LITERATURE SURVEY ON ELECTRONIC AMPLIFIER

CHAPTER ONE

INTRODUCTION

1.1 GENERAL INTRODUCTION

Amplifier is an electronic device that increase the power of the signal, the amplification of weak signal into stronger signal it is of fundamental importance in almost any electronic system. Sometimes this is obvious, as in a radio receiver, where the objective is to select and amplify the weak signal from the antenna and to present the detected signal via loudspeaker. An amplifier can either be a piece of equipment or an electrical circuit contain with another device. Amplification is fundamental to modern electronics and they are widely used in almost all electronic equipment. According to the history, the first practical device that could amplify was the triode vacuum tube, which invented in 1906 by lee de forest which lead the first amplifier around the world in 1912. vacuum tubes were used in almost all amplifiers until the 1960s-1970s when transistors is introduced at 1947 and vacuum was replaced by the transistor. Nowadays almost all of amplifiers was used by transistor, but vacuum tubes continue to be use in some application, the development of audio communication in form of telephone was first invented in 1876, created the need to increase the amplitude of electrical signals to extend the transmission of signals to a long distances. The shreeve mechanical repeater and the vacuum tube were the only amplifying devices, other than specialized power devices such as the magnetic amplifier and amplidyne, for 40 years. Power control circuitry used magnetic amplifiers until the latter half of the twentieth century when power semiconductor devices became more economical with higher operating speeds. Shreeve repeaters were used as adjustable amplifiers in telephone subscriber sets for the hearing impaired until the transistor provided smaller and higher quality amplifiers in the 1950s. The replacement of bulky electron tubes with transistors during the 1960s and 1970s created another revolution in electronics, making possible a large class of portable electronic devices, such as the transistor radio developed in 1954. Today, use of vacuum tubes is limited for some high power applications, such as radio transmitters.

The terms amplifier and amplification, derived from the Latin amplificare, (to enlarge or expand), were first used for this new capability around 1915 when triodes became widespread.

After many centuries it was found that negative resistance mercy lamp could amplify and were also tired in repeater, the actual development of thermionic valves starting around ca.1902, which provided an entirely electronic method of amplifying signals. The first practical version of such devices was the audio triode, invented in 1906 by lee De forest which lead to the first amplifier around 1912. in analogy to previous types of relays in telegraphy and telephony, the amplifying vacuum tube was first called an electron relays. In the first extensive commercial use of vacuum tube, such as repeaters powered the first transcontinental telephone line for commercial services in 1915, much of the mathematical theory of amplifiers was developed at BELL TELEPHONE LABORATORIES during 1920s to 1940s.

Beginning of 1970s more and more transistors were connected on a single chip by creating higher scales of integration (small-scale, medium-scale, large-scale). Many commercially available today are base on integrated circuits. In the early 20 ^th century, advance digital electronic were introduce such as D-amplifier.

1.2 AIM AND OBJECTION

1.2.1 AIM

The aim of this work is to design and study of the feedback amplifiers.

1.2.2 OBJECTIVES

The objectives of this work are:

1. To study how amplifiers work.
2. To design and study the feedback amplifiers.
3. To know the application of the feedback amplifiers.
4. To know the types and uses of feedback amplifiers.

1.3 SCOPE AND LIMITATION

1.3.1 SCOPE

This literature was based on the study of the working principle of the feedback amplifiers.

1.3.2 LIMITATION

This literature was limited to study the various types, classification, application of the feedback amplifiers.

1.4 MOTIVATION

The most important part of this work is to see how actually amplifier is, how they work and how is possible to transmit audio from low frequency to a higher frequency using electronic component.

CHAPTER TWO

FEEDBACK AMPLIFIERS

2.1 INTRODUCTION

An amplifier is an electronic device that can amplify sound from low frequency response to the high frequency response, an amplifier can either be a separate piece of equipment or an electronic circuit contain within another device. Amplification is a fundamental to modern electronic and amplifier are widely used in almost all electronic devices.

