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Objectives: On completion of this period you would be able to : • • • • • Define feedback. Need for feedback. Types of feedback. Define negative and positive feedback. Advantages and disadvantages of negative and positive feedback. • Compare negative and positive feed back. 9AEI303.33 1 Topics to be discussed: working of amplifiers and their parameters. Topics under discussion: what is Feed back? Comparison of different types of feed back. 9AEI303.33 2 What is Feed back? • The process by which a fraction of output energy is injected back to the input is called feed back. • The amplifiers using this technique are called feed back amplifiers. 9AEI303.33 3 Characteristics of basic amplifiers Important characteristics of an amplifier are its : • Voltage gain. • Input impedance. • Output impedance. • Band width. These parameters are more or less fixed for a basic amplifier. 9AEI303.33 4 Need for feed back • The parameters of a basic amplifier are required to changed as per the need. • These changes can be brought out efficiently by introducing feed back in the amplifier. 9AEI303.33 5 Types of feed back Depending upon the effect of feed back signal there are two types of feed back. • Negative feed back. • Positive feed back. 9AEI303.33 6 What is negative feed back? When the feed back signal decreases the net input signal i.e., the feedback signal is 1800out of phase with respect to input signal, it is called negative or Degenerative feed back. 9AEI303.33 7 What is positive feed back? When the feed back signal increases the net input signal i.e., the feedback signal is in phase with the input signal, it is called Positive or Regenerative feed back. 9AEI303.33 8 Advantages of Negative Feed back: • Improves stability in gain. • Reduces distortion. • Reduces the noise level at the output. 9AEI303.33 9 Disadvantages of negative feed back • decreases the gain. • decreases the distortion. • decreases the noise. Due to these features: it is used in amplifiers. 9AEI303.33 10 Disadvantages of positive feed back • Increases the distortion. • Increases the noise. • Poor stability. Due to these features: • it is seldom used in amplifiers. • it is used in oscillators. 9AEI303.33 11 Comparison of positive and negative feed back Negative feed back 1800out of phase with the input signal. Positive feed back In phase with the input signal Net input signal Decreases Increases Gain Decreases Increases Noise Decreases Increases Stability Improved Poor Uses Amplifiers oscillators Feed back signal 9AEI303.33 12 Principle of feed back. + Rs vs - + Ii Vi Mixing net work + Vf Basic amplifi er A I V Sampli ng networ k Io Vo RL - Feed back network . b Block Diagram of Feed Back Amplifier 9AEI303.34 13 • Feed back amplifier is one in which a part of the output of an amplifier is fed back to the input. • It consists of basic amplifier with a gain A. feed back network. sampling network. mixing network. 9AEI303.34 14 Basic amplifier • This stage simply amplifies the signal that is present at its input . • The voltage gain of this amplifier is “A.” 9AEI303.34 15 Feed back network It is usually a passive two port network which may contain resistors , capacitors & inductors. 9AEI303.34 16 Sampling Network • Using this network, we sample the output voltage or current. The sampled energy is fed to the feedback network linearly in series or shunt with the output network. Sampled Signal is Voltage 9AEI303.34 Sampled Signal is Current 17 Mixing network • This network mixes the feedback energy with the applied input. The feedback signal is connected in series or in shunt with the input signal. SHUNT FEEDBACK SERIES FEEDBACK 9AEI303.34 18 Gain of negative feed back amplifier mixer A b A gain of amplifier without feedback difference signal Xi Xs input signal. feed back amplifier Af 9AEI303.34 Xo output signal 19 Negative feed back is applied by feeding the fraction of the output voltage bXo back to the amplifier input. Xi = Xs-Xf Xf = ß.Xo Xo = A.Xi Substituting in Xi Xo =( Xs- ß.Xo)A Xo(1+A. ß) = A.Xs Xo/Xs = A/(1+A. ß) Xo/Xs is called the gain of the feed back amplifier. Af =A/(1+A.b) 9AEI303.34 20 Advantages of negative feed back amplifier. • Increased stability. • Increased band width. • Less distortion . • Reduces non linear distortion and noise. • Input and output impedance can be modified as desired. • High fidelity i.e., more linear operation. 