feed back amplifiers

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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.

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Topics to be discussed:

working of amplifiers and their parameters.

Topics under discussion: what is Feed back?

Comparison of different types of feed back.

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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.

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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.

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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.

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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.

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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.

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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.

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Advantages of Negative Feed back:

• Improves stability in gain.

• Reduces distortion.

• Reduces the noise level at the output.

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• decreases the gain.• decreases the distortion.• decreases the noise.

Disadvantages of negative feed back

Due to these features:

it is used in amplifiers.

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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.

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Comparison of positive and negative feed back Negative feed back1800out of phase with the input signal.

Decreases

Decreases

Decreases

Improved

Amplifiers

Positive feed backIn phase with the input signal

Increases

Increases

Increases

Poor

oscillators

Feed back signal

Net input signal

Gain

Noise

Stability

Uses

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Principle of feed back.

Basic amplifier A

Mixing net work

Sampling network

Feed back network.

Rs

vs Vi V VoIoI

RL

Vf

+

-

++

--

Ii

Block Diagram of Feed Back Amplifier

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• 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.

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Basic amplifier

• This stage simply amplifies the signal that is present at its input .

• The voltage gain of this amplifier is “A.”

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Feed back network

It is usually a passive two port network which may contain resistors , capacitors & inductors.

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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 Sampled Signal is Current

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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.

SERIES FEEDBACK SHUNT FEEDBACK

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Gain of negative feed back amplifier

A

mixer

A gain of amplifier without feedback Xi difference signal

Xs input signal. Af feed back amplifier

Xo output signal gain.

Xf feed back signal

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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.)

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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.

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Disadvantages of negative feedback

Reduces the gain of an amplifier.

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Basis of classification

Classification of negative feed back amplifiers is based on the method of mixing and sampling employed.

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Classification

• Voltage series feedback amplifier.• Current series feedback amplifier.• Current shunt feedback amplifier.• Voltage shunt feedback amplifier.

Negative feed back amplifiers are classified into:-

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Voltage series feedback amplifier

Basic amplifi

er A

Vs

Feed back

Vf

Vi Vo RL Vo

This connection increases the input resistance and decreases the output resistance .

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Current series feedback amplifier

This connection increases both input resistance and output resistance

Basic amplifi

er A

VsVi RL

Vf

Feed back

Io Io

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Current shunt feedback amplifier

This connection decreases input resistance and increases the output resistance.

Basic amplifi

er A

Feed back

Io RLIi

If

IoIs

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Voltage shunt feedback amplifier

This connection decreases both input resistance and output resistance.

Feed back

RL

Basic amplifi

er A

Vo Vo

Is Ii

If

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Signals in feedback amplifierssignal Type of feedback

Voltage series

Current series

Current shunt

Voltage shunt

Output signal

Input signal

Feedback signalDifferent signalGain A

Feedback factor

Voltage Voltage

Voltage

Voltage

Voltage

Voltage

Voltage

Voltage

Current Current

Current

Current Current

Current

Current

Current

Voltage gain Av

Trans conductance Gm

Trans resistance Rm

Current gain Ai

Voltage ratio Vf/Vo

Current ratio If/Io

Resistance Vf/Io

Conductance If/Vo

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• 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.

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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.

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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 = Vf/Io= -Io.RE/Io= -RE.

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Current series feedback

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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.

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Current shunt feedback

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• Feedback signal If =RE Io/ (Rf + RE.)

• Feedback factor β = If/Io = RE/(Rf +RE.)

If,

Rf >>RE

= RE/Rf

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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 = Vf/Vo = 1.

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Voltage series feedback

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S .No Characteristics Voltage series

Voltage shunt

Current series

Current shunt

1 Stability of transfer gain

Improves Av Improves Rm

Improves Gm

Improves Ai

2 Frequency & phase distortion

decreases 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

Comparison of different feedback amplifiers

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Voltage amplifiersVoltage 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.

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• 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.

Voltage amplifiersVoltage amplifiers

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Power amplifiersPower 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.

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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

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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

The devices have to supply large currents and are of low output resistance

8 Cascading and cascoding is used

Push pull complimentary symmetry and Darlington circuits are used

9 Heat generated is very less. No cooling arrangements are required

Considerable heat is generated. Cooling arrangements are required

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S.No Voltage amplifiers Power amplifiers

10 Output must be free from distortion

Reasonable distortion is permitted.

11 Their operation is restricted to linear portion.

Non linear operation is also allowed

12 Used as preamplifier Used in output stage

Differences between power and voltage amplifiers

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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.

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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.

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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.

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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.

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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

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Class A Power Amplifier

The period of conduction is 3600 (Full Cycle)

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Class B Power Amplifier

• Period of conduction is 1800 (Half Cycle)

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Class C Power Amplifier

• Period of Conduction is less than 1800

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Class AB Power Amplifier

• Period of Conduction is greater than 180, less than 3600.

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Classification based on Configuration

• CE Amplifier. Uses CE configuration

• CB Amplifier. Uses CB configuration

• CC Amplifier. Uses CC configuration

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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.

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Class-B push-pull Amplifier

Fig.27.1

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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.

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• 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.

Construction

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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.

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• 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.

Operation

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• 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.

Operation

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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.

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Disadvantages

• Higher harmonic distortion.

• Self – bias cannot be used.

• Supply voltages must have good regulation.

• Transformers are bulky and expensive.

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Class-B Operation

t0 t1 t2

Vi

nAv

Vout t0t1

In class-B operation collector current flows only 1800.(Half cycle).

Fig 11.1

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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.

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Q-point location for class-B operation

The Q-point is at cut off.

Fig 11.2

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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.

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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).

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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.

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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.

Applications of Power Amp

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Audio frequency power amplifiers are used in

• Telephone circuits.

• Repeater circuits.

Applications:

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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.

Applications:

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Power amplifiers are also used in

• Industrial electronic equipment.

• Medical electronic equipment.

Applications:

feed back amplifiers

Engineering