Amplifier can be categorized in different ways, one is by the frequency of electronic signal being amplified. For example, audio amplifier, amplify signals. In the audio (sound) range of less than 20KHZ, RF amplifiers amplify frequencies in the radio frequency range between 20KHZ and 300GHZ and servo amplifiers and instrumental amplifiers may work with very low frequency down to direct current. It can also be categorized by their physical placement in the signal chain.

Feedback amplifiers is one of the fundamental process in electronic also. It defined as process whereby a portion of the output signal is fed to the input signal in order to form a part of the system-output concentration.

Feedback is used in operating point of transistors insensitive to the both manufacture variation as well as temperature. there is another types of feedback called positive or regenerative feedback in which the overall gain is increased, positive feedback is useful in oscillators and while establishing the two stable state of flip-flop.

2.2 TYPES OF FEEDBACK AMPLIFIER

feedback amplifier are classified into four(4) groups :-

1- voltage-shunt feedback.

In voltage-shunt feedback the feedback consist of single resistance R_F. Voltage developed across and feedbackR_L. It sample to input. Through R_F. The shunt connection at input and output terminal reduces input and output impedance. The amplifier works as trans-resistance type voltage amplifier. (2.1)

2- voltage-series feedback.

In voltage-series feedback, the voltage across the load resistance is sampled and feedback to input through resistance R_F and R_E (potential divider). Sample voltage is proportional to output voltage and feedback series with the input voltage.

Series connection at input is increases input resistance and shunt connection at output reduces output resistance. The resulting amplifier is true voltage amplifier.

Thus

(2.2)

(2.3)

so (2.4)

3- current-shunt feedback

In current-shunt feedback, the feedback signal is proportional to the output current and feed to input in shunt. The series connection at input, decrease input resistance and the amplifier works as true current amplifier.

Thus (2.5)

4- current-series feedback

In current-series feedback, the signal is proportional to the load current and fed to the output through a resistance R_E in series with the input signal. The series connection at the input and the output increase the input and output impedance. The amplifier circuit works as trans-conductance type amplifier.

Thus,

(2.6)

(2.7)

(2.8)

(2.9)

2.3 CLASSIFICATION OF FEEDBACK AMPLIFIER

Amplifier are classified into several classes, they can be classified into: Depending on the portion of wave amplified, these are classified into four (4) classes. They are

Class A AMPLIFIER

Class B AMPLIFIER

Class AB AMPLIFIER

Class C AMPLIFIER

2.3.1 CLASS A AMPLIFIER

Class A Amplifiers are the most common type of amplifier class due to their simple design. Class A, literally means “the best class” of amplifier due mainly to their low signal distortion levels and are probably the best sounding of all the amplifier classes mentioned here. The class A amplifier has the highest linearity over the other amplifier classes and as such operates in the linear portion of the characteristics curve.

Generally class A amplifiers use the same single transistor (Bipolar, FET, IGBT, etc) connected in a common emitter configuration for both halves of the waveform with the transistor always having current flowing through it, even if it has no base signal. This means that the output stage whether using a Bipolar, MOSFET or IGBT device, is never driven fully into its cut-off or saturation regions but instead has a base biasing Q-point in the middle of its load line, Then the transistor never turns “OFF” which is one of its main disadvantages.

fig(2.1) Class A Amplifier

To achieve high linearity and gain, the output stage of a class A amplifier is biased “ON” (conducting) all the time. Then for an amplifier to be classified as “Class A” the zero signal idle current in the output stage must be equal to or greater than the maximum load current (usually a loudspeaker) required to produce the largest output signal. As a class A amplifier operates in the linear portion of its characteristic curves, the single output device conducts through a full 360 degrees of the output waveform. Then the class A amplifier is equivalent to a current source.

Since a class A amplifier operates in the linear region, the transistors base (or gate) DC biasing voltage should by chosen properly to ensure correct operation and low distortion. However, as the output device is “ON” at all times, it is constantly carrying current, which represents a continuous loss of power in the amplifier.