9AEI303.34 21 Disadvantages of negative feedback Reduces the gain of an amplifier. 9AEI303.34 22 Basis of classification Classification of negative feed back amplifiers is based on the method of mixing and sampling employed. 9AEI303.35 23 Classification Negative feed back amplifiers are classified into:• • • • Voltage series feedback amplifier. Current series feedback amplifier. Current shunt feedback amplifier. Voltage shunt feedback amplifier. 9AEI303.35 24 Voltage series feedback amplifier Basic V amplifie r Vs i Vo R L A Vf Feed back b This connection increases the input resistance and decreases the output resistance . 9AEI303.35 25 Current series feedback amplifier Vs Vi V f Basic amplifie r A Io R L Feed back b This connection increases both input resistance and output resistance 9AEI303.35 26 Current shunt feedback amplifier Basic I amplifie r A Is i I f Io R L Feed back b This connection decreases input resistance and increases the output resistance. 9AEI303.35 27 Voltage shunt feedback amplifier Is I I i f Basic amplifie r A Vo R L Feed back b This connection decreases both input resistance and output resistance. 9AEI303.35 28 Signals in feedback amplifiers signal Voltage series Output signal Type of feedback Current series Current shunt Voltage shunt Voltage Current Current Voltage Voltage Voltage Current Current Feedback signal Voltage Voltage Current Current Different signal Voltage Voltage Current Current Voltage gain Av Trans conductan ce Gm Resistan ce Vf/Io Current gain Ai Trans resistanc e Rm Conducta nce If/Vo Input signal Gain A Feedback factor b Voltage ratio Vf/Vo 9AEI303.35 Current ratio If/Io 29 Voltage shunt feedback amplifier • A resistor is connected between the collector and base of the transistor. • The output voltage Vo is much greater than the input voltage Vi and is 180 degrees out of phase with Vi. • The portion of output is connected through the feedback resistor Rf to the base. 9AEI303.36 30 30 Voltage shunt feedback amplifier 9AEI303.36 31 31 Hence: • The feedback current If=(Vi-V0)/Rf ~V0/Rf = β V0. • Feedback factor β = -1/Rf. • The feedback current is proportional to the output voltage, this circuit is an example of voltage-shunt feedback amplifier. 9AEI303.36 32 32 Current series feedback • The feedback signal is the voltage Vf across RE and the sampled signal is the load current Io. • The feedback signal Vf= Io RE. • The feedback voltage is directly proportional to the output current. • The feedback factor b = Vf/Io= -Io.RE/Io= -RE. 9AEI303.36 33 33 Current series feedback 9AEI303.36 34 34 Current shunt feedback • A two transistor CE amplifier with feedback from the second emitter to the base of first through the resistor Rf. • The input current is the difference of the current at the base of transistor due to Vs and the current If. • This is smaller than the magnitude of current without feedback. 9AEI303.36 35 35 Current shunt feedback 9AEI303.36 36 36 • Feedback signal If =RE Io/ (Rf + RE.) • Feedback factor β = If/Io = RE/(Rf +RE.) If, Rf >>RE b = RE/Rf 9AEI303.36 37 37 Voltage series feedback • Common collector amplifier is also known as emitter follower. • The output voltage Vo is developed across the emitter resistor RE and is fedback to the input in series. • The feedback factor b = Vf/Vo = 1. 9AEI303.36 38 38 Voltage series feedback 9AEI303.36 39 39 Comparison of different feedback amplifiers S .No Characteristics Current series Current shunt Stability of transfer Improves Av Improves gain Rm Improves Improves Ai 2 Frequency & phase decreases distortion decreases decreases decreases 3 Nonlinear distortion decreases decreases decreases decreases 4 Noise decreases decreases decreases decreases 5 Bandwidth increases increases increases increases 6 Output resistance decreases decreases increases increases 7 Input resistance increases decreases increases decreases 1 Voltage series Voltage shunt 9AEI303.36 Gm 40 40 Voltage amplifiers • The primary function of a voltage amplifier is to raise the voltage level of the signal. • The voltage amplifiers are designed to achieve maximum voltage amplification. • Small signal amplifiers give large output voltage taking small input signal amplitudes. 