Detailed Notes: Feedback Amplifiers | Analog Circuits - Electronics and Communication Engineering (ECE) PDF Download

 Page 1


feedback amplifiers, effect of feedback on amplifier characteristics, voltage series, voltage shunt, 
current series and current shunt feedback configurations, simple problems; Oscillators: Condition 
for Oscillations, RC type Oscillators RC phase shift and Wien-bridge Oscillators, LC type 
Oscillators, generalized analysis of LC Oscillators, Hartley and Colpitts oscillators. 
 
INTRODUCTION TO FEEDBACK AMPLIFIERS 
Feedback is a common phenomenon in nature. It plays an important role in electronics & control 
systems. Feedback is a process whereby a portion of the output signal of the amplifier is feedback 
to the input of the amplifier. The feedback signal can be either a voltage or a current, being applied 
in series or shunt respectively with the input signal.  
The path over which the feedback is applied is the feedback loop. There are two types of feedback 
used in electronic circuits. (i) If the feedback voltage or current is in phase with the input signal and 
adds to its magnitude, the feedback is called positive or regenerative feedback.(ii) If the feedback 
voltage or current is opposite in phase to the input signal and opposes it , the feedback is called 
negative or regenerative feedback. 
CLASSIFICATION OF AMPLIFIERS: 
Before analyzing the concept of feedback, it is useful to classify amplifiers based on the 
magnitudes of the input & output impedances of an amplifier relative to the sources & load 
impedances respectively as (i) voltage (ii) current (iii) Tran conductance (iv) Tran resistance 
amplifiers. 
VOLTAGE AMPLIFIER: 
The above figure shows a Thevenin’s equivalent circuit of an amplifier. If the input resistance of 
the amplifier Ri is large compared with the source resistance Rs, then Vi = Vs. If the external load 
RL is large compared with the output resistance R0 of the amplifier, then V0 = AV VS .This type 
of amplifier provides a voltage output proportional to the input voltage & the proportionality factor 
doesn’t depend on the magnitudes of the source and load resistances. Hence, this amplifier is 
known as voltage amplifier. An ideal voltage amplifier must have infinite resistance Ri and zero 
output resistance. 
 
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feedback amplifiers, effect of feedback on amplifier characteristics, voltage series, voltage shunt, 
current series and current shunt feedback configurations, simple problems; Oscillators: Condition 
for Oscillations, RC type Oscillators RC phase shift and Wien-bridge Oscillators, LC type 
Oscillators, generalized analysis of LC Oscillators, Hartley and Colpitts oscillators. 
 
INTRODUCTION TO FEEDBACK AMPLIFIERS 
Feedback is a common phenomenon in nature. It plays an important role in electronics & control 
systems. Feedback is a process whereby a portion of the output signal of the amplifier is feedback 
to the input of the amplifier. The feedback signal can be either a voltage or a current, being applied 
in series or shunt respectively with the input signal.  
The path over which the feedback is applied is the feedback loop. There are two types of feedback 
used in electronic circuits. (i) If the feedback voltage or current is in phase with the input signal and 
adds to its magnitude, the feedback is called positive or regenerative feedback.(ii) If the feedback 
voltage or current is opposite in phase to the input signal and opposes it , the feedback is called 
negative or regenerative feedback. 
CLASSIFICATION OF AMPLIFIERS: 
Before analyzing the concept of feedback, it is useful to classify amplifiers based on the 
magnitudes of the input & output impedances of an amplifier relative to the sources & load 
impedances respectively as (i) voltage (ii) current (iii) Tran conductance (iv) Tran resistance 
amplifiers. 
VOLTAGE AMPLIFIER: 
The above figure shows a Thevenin’s equivalent circuit of an amplifier. If the input resistance of 
the amplifier Ri is large compared with the source resistance Rs, then Vi = Vs. If the external load 
RL is large compared with the output resistance R0 of the amplifier, then V0 = AV VS .This type 
of amplifier provides a voltage output proportional to the input voltage & the proportionality factor 
doesn’t depend on the magnitudes of the source and load resistances. Hence, this amplifier is 
known as voltage amplifier. An ideal voltage amplifier must have infinite resistance Ri and zero 
output resistance. 
 