Due to this continuous loss of power class A amplifiers create tremendous amounts of heat adding to their very low efficiency at around 30%, making them impractical for high-power amplifications. Also due to the high idling current of the amplifier, the power supply must be sized accordingly and be well filtered to avoid any amplifier hum and noise. Therefore, due to the low efficiency and over heating problems of Class A amplifiers, more efficient amplifier classes have been developed.

2..3.2 CLASS B AMPLIFIER

Class B amplifiers were invented as a solution to the efficiency and heating problems associated with the previous class A amplifier. The basic class B amplifier uses two complimentary transistors either bipolar of FET for each half of the waveform with its output stage configured in a “push-pull” type arrangement, so that each transistor device amplifies only half of the output waveform.

In the class B amplifier, there is no DC base bias current as its quiescent current is zero, so that the dc power is small and therefore its efficiency is much higher than that of the class A amplifier. However, the price paid for the improvement in the efficiency is in the linearity of the switching device.

fig(2.2) CLASS B AMPLIFIER.

When the input signal goes positive, the positive biased transistor conducts while the negative transistor is switched “OFF”. Likewise, when the input signal goes negative, the positive transistor switches “OFF” while the negative biased transistor turns “ON” and conducts the negative portion of the signal. Thus the transistor conducts only half of the time, either on positive or negative half cycle of the input signal.

Then we can see that each transistor device of the class B amplifier only conducts through one half or 180 degrees of the output waveform in strict time alternation, but as the output stage has devices for both halves of the signal waveform the two halves are combined together to produce the full linear output waveform.

This push-pull design of amplifier is obviously more efficient than Class A, at about 50%, but the problem with the class B amplifier design is that it can create distortion at the zero-crossing point of the waveform due to the transistors dead band of input base voltages from -0.7V to +0.7.

We remember from the transistors that it takes a base-emitter voltage of about 0.7 volts to get a bipolar transistor to start conducting. Then in a class B amplifier, the output transistor is not “biased” to an “ON” state of operation until this voltage is exceeded. This means that the the part of the waveform which falls within this 0.7 volt window will not be reproduced accurately making the class B amplifier unsuitable for precision audio amplifier applications.

To overcome this zero-crossing distortion (also known as crossover distortion) class AB amplifiers were developed.

2.3.3 CLASS AB AMPLIFIER

As its name implies, the Class AB Amplifier is a combination of the “Class A” and the “Class B” type amplifiers that stated above. The AB classification of amplifier is currently one of the most common used types of audio power amplifier design. The class AB amplifier is a variation of a class B amplifier as described above, except that both devices are allowed to conduct at the same time around the waveforms crossover point eliminating the crossover distortion problems of the previous class B amplifier.

The two transistors have a very small bias voltage, typically at 5 to 10% of the quiescent current to bias the transistors just above its cut-off point. Then the conducting device, either bipolar of FET, will be “ON” for more than one half cycle, but much less than one full cycle of the input signal. Therefore, in a class AB amplifier design each of the push-pull transistors is conducting for slightly more than the half cycle of conduction in class B, but much less than the full cycle of conduction of class A. In other words, the conduction angle of a class AB amplifier is somewhere between 180^o and 360^o depending upon the chosen bias point as shown in fig(3).

fig(2.3) Class AB Amplifier

The advantage of this small bias voltage, provided by series diodes or resistors, is that the crossover distortion created by the class B amplifier characteristics is overcome, without the inefficiencies of the class A amplifier design. So the class AB amplifier is a good compromise between class A and class B in terms of efficiency and linearity, with conversion efficiencies reaching about 50% to 60%.

2.3.4 CLASS C AMPLIFIER

The Class C Amplifier design has the greatest efficiency but the poorest linearity of the classes of amplifiers mentioned here. The previous classes, A, B and AB are considered linear amplifiers, as the output signals amplitude and phase are linearly related to the input signals amplitude and phase.