9AEI303.39 41 Voltage amplifiers • The first few stages in the multistage amplifier are used to achieve only voltage amplification. • Only very little power can be drawn from its output. 9AEI303.39 42 Power amplifiers • A transistor amplifier which raises the power level of the signals is known as transistor power amplifier. • This amplifier can feed large amount of power to the load. • Power amplifiers are also called large signal amplifiers. • In general the last stage of multi stage amplifier is the power amplifier. 9AEI303.39 43 Differences between power and voltage amplifiers S.No Voltage amplifiers Power amplifiers 1 These are small signal amplifiers These are large signal amplifiers 2 Power output is small Voltage output is small 3 The transistor with high β is used The transistor with low ß is used 4 Collector current is less Collector current is high 9AEI303.39 44 5 Input signal voltages are very low in magnitude Input signal voltages are high in magnitude 6 R-C coupling is used Transformer coupling is used 7 The devices used are low current devices with large output resistance Cascading and cascoding is used The devices have to supply large currents and are of low output resistance Push pull complimentary symmetry and Darlington circuits are used Considerable heat is generated. Cooling arrangements are required 8 9 Heat generated is very less. No cooling arrangements are required 9AEI303.39 45 Differences between power and voltage amplifiers S.No Voltage amplifiers Power amplifiers 10 Output must be free from distortion Reasonable distortion is permitted. 11 Their operation is restricted Non linear operation is also to linear portion. allowed 12 Used as preamplifier Used in output stage 9AEI303.39 46 The characteristics that make an amplifier as voltage amplifier are: • Higher value of β (>100) – thin base. • Low input resistance. • High collector load resistance. • Low collector current. • R-C coupling. 9AEI303.39 47 The characteristics that make an amplifier as power amplifier are: • Thick base i.e., smaller value of ß. • Collector current is high. • Low collector load resistance. • Transformer coupling. 9AEI303.39 48 Power Amplifiers • Converts DC power from supply into AC power. • They are large signal amplifiers. • Larger portion of load line is used during signal operation than small signal amplifiers. • Classification is based on percentage of input cycle for which amplifier operates in its linear region. 9AEI303.40 49 Classification of Amplifiers based on Frequency • Audio frequency or AF power amplifiers used in the audio range i.e. 20 Hz to 20 KHz. Example : Public Address System etc. • Radio frequency or RF power amplifiers used in the radio frequency range from 30KHz to 3MHz. Example : Radio receiver etc. • Video frequency or VF power amplifiers used in the video frequency range > 3MHz. Example : TV receiver etc. 9AEI303.40 50 Classification according to Period of Conduction • Class A power amplifier. Conduction angle θ = 360o i.e. full cycle • Class B power amplifier. Conduction angle θ = 180o i.e. half cycle • Class C power amplifier. Conduction angle θ <180o i.e. less than half cycle • Class AB power amplifier. Conduction angle 360o< θ >180o i.e. more than half cycle 9AEI303.40 51 Class A Power Amplifier The period of conduction is 3600 (Full Cycle) 9AEI303.40 52 Class B Power Amplifier • Period of conduction is 1800 (Half Cycle) 9AEI303.40 53 Class C Power Amplifier • Period of Conduction is less than 1800 9AEI303.40 54 Class AB Power Amplifier • Period of Conduction is greater than 180, less than 3600. 9AEI303.40 55 Classification based on Configuration • CE Amplifier. Uses CE configuration • CB Amplifier. Uses CB configuration • CC Amplifier. Uses CC configuration 9AEI303.40 56 Class b power amp • An Amplifier is biased at cut-off (class-B) • We can get more output power for a given amount of input power • It is more difficult to implement the circuit in order to get a linear reproduction of the input waveform. 