CURRENT AMPLIFIER: 
 
Above figure shows a Norton’s equivalent circuit of a current amplifier. If the input resistance of 
the amplifier Ri is very low compared to the source resistance RS, then Ii = IS. If the output 
resistance of the amplifier R0 is very large compared to external load RL, then IL = AiIi = Ai IS. 
This   amplifier   provides   an   output   current   proportional   to   the   signal   current   and the 
proportionally is dependent of the source and load resistance. Hence, this amplifier is called a 
current amplifier. An ideal current amplifier must have zero input resistance & infinite output 
resistance. 
TRANSCONDUCTANCE AMPLIFIER: 
 
The above figure shows the equivalent circuit of a transconductance amplifier. In this circuit, the 
output current I0 is proportional to the signal voltage VS and the proportionality factor is 
independent of the magnitudes of source and load resistances. An ideal transconductance amplifier 
must have an infinite resistance Ri & infinite output resistance R0. 
TRANSRESISTANCE AMPLIFIER: 
Figure above shows the equivalent circuit of a transconductance amplifier. Here, the output voltage 
V0 is proportional to the signal current IS  and the proportionality factor is  independent of 
magnitudes of source and loads resistances. If RS >>Ri , then Ii = IS , Output voltage V0 = RmIS 
An ideal transconductance amplifier must have zero input resistance and zero output resistance. 
 
 
 
 
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feedback amplifiers, effect of feedback on amplifier characteristics, voltage series, voltage shunt, 
current series and current shunt feedback configurations, simple problems; Oscillators: Condition 
for Oscillations, RC type Oscillators RC phase shift and Wien-bridge Oscillators, LC type 
Oscillators, generalized analysis of LC Oscillators, Hartley and Colpitts oscillators. 
 
INTRODUCTION TO FEEDBACK AMPLIFIERS 
Feedback is a common phenomenon in nature. It plays an important role in electronics & control 
systems. Feedback is a process whereby a portion of the output signal of the amplifier is feedback 
to the input of the amplifier. The feedback signal can be either a voltage or a current, being applied 
in series or shunt respectively with the input signal.  
The path over which the feedback is applied is the feedback loop. There are two types of feedback 
used in electronic circuits. (i) If the feedback voltage or current is in phase with the input signal and 
adds to its magnitude, the feedback is called positive or regenerative feedback.(ii) If the feedback 
voltage or current is opposite in phase to the input signal and opposes it , the feedback is called 
negative or regenerative feedback. 
CLASSIFICATION OF AMPLIFIERS: 
Before analyzing the concept of feedback, it is useful to classify amplifiers based on the 
magnitudes of the input & output impedances of an amplifier relative to the sources & load 
impedances respectively as (i) voltage (ii) current (iii) Tran conductance (iv) Tran resistance 
amplifiers. 
VOLTAGE AMPLIFIER: 
The above figure shows a Thevenin’s equivalent circuit of an amplifier. If the input resistance of 
the amplifier Ri is large compared with the source resistance Rs, then Vi = Vs. If the external load 
RL is large compared with the output resistance R0 of the amplifier, then V0 = AV VS .This type 
of amplifier provides a voltage output proportional to the input voltage & the proportionality factor 
doesn’t depend on the magnitudes of the source and load resistances. Hence, this amplifier is 
known as voltage amplifier. An ideal voltage amplifier must have infinite resistance Ri and zero 
output resistance. 
 
CURRENT AMPLIFIER: 
 
Above figure shows a Norton’s equivalent circuit of a current amplifier. If the input resistance of 
the amplifier Ri is very low compared to the source resistance RS, then Ii = IS. If the output 
resistance of the amplifier R0 is very large compared to external load RL, then IL = AiIi = Ai IS. 
This   amplifier   provides   an   output   current   proportional   to   the   signal   current   and the 
proportionally is dependent of the source and load resistance. Hence, this amplifier is called a 
current amplifier. An ideal current amplifier must have zero input resistance & infinite output 
resistance. 
TRANSCONDUCTANCE AMPLIFIER: 
 
The above figure shows the equivalent circuit of a transconductance amplifier. In this circuit, the 
output current I0 is proportional to the signal voltage VS and the proportionality factor is 
independent of the magnitudes of source and load resistances. An ideal transconductance amplifier 
must have an infinite resistance Ri & infinite output resistance R0. 
TRANSRESISTANCE AMPLIFIER: 
Figure above shows the equivalent circuit of a transconductance amplifier. Here, the output voltage 
V0 is proportional to the signal current IS  and the proportionality factor is  independent of 
magnitudes of source and loads resistances. If RS >>Ri , then Ii = IS , Output voltage V0 = RmIS 
An ideal transconductance amplifier must have zero input resistance and zero output resistance. 
 
 
 
 
THE FEEDBACK CONCEPT: 
In each of the above discussed amplifiers, we can sample the output voltage or current by means of 
a suitable sampling network & this sampled portion is feedback to the input through a feedback 
network as shown below. 
 
