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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. 1.
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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
Negative feed back 1800out of phase with the input signal. Decreases Decreases Decreases Improved Amplifiers Positive feed back In phase with the input signal Increases Increases Increases Poor oscillators Comparison of positive and negative feed back Feed back signal Net input signal Gain Noise Stability Uses 9AEI303.33 12
Principle of feed back. + + Rs Io Ii I Mixing net work Basic amplifier A V Sampling network Vo Vi RL vs - - + Feed back network. b Vf - 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 Current Sampled Signal is Voltage 9AEI303.34 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 Xi difference signal Xs input signal. Af feed back amplifier Xo output signal gain. Xf feed back signal 9AEI303.34 19
Negative feed back is applied by feeding the fraction of the output voltage Xo 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 amplifier A Vi Vo RL Vs bVo Feed back b Vf This connection increases the input resistance and decreases the output resistance . 9AEI303.35 25
Current series feedback amplifier Basic amplifier A Vi Vs Io RL bIo Feed back b Vf This connection increases both input resistance and output resistance 9AEI303.35 26
Current shunt feedback amplifier Basic amplifier A Is Ii bIo Io RL Feed back b If This connection decreases input resistance and increases the output resistance. 9AEI303.35 27
Voltage shunt feedback amplifier Basic amplifier A Is Ii Vo RL bVo Feed back b If This connection decreases both input resistance and output resistance. 9AEI303.35 28
Signals in feedback amplifiers Voltage Current Current Voltage Current Voltage Voltage Current Voltage Voltage Current Current Voltage Current Current Voltage Trans conductance Gm Current gain Ai Voltage gain Av Trans resistance Rm Voltage ratio Vf/Vo Resistance Vf/Io Current ratio If/Io Conductance If/Vo 9AEI303.35 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
Voltage shunt feedback amplifier 9AEI303.36 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
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
Current series feedback 9AEI303.36 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
Current shunt feedback 9AEI303.36 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
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
Voltage series feedback 9AEI303.36 39
Comparison of different feedback amplifiers 9AEI303.36 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 9AEI303.39 44
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Differences between power and voltage amplifiers 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