B
Class AB Class C
 Class A Power Amplifier

An amplifier that conducts during the full cycle, or has a conducting angle of
360 degrees is known as a Class A power amplifier. It is the simplest and
most common type of power amplifier, because of low signal distortion
levels.
• Transistor is so biased that the output current flows for the entire cycle of the
input signal. Thus the operating point is so selected that the transistor operates
only over the linear region of its load line. So such an amplifier can amplify input
signal of small amplitude.
• As the transistor operates over the linear portion of load line, the output
waveform is exactly similar to input waveform. So class A amplifiers are
characterized by a high fidelity of the output. Such amplifiers are used where
freedom from distortion is prime aim.
• Operation is restricted only over a small central region of the load line so such
amplifiers can be used for amplifying signals of small amplitude. Also ac power
output per transistor is small.
• The maximum possible overall efficiency with resistive load is 25%. The maximum
possible collector efficiency with resistive load is 50%. In case an output
transformer is used, both of these efficiencies are 50%.
Class A Power Efficiency
 Class B Power Amplifier

Class B Power Amplifiers, unlike Class A, work for only half of each input cycle, which
means they have a conducting angle of 180 degrees. In simple words, these amplifiers
amplify only half of the input cycle.
• The transistor bias and signal amplitude are such that output current flows only during positive half
cycle of the input signal.

• At zero signals, the collector current is zero and no biasing system is required in class B amplifiers.

• The operating point is selected at collector cut-off voltage; Because of total absence of negative half
cycle from the output the signal distortion is high.

• Zero signal input represents the best condition for class B amplifiers because of zero collectors
current.

• The transistor dissipates more power with increase in signal strength.

• In comparison to class A amplifiers average current is less, power dissipation is less. So overall
efficiency is increased. The theoretical efficiency in class B operation is about 78.5% while it is only
50% in class A operation.
Class B Power Efficiency
Class B: Crossover Distortion
 Class AB Power Amplifier

Class AB amplifiers are a combination of class A and class B amplifiers. This class of
amplifiers are designed to reduce the less efficiency problem of class A amplifiers and
distortion of signal at crossover region in class B amplifiers.
• Each transistor conducts for more than a half cycle, they conduct less than a full
cycle completely. So the conduction angle is somewhere around 180 degrees and
360 degrees, commonly shown as 270 degrees.
• In class AB power amplifiers, the biasing circuit is so adjusted that the
operating point Q lies near the cut-off voltage.
• During a small portion of negative half cycle and for complete positive
half cycle of the signal, the input circuit remains forward biased–and
hence collector current flows.
• But during a small portion (less than half cycle) of the negative cycle”‘
the input circuit is reverse biased and, therefore, no collector current
flows during this period.
• Class AB operation needs a push-pull connection to achieve a full
output cycle.
Transformer coupled Class A Power amplifier
• These amplifiers are known as RC coupled power amplifiers. It is
known that efficiency of RC amplifiers is very poor.
• In class A, efficiency is 25%. To improve efficiency, the transformer
coupled class A power amplifier is preferred.
• In these amplifiers, a transformer is used to couple AC power to the
load.
• By adjusting the turn ratio of the primary windings to the secondary
windings, one can match the source and load impedance for
maximum power transfer.
• This makes transformer coupled power amplifiers more efficient as
compared to RC coupled power amplifiers, as maximum power
transfer can take place.
Characteristics of power MOSFET
Temperature Effect of Power
MOSFET
• Value of VGS in the range of 4V to 6V for most power MOSFETs at
which the temperature coefficient of iD is zero.
• Higher values of vGS , iD exhibits a negative temperature coefficient.
This is significant property.
• It implies that a MOSFET operating beyond the zero temperature
coefficient point does not suffer from the possibility of thermal
runaway.
• As an application of power MOSFETS, a class AB output stage utilising
a pair of complementary MOSFETS and employing BJTs for biasing and
in the driver stage shown in below.
DC to DC Converters
• It is an electronic circuit which converts dc from one voltage level to
other.
• Normally dc to dc converters use the switching techniques to convert
the pulsating dc to smooth & constant dc. This is called Switched
Mode power supply.
• The power received from main is rectified and filtered as high voltage
d.c. It is then switched at a high rate of speed approximately 15kHz to
50kHz and fed to the primary side of a step down transformer
• The conversion is achieved by storing the input energy temporarily
and then releasing that energy to output at difference voltage
• The step down transformer is only a fraction of the size of a
comparable 50Hz units thus relieving the size and weight problems
• The output at the secondary side of the transformer is rectified and
filtered. Then it is sent to the output of power supply
• A sample of this output is sent back to the switch to control the output
voltage
Switching Power Supply
Switching Power supply waveforms
Switched Mode Converter
Buck Converters or step down switching regulator
Boost or step up switching regulator
Buck Boost or voltage inverter type
Different types of SMPS (or) DC to
DC converters
• Forward converter
• Flyback converter
• Push pull DC to DC converter
Forward converter
• The choke carries the current both when the transistor is conducting
as well as it is not .
• The diode carries the current during the OFF period of the transistor.
Therefore energy flows into load during both ON and OFF periods
• The choke stores energy during ON period and also passes some
energy into the output load.
• The diode serves two functions:
Flyback converter
• The energy stored entirely in the form of magnetic flux of the inductor
during ON period of the switch
• The energy is discharged into the output circuit when the switch is in
open circuit
• The output voltage depends on the duty cycle
Transformer coupled Push pull DC to DC converter