All the input of the amplifier, the feedback signal is combined with the source signal through a unit 
called mixer. The signal source shown in the above figure can be either a voltage source VS or a 
current source. The feedback connection has three networks. 
1. Sampling network 
2. Feedback network 
3. Mixer network 
SAMPLING NETWORK: 
There are two ways to sample the output, depending on the required feedback parameter. The 
output voltage is sampled by connecting the feedback network in shunt with the output. This is 
called as voltage sampling.  
 
 
R L 
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feedback amplifiers, effect of feedback on amplifier characteristics, voltage series, voltage shunt, 
current series and current shunt feedback configurations, simple problems; Oscillators: Condition 
for Oscillations, RC type Oscillators RC phase shift and Wien-bridge Oscillators, LC type 
Oscillators, generalized analysis of LC Oscillators, Hartley and Colpitts oscillators. 
 
INTRODUCTION TO FEEDBACK AMPLIFIERS 
Feedback is a common phenomenon in nature. It plays an important role in electronics & control 
systems. Feedback is a process whereby a portion of the output signal of the amplifier is feedback 
to the input of the amplifier. The feedback signal can be either a voltage or a current, being applied 
in series or shunt respectively with the input signal.  
The path over which the feedback is applied is the feedback loop. There are two types of feedback 
used in electronic circuits. (i) If the feedback voltage or current is in phase with the input signal and 
adds to its magnitude, the feedback is called positive or regenerative feedback.(ii) If the feedback 
voltage or current is opposite in phase to the input signal and opposes it , the feedback is called 
negative or regenerative feedback. 
CLASSIFICATION OF AMPLIFIERS: 
Before analyzing the concept of feedback, it is useful to classify amplifiers based on the 
magnitudes of the input & output impedances of an amplifier relative to the sources & load 
impedances respectively as (i) voltage (ii) current (iii) Tran conductance (iv) Tran resistance 
amplifiers. 
VOLTAGE AMPLIFIER: 
The above figure shows a Thevenin’s equivalent circuit of an amplifier. If the input resistance of 
the amplifier Ri is large compared with the source resistance Rs, then Vi = Vs. If the external load 
RL is large compared with the output resistance R0 of the amplifier, then V0 = AV VS .This type 
of amplifier provides a voltage output proportional to the input voltage & the proportionality factor 
doesn’t depend on the magnitudes of the source and load resistances. Hence, this amplifier is 
known as voltage amplifier. An ideal voltage amplifier must have infinite resistance Ri and zero 
output resistance. 
 
CURRENT AMPLIFIER: 
 
Above figure shows a Norton’s equivalent circuit of a current amplifier. If the input resistance of 
the amplifier Ri is very low compared to the source resistance RS, then Ii = IS. If the output 
resistance of the amplifier R0 is very large compared to external load RL, then IL = AiIi = Ai IS. 
This   amplifier   provides   an   output   current   proportional   to   the   signal   current   and the 
proportionally is dependent of the source and load resistance. Hence, this amplifier is called a 
current amplifier. An ideal current amplifier must have zero input resistance & infinite output 
resistance. 
TRANSCONDUCTANCE AMPLIFIER: 
 
The above figure shows the equivalent circuit of a transconductance amplifier. In this circuit, the 
output current I0 is proportional to the signal voltage VS and the proportionality factor is 
independent of the magnitudes of source and load resistances. An ideal transconductance amplifier 
must have an infinite resistance Ri & infinite output resistance R0. 
TRANSRESISTANCE AMPLIFIER: 
Figure above shows the equivalent circuit of a transconductance amplifier. Here, the output voltage 
V0 is proportional to the signal current IS  and the proportionality factor is  independent of 
magnitudes of source and loads resistances. If RS >>Ri , then Ii = IS , Output voltage V0 = RmIS 
An ideal transconductance amplifier must have zero input resistance and zero output resistance. 
 
 
 
 
THE FEEDBACK CONCEPT: 
In each of the above discussed amplifiers, we can sample the output voltage or current by means of 
a suitable sampling network & this sampled portion is feedback to the input through a feedback 
network as shown below. 
 
All the input of the amplifier, the feedback signal is combined with the source signal through a unit 
called mixer. The signal source shown in the above figure can be either a voltage source VS or a 
current source. The feedback connection has three networks. 
1. Sampling network 
2. Feedback network 
3. Mixer network 
SAMPLING NETWORK: 
There are two ways to sample the output, depending on the required feedback parameter. The 
output voltage is sampled by connecting the feedback network in shunt with the output. This is 
called as voltage sampling.  
 
