POWER AMPLIFIER

A SEMINAR REPORT

BY

OGUNWALE IKEOLUWA REBECCA

SUBMITTED TO:

DEPARTMENT OF SCIENCE TECHNOLOGY,

SCHOOL OF SCIENCE AND COMPUTER STUDIES

THE FEDERAL POLYTECHNIC ADO-EKITI

IN PARTIAL FULFILLMENT OF THE AWARD OF

OCTOBER, 2022

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 ABSTRACT

Amplifier circuits form the basis of most electronic systems, many of which need to

produce high power to drive some output device. Basically there are three classes of

power amplifiers. Class A amplifiers are the most common form of power amplifier

but only have an efficiency rating of less than 40%.Class B amplifiers are more

efficient than Class A amplifiers at around 70% but produce high amounts of

distortion. The output of a class C amplifier is biased for operation at less than 180 of

the cycle and will operate only with a tuned (resonant) circuit, which provides a full

cycle of operation for the tuned or resonant frequency.

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1.0 INTRODUCTION

An Amplifier receives a signal from some pickup transducer or other input source and

provides a larger version of the signal to some output device or to another amplifier

stage. An input transducer signal is generally small and needs to be amplified

sufficiently to operate an output. In small signal amplifiers, the main factors are

usually amplification linearity and magnitude of gain, since signal voltage and current

are small in a small-signal amplifier, the amount of power-handling capacity and

power efficiency are of little concern (Aref et al., 2015). A voltage amplifier provides

voltage amplification primarily to increase the voltage of the input signal. Large-

signal or power amplifiers, on the other hand, primarily provide sufficient power to an

output load to drive a speaker or other power device. Presently, we concentrate on

those amplifier circuits used to handle large-voltage signals at moderate to high

current levels. The main features of a large-signal amplifier are the circuit's power

efficiency, the maximum amount of power that the circuit is capable of handling, and

the impedance matching to the output device. One method used to categorize

amplifiers is by class. Basically, amplifier classes represent the amount the output

signal varies over one cycle of operation for a full cycle of input signal. A brief

description of amplifier classes is provided in this paper (Jayamon et al., 2016).

1.1 POWER AMPLIFIERS

Power amplifiers are classified according to their mode of operation i.e. the portion of

the input cycle during which the collector current is expected to flow. On this basis the

power amplifiers are classified as:

1. Class A Power amplifier

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 2. Class B Power Amplifier

3. Class C Power Amplifier

1.2 PRINCIPLES OF POWER AMPLIFIERS

In general, the efficiency is an important parameter in order to obtain high gain at the

output of the amplifier and in power amplifiers circuits there are some factors that can

affect the performance of the amplifiers. These are collector efficiency, power

dissipation capability and distortion (Kang et al., 2017).

• Distortion: Distortion can be defined as a change in the shape of output waveform

while it is compared with input waveform. When the power amplifiers compare to the

voltage amplifiers, they can handle larger signals. Therefore, in the power amplifiers,

the distortion is an important problem that should be taken into consideration while

designing.

• Collector Efficiency: The collector efficiency is indicated the ratio of AC output

power to the DC input power of a power amplifier. It reveals us the percentage of DC

power converted to AC power by an amplifier. As a mathematical expression, it can

be defined as;

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• Power Dissipation Capability: In power amplifiers, transistors are used as a

switching device and they can carry large current during circuit operation. The large

current heats up the collector junction. The rises in temperature can affect the

operating conditions of transistor (Kim, 2018).

Figure 1: Types of power amplifiers

1.3 BASIC CONCEPT OF POWER AMPLIFIERS

1.3.1 Class A

The purpose of class A bias is to make the amplifier relatively free from distortion by

keeping the signal waveform out of the region between 0V and about 0.6V where the

transistor’s input characteristic is nonlinear. Class A design produces good linear

amplifiers, but are wasteful of power. The output power they produce is theoretically

50%, but practically only about 25 to 30%, compared with the DC power they

consume from the power supply.

1.3.2 Class B

A class B circuit provides an output signal varying over one-half input signal. There is

no standing bias current (the quiescent current is zero) and therefore the transistor

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conducts for only half of each cycle of the signal waveform. This dramatically

increases efficiency, compared with class A. Theoretically nearly 80% efficiency can

be achieved with this bias and in practical circuits, efficiencies of 50% to 60% are

possible.

1.3.3 Class C

The collector current flows for less than half-cycle of the input signal, it is called class

C power amplifier. In C power amplifier, the base is negatively biased, so that

collector current does not flow just when the

Positive half-cycle of the signal starts. Such amplifiers are never used for power

amplification but as tuned

Amplifier i.e. to amplify a narrow band of frequencies near the resonant frequency

1.4 BRIEF DESCRIPTION OF POWER AMPLIFIERS

1.4.1 Operation and Characteristics Curve of Class A

1.4.1.1 Series Fed Class A Amplifier

This simple fixed-bias circuit connection shown in Figure. 1 can be used to discuss the

main features of a class A series-fed amplifier. The beta of a power transistor is

generally less than 100, the overall amplifier circuit using power transistors that are

capable of handling large power or current while not providing much voltage gain.

Figure 2 is the characteristics curve of class A amplifier where the Q point is shown

which has been determined by fixed biasing the circuit (Park and Park, 2016).