However, the class C amplifier is heavily biased so that the output current is zero for more than one half of an input sinusoidal signal cycle with the transistor idling at its cut-off point. In other words, the conduction angle for the transistor is significantly less than 180 degrees, and is generally around the 90 degrees area. While this form of transistor biasing gives a much improved efficiency of around 80% to the amplifier, it introduces a very heavy distortion of the output signal. Therefore, class C amplifiers are not suitable for use as audio amplifiers.

fig(2.4) Class C Amplifier

Due to its heavy audio distortion, class C amplifiers are commonly used in high frequency sine wave oscillators and certain types of radio frequency amplifiers, where the pulses of current produced at the amplifiers output can be converted to complete sine waves of a particular frequency by the use of LC resonant circuits in its collector circuit.

Other classes of amplifier are ;

2.3.5 Class D Amplifier

A Class D audio amplifier is basically a non-linear switching amplifier or PWM amplifier. Class-D amplifiers theoretically can reach 100% efficiency, as there is no period during a cycle were the voltage and current waveforms overlap as current is drawn only through the transistor that is ON.

2.3.6 Class F Amplifier

Class-F amplifiers boost both efficiency and output by using harmonic resonators in the output network to shape the output waveform into a square wave. Class-F amplifiers are capable of high efficiencies of more than 90% if infinite harmonic tuning is used.

2.3.7Class G Amplifier

Class G offers enhancements to the basic class AB amplifier design. Class G uses multiple power supply rails of various voltages and automatically switches between these supply rails as the input signal changes. This constant switching reduces the average power consumption, and therefore power loss caused by wasted heat.

2.3.8 Class I Amplifier

The class I amplifier has two sets of complementary output switching devices arranged in a parallel push-pull configuration with both sets of switching devices sampling the same input waveform. One device switches the positive half of the waveform, while the other switches the negative half similar to a class B amplifier. With no input signal applied, or when a signal reaches the zero crossing point, the switching devices are both turned ON and OFF simultaneously with a 50% PWM duty cycle canceling out any high frequency signals.

To produce the positive half of the output signal, the output of the positive switching device is increased in duty cycle while the negative switching device is decreased by the same and vice versa. The two switching signal currents are said to be interleaved at the output, giving the class I amplifier the named of: “interleaved PWM amplifier” operating at switching frequencies in excess of 250kHz.

2.3.9Class S Amplifier

A class S power amplifier is a non-linear switching mode amplifier similar in operation to the class D amplifier. The class S amplifier converts analogue input signals into digital square wave pulses by a delta-sigma modulator, and amplifies them to increases the output power before finally being demodulated by a band pass filter. As the digital signal of this switching amplifier is always either fully “ON” or “OFF” (theoretically zero power dissipation), efficiencies reaching 100% are possible.

2.4 Class T Amplifier

The class T amplifier is another type of digital switching amplifier design. Class T amplifiers are starting to become more popular these days as an audio amplifier design due to the existence of digital signal processing (DSP) chips and multi-channel surround sound amplifiers as it converts analogue signals into digital pulse width modulated (PWM) signals for amplification increasing the amplifiers efficiency. Class T amplifier designs combine both the low distortion signal levels of class AB amplifier and the power efficiency of a class D amplifier.

We have seen here a number of classification of amplifiers ranging from linear power amplifiers to non-linear switching amplifiers, and have seen how an amplifier class differs along the amplifiers load line. The class AB, B and C amplifiers can be defined in terms of the conduction angle (θ), as follows:Amplifier Class by Conduction Angle.