9AEI303.41 57 Class-B push-pull Amplifier Fig.27.1 9AEI303.41 58 Construction • Two Transistors are connected in push-pull arrangement. • It consists of two centre-tapped transformers T1&T2 and two identical transistors Q1&Q2. • Transformer T1 is secondary centre tapped input transformer and is a phase splitter • It is required to produce two signal voltages that are 1800 out of phase. • T2 is an output primary center tapped transformer. 9AEI303.41 59 Construction • T2 is required to couple the a.c. output signal from the collector to the load. • Transistor Q1 and Q2 are biased at cutoff. • To get a balanced circuit, two emitters are connected to the centre tap of T1 and Vcc supply to the centre tap of T2. 9AEI303.41 60 Operation • With no input signal transistors Q1 & Q2 are cut off. • No current is drawn from the Vcc supply. • Input signal is applied through center tapped transformer. • The inputs supplied to the bases of two transistors are out of phase. • Thus the two transistors conduct on alternate half cycles of the input signal. 9AEI303.41 61 Operation • Current for each transistor flows opposite directions through Secondary of T2 transformer. • The magnetic flux set up by each currents result in opposite flux. • Net flux is Zero. • Transformer is not required to handle a large flux due to d.c. currents. 9AEI303.41 62 Operation • During positive half cycle of the input signal, base of Q1 is positive and Q2 is negative. • Q1 conducts and Q2 is off. • During negative half cycle of the input signal, base of Q1 is negative and Q2 is positive. • Q1 is off and Q2 conducts. • Each transistor handles one half of the input signal. • Output transformer joins these two halves and produces full sine wave. 9AEI303.41 63 Advantages of Class B push – pull power amplifier • Possible to obtain greater power output. • Efficiency is higher (78.5%) • Negligible power loss at no signal. 9AEI303.41 64 Disadvantages • Higher harmonic distortion. • Self – bias cannot be used. • Supply voltages must have good regulation. • Transformers are bulky and expensive. 9AEI303.41 65 Class-B Operation Vi n t0 t1 Av t2 Vout t0 t1 Fig 11.1 In class-B operation collector current flows only 1800.(Half cycle). 9AEI303.42 66 Class-B operation • More output power is obtained than class-A amplifier. • The transistor dissipates no power with zero input signal. • Overall efficiency is 75%. • Power dissipated by the transistor is less in class-B Amplifier. • Average current drawn by the circuit is smaller than that of class-A amplifier. 9AEI303.42 67 Q-point location for class-B operation Fig 11.2 The Q-point is at cut off. 9AEI303.42 68 Q-point location for class-B operation • Class-B amplifier is biased at the cut off point, Icq=0 and Vceq=Vcc • It is brought out of cutoff and operates in its linear region when input signal drives the transistor into conduction. • The output is not a replica of the input. 9AEI303.42 69 Max value of A.C. o/p power in class-B Amplifier • Location of a Q-point for class-B amplifier along with maximum variation of Vce and Ic with largest possible signal. • In a class-B amplifier power is developed only during one half cycle of the input signal. • PO(ac)=1/2[VCP/√2].[ICP/√2]=1/(4*VCP.ICP). 9AEI303.42 70 Advantages of push-Pull amplifier • Circuit efficiency of a class-B push-pull amplifier is 78.5%. • No power is drawn from the d.c. supply under no signal condition. • Eliminates even order harmonics in a.c. output signal. • Due to the absence of even harmonics the circuit gives more output per device, for a given amount of distortion. • No d.c. component in the out put signal. 9AEI303.42 71 Applications of Power Amp • • • • • • The AF power amplifiers are used as output stages in the audio equipment such as Radio receiver. Tape recorder. TV receiver. Public address system. Stereo system. Sound system in cinema theatre. 9AEI303.43 72 Applications: Audio frequency power amplifiers are used in • Telephone circuits. • Repeater circuits. 9AEI303.43 73 Applications: Radio frequency power amplifiers are used at the output of broadcast transmitters for feeding large power to the antenna. • • • • Radio transmitter. TV transmitter. Radar transmitter. Satellite transmitter. 9AEI303.43 74 Applications: Power amplifiers are also used in • Industrial electronic equipment. • Medical electronic equipment. 9AEI303.43 75