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 Figure 2. Fixed Bias Class A amplifier

Figure 3. Characteristics Curve of Class A

1.4.1.2 Voltage Divider Biased Class A Amplifier

It is same as fixed biased class A amplifier but the change is here as shown in Figure 4

we are using voltage divider bias. The change in efficiency due to the change of

biasing state is very low and delivers small power outputs for a large drain on the DC

power supply. A Class A amplifier stage passes the same load current even when no

input signal is applied so large heat sinks are needed for the output transistors (Sim et

al., 2015).

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 Figure 4. Voltage divider Class A amplifier

1.4.1.3 Transformer Coupled Class A amplifier

To improve the full power efficiency of the Class A amplifier it is possible to design

the circuit with a transformer connected directly in the Collector circuit to form a

circuit called a Transformer Coupled Amplifier. As the Collector current, Ic is reduced

to below the quiescent Q-point set up by the base bias voltage, due to variations in the

base current, the magnetic flux in the transformer core collapses causing an induced

emf in the transformer primary windings (Son et al., 2009). This causes an

instantaneous collector voltage to rise to a value of twice the supply voltage 2Vcc

giving a maximum collector current of twice Ic when the collector voltage is at its

minimum. Then the efficiency of this type of Class A amplifier configuration can be

calculated as follows.

The R.M.S. Collector voltage is given as:

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The R.M.S. Collector current is given as:

The R.M.S Power delivered to the load (Pac) is therefore given as:

The average power drawn from the supply (Pdc) is given by:

Therefore the efficiency of a Transformer-coupled Class A amplifier is given as:

Figure 5 Transformer Coupled Class A amplifier

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1.4.1.4 Darlington Transistor Configuration

Another simple way to increase the current handling capacity of the circuit while at

the same time obtain a greater power gain is to replace the single output transistor

with a Darlington Transistor shown in Figure 6. These types of devices are basically

two transistors within a single package, one small “pilot” transistor and another larger

“switching” transistor. The big advantage of these devices are that the input

impedance is suitably large while the output impedance is relatively low, thereby

reducing the power loss and therefore the heat within the switching device. The

overall current gain Beta (β) or hfe value of a Darlington device is the product of the

two individual gains of the transistors multiplied together and very high β values

along with high Collector currents are possible compared to a single transistor circuit

(Son et al., 2019).

Figure 6 Darlington Transistor Class A amplifier

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1.4.2 Operation and Characteristics Curve of Class B amplifier

1.4.2.1 Push Pull Configuration of Class B

In class B push pull configuration circuit, two transistors are biased at the cutoff point.

The Class B configuration can provide better power output and has higher efficiency

(up to 78.5%). Since the transistor are biased at the cutoff point, they consumes no

power during idle condition and this adds to the efficiency. The advantages of Class B

push pull amplifiers are, ability to work in limited power supply conditions (due to the

higher efficiency), absence of even harmonics in the output, simple circuitry when

compared to the Class A configuration (Wu et al., 2018).

Figure 7 Push Pull Configuration of Class B

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 Figure 8 Characteristics Curve of Class B

1.4.2.2 Class B amplifier Pre biasing by diodes

The Class B amplifier circuit contained complimentary transistors for each half of the

waveform and while Class B amplifiers have a much high gain than the Class A types,

one of the main disadvantages of class B type push-pull amplifiers is that they suffer

from an effect known commonly as crossover distortion (Yoo et al., 2017). A simple

way to eliminate crossover distortion in a Class B amplifier is to add two small

voltage sources to the circuit to bias both the transistors at a point slightly above their

cut-off point. This then would give us what is commonly called a class AB amplifier

circuit.

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 Figure 9 Class B amplifier with pre biasing diodes

1.4.3 Operation and Characteristics Curve of Class C

Class C power amplifier is a type of amplifier where the active element (transistor)

conduct for less than one half cycle of the input signal. Less than one half cycle means

the conduction angle is less than 180° and its typical value is 80° to 120°. The reduced

conduction angle improves the efficiency to a great extend but causes a lot of

distortion. Theoretical maximum efficiency of a Class C amplifier is around 90%. In

this circuit, a RF tune resonant circuit is connected to the BJT. Mainly tuned resonant

circuit is used as it has radio frequency application. It ease the grasp of signal and

gives output fully without distortion thus highly efficient. The characteristics curve

shows how the circuit works at q point and what the changes in outputs and inputs are.

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 Figure 10 Class C Power Amplifier

Figure 11 Characteristics curve of Class C

1.5 CONCLUSION

In electronics, the amplifier is a term that is used to increase the amplitude of signal

and the amplifiers are divided into 3 types according to their frequency range, mode of

operation and driving output. In mode of operation there are several classes such as

class A, B, C, AB and etc. Some of these amplifiers are used to increase the sound

level. Therefore they are commonly called audio amplifier, some of them are used for
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wireless power transfer. There are some factors which should be into consideration to

design power amplifier. These factors are distortion, collector efficiency and power

dissipation capability which affects the performance of the power amplifiers. In this

paper, several power amplifier classes and their working principles have been

discussed and some parameters that affects the efficiency of power amplifiers have

been mentioned. Also some experiments of class AB amplifier based on different

circuit configuration have been conducted and the output waveforms for different

circuit topology have been obtained successfully.

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Kang S, Baek D, and Hong S. (2017). Power amplifier with a parallelcascoded

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