Amplifier Class	Description	Conduction Angle
Class-A	Full cycle 360o of Conduction	Θ =2π
Class-B	Half cycle 180o of Conduction	Θ = π
Class-AB	Slightly more than 180o of conduction	Π < θ < 2π
Class-C	Slightly less than 180o of conduction	Θ < π
Class-D to T	ON-OFF non-linear switching	Θ = 0

Table (2.1) classification of amplifiers by conducting angle

Amplifies can be further classified based on the signal they amplify. They are as follows :

1. AUDIO FREQUENCY AMPLIFIERS (A.F. AMPLIFIERS)

Audio Frequency Amplifiers is the amplifier that amplify the audio frequencies, Generally audio frequencies are in the range of 20 Hz to 20 kHz. Some of the HI-FI audio amplifiers may amplify up to 100 kHz.

These are used to supply audio frequency power to operate the loud speakers. Most of the modern audio amplifiers are based on solid state dives such as transistors, in early stages they are made of vacuum tubes.

2. Intermediate Frequency Amplifiers (I.F. Amplifiers):

The Intermediate frequencies are amplified by this amplifier. These amplifiers are used in TV, radio and radar. They provide the maximum voltage amplification of a radio, TV or radar signal, before the video or audio information carried by the signal is demodulated.

Their frequency of operation is lower than that of the received radio signal, but higher than the audio or video signals eventually produced by the system. The type of equipment decides the frequency at which I.F. amplifiers operate.

3. Radio Frequency Amplifiers (R.F. Amplifiers):

This amplifier increases the power of low-frequency radio signal. These are used to drive antenna of a transmitter. Radio Frequency amplifiers are tuned amplifiers whose the frequency of operation is controlled by the tuned circuit. This circuit can be adjusted depending on the amplifier purpose. Input resistance is generally low, as is gain.

A special feature of RF amplifiers is low noise performance. So they are used in the earliest stages of a receiver. The background noise generally produced by any electronic device is kept to a low value as the amplifier handles very low amplitude signals from the antenna. Hence low noise FET transistors used in these stages.

4. Ultrasonic Amplifiers:

Ultrasonic amplifiers amplify the ultrasonic waves. These are in the frequency range of around 20 kHz up to about 100 kHz. They are used for specific purposes such as ultrasonic cleaning, ultrasound scanning, remote control systems etc. Each type will operate at narrow band of frequencies within the ultrasonic range.

5. Wideband Amplifiers:

Wide band amplifiers will amplify a band of frequencies. They amplify from DC to several tens of MHz They are used in equipment such as oscilloscopes etc. These are used where there is a need to accurately measure signals over a wide range of frequencies. Because of their wide bandwidth, gain is low.

6. Direct Coupled Amplifiers (DC Amplifiers):

Direct coupled or DC amplifiers are used to amplify very low frequency signals. The output of one stage is coupled with the input of the next stage in these amplifiers. This amplifier amplifies the DC frequency that is zero frequency. They are mostly used in many electrical control systems and measuring instruments.

7. Video Amplifiers:

Video amplifiers are used to improve the video signal and display it with high resolution. The video signal carries all the information about picture in TV and radar systems. They are a special type of wide band amplifier. They are used specifically for signals that are to be applied to video equipment.

The bandwidth of video amplifiers depends on use. In TV receivers they extend from 0Hz to 6MHz and is still wider in radar. These amplifiers are used to amplify the signals received from DVDs, computer monitors. They can also be used to amplify the video quality in small TVs that are installed in the vehicles.

8. Buffer Amplifiers :

Buffer amplifiers are commonly used for electrical impedance transformation from one circuit to another. They have amplifier gain of 1.They are used for isolating the circuits from each other. They have high impedance at the input and low impedance at the output .

Therefore can be used as impedance matching device. This implies that signals are not attenuated between circuits, which happens when a circuit with high output impedance feeds a signal directly to another circuit having a low input impedance.

9. Operational Amplifiers :

Operational amplifiers are high gain electronic voltage amplifiers. The operational amplifiers are used to perform mathematical operations on voltages . They are used in the form of ICs initially they were developed with the vacuum tubes. The operational amplifier has two input terminal mainly.

They are inverting and non-inverting. They can be used as inverting amplifiers, non-inverting amplifiers, summing amplifiers, difference amplifier etc.

10. Transistor Amplifiers:

Transistor is an electronic device. This is also used as an amplifier. Transistor amplifiers amplify the voltage or current of the input signal.

There are two types of transistor devices.

1) BJT (Bipolar junction transistors).

2)FET (Field Effect Transistors).

Transistor amplifiers are analyzed in different configurations. They are Common Base, Common Emitter and Common Collector using BJT. By using FET, transistor amplifiers are analyzed in the following configurations, Common Gate Common Source and Common Drain.

In Bipolar Junction Transistors, small current at the terminal of base can control the current at emitter and collector, while in Field Effect Transistors (FET) small voltage at the gate can control the voltage at source and drain.

2.5 APPLICATION OF FEEDBACK AMPLIFIER

Amplifiers has many uses nowaday because most of the electronic devices are connected with an electronic amplifiers, some of its application are as follows :-

• amplifiers are used as a device that increase sound from low frequencies to the high frequencies. e.g electronic amp in a set of loud speaker.
• Amplifiers are used in hi-fi system. e.g. class AB amplifier.
• Amplifiers are used as a sound device in a mobile devices and personal computers.
• Amplifiers are used cars audio sub-woofer amplifiers.
• Amplifiers can be used as tools that hooked up to take the readings from human body at the hospitals, patient is going to be connected to sensor that run by instrumental amplifiers. These circuit is used in nearly every medical devices. Most of the biomedical sensors are with very high impedance and generate tiny signals such as blood pressure sensor, ultra sound transducers, polarized and non polarize electrons and radiation thermometric transducers.
  The required is with very high impedance presented by an instrumentation amplifier because the characteristic of potentials can subjected to loading effect, which can cause distortion of the signals. An easily recognizable medical application of amplifiers is like in electrocardiography medical which can maintain the changes in the hearts dipole electric field.
  In conclusion the electronic amplifiers have uses in nearly every field of electronics, they fulfill a specific role in circuit needing the advantages of high input impedance with good gain while providing common mode noise rejection and fully differential inputs. With such widespread use, this is a device that every engineer should have in his tool belt.

CHAPTER THREE

SUMMARY AND CONCLUSION

3.1 SUMMARY

Then we have seen that the quiescent DC operating point (Q-point) of an amplifier determines the amplifier classification. By setting the position of the Q-point at half way on the load line of the amplifiers characteristics curve, the amplifier will operate as a class A amplifier. By moving the Q-point lower down the load line changes the amplifier into a class AB, B or C amplifier. Then the class of operation of the amplifier with regards to its DC operating point can be given as:

Fig (3.1) Amplifier Classes and Efficiency

As well as audio amplifiers there are a number of high efficiency Amplifier Classes relating to switching amplifier designs that use different switching techniques to reduce power loss and increase efficiency. Some amplifier class designs listed below use RLC resonators or multiple power-supply voltages to reduce power loss, or are digital DSP (digital signal processing) type amplifiers which use pulse width modulation (PWM) switching techniques.

3.2 CONCLUSION

The study of feedback amplifier is very wide field which is never been emphasizes unless is to take some part of it. The feedback amplifiers was discuss in this literature like, its types, classification and some of its applications.

REFERENCE

• Feedback Amplifiers ; theory and design (kluwer academy) p.64, ISBN 0-7923-7643-9.
• ELEC 312 lecture note chapter 4 p. 17-20, by Rabin Raut, phd.
• Feedback Amps p.16-19, by professor W. marshal leach jr. (Geoge institute of technology, school of electrical and computer engineering).
• Feedback Amplifiers chapter 9, by De bashis (Associate professor west Bengal university of technology).
• Laboratory manual electronic circuit & Analysis by C.S Swapna (assistant professor and R.Sanvers pillai). At MLR Institute of technology & management), Dundigal, Quhbullapur (M), Hyderabad.
• Electronic Amplifier circuit Theory and Design. By Professor Joseph mypettil and associate professor Malcolm myers, (Stanford university).