 
R L 
FEEDBACK NETWORK: 
This is usually a passive two-port network consisting of resistors, capacitors and inductors. In case 
of a voltage shunt feedback, it provides a fraction of the output voltage as feedback signal Vf to the 
input of the mixer.  
MIXER: 
There are two ways of mixing the feedback signal with the input signal with the input signal as 
shown in figure . below. 
 
When the feedback voltage is applied in series with the input voltage through the feedback network 
as shown in figure 6.7 (a) above, it is called series mixing. Otherwise, when the feedback voltage is 
applied in parallel to the input of the amplifier as shown in figure (b) above, it is called shunt 
feedback. 
GAIN OR TRANSFER RATIO: 
The ratio of the output signal to the input signal of the basic amplifier is represented by the symbol 
A , with proper suffix representing the different quantities. 
TYPES OF FEEDBACK: 
Feedback amplifiers can be classified as positive or negative feedback depending on how the 
feedback signal gets added to the incoming signal. If the feedback signal is of the same sign as the 
incoming signal, they get added & this is called as positive feedback. On the other hand, if the 
feedback signal is in phase inverse with the incoming signal, they get subtracted from each other; it 
will be called as negative feedback amplifier. Positive feedback is employed in oscillators whereas 
negative feedback is used in amplifiers. 
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feedback amplifiers, effect of feedback on amplifier characteristics, voltage series, voltage shunt, 
current series and current shunt feedback configurations, simple problems; Oscillators: Condition 
for Oscillations, RC type Oscillators RC phase shift and Wien-bridge Oscillators, LC type 
Oscillators, generalized analysis of LC Oscillators, Hartley and Colpitts oscillators. 
 
INTRODUCTION TO FEEDBACK AMPLIFIERS 
Feedback is a common phenomenon in nature. It plays an important role in electronics & control 
systems. Feedback is a process whereby a portion of the output signal of the amplifier is feedback 
to the input of the amplifier. The feedback signal can be either a voltage or a current, being applied 
in series or shunt respectively with the input signal.  
The path over which the feedback is applied is the feedback loop. There are two types of feedback 
used in electronic circuits. (i) If the feedback voltage or current is in phase with the input signal and 
adds to its magnitude, the feedback is called positive or regenerative feedback.(ii) If the feedback 
voltage or current is opposite in phase to the input signal and opposes it , the feedback is called 
negative or regenerative feedback. 
CLASSIFICATION OF AMPLIFIERS: 
Before analyzing the concept of feedback, it is useful to classify amplifiers based on the 
magnitudes of the input & output impedances of an amplifier relative to the sources & load 
impedances respectively as (i) voltage (ii) current (iii) Tran conductance (iv) Tran resistance 
amplifiers. 
VOLTAGE AMPLIFIER: 
The above figure shows a Thevenin’s equivalent circuit of an amplifier. If the input resistance of 
the amplifier Ri is large compared with the source resistance Rs, then Vi = Vs. If the external load 
RL is large compared with the output resistance R0 of the amplifier, then V0 = AV VS .This type 
of amplifier provides a voltage output proportional to the input voltage & the proportionality factor 
doesn’t depend on the magnitudes of the source and load resistances. Hence, this amplifier is 
known as voltage amplifier. An ideal voltage amplifier must have infinite resistance Ri and zero 
output resistance. 
 
CURRENT AMPLIFIER: 
 
Above figure shows a Norton’s equivalent circuit of a current amplifier. If the input resistance of 
the amplifier Ri is very low compared to the source resistance RS, then Ii = IS. If the output 
resistance of the amplifier R0 is very large compared to external load RL, then IL = AiIi = Ai IS. 
This   amplifier   provides   an   output   current   proportional   to   the   signal   current   and the 
proportionally is dependent of the source and load resistance. Hence, this amplifier is called a 
current amplifier. An ideal current amplifier must have zero input resistance & infinite output 
resistance. 
TRANSCONDUCTANCE AMPLIFIER: 
 
The above figure shows the equivalent circuit of a transconductance amplifier. In this circuit, the 
output current I0 is proportional to the signal voltage VS and the proportionality factor is 
independent of the magnitudes of source and load resistances. An ideal transconductance amplifier 
must have an infinite resistance Ri & infinite output resistance R0. 
TRANSRESISTANCE AMPLIFIER: 
Figure above shows the equivalent circuit of a transconductance amplifier. Here, the output voltage 
V0 is proportional to the signal current IS  and the proportionality factor is  independent of 
magnitudes of source and loads resistances. If RS >>Ri , then Ii = IS , Output voltage V0 = RmIS 
An ideal transconductance amplifier must have zero input resistance and zero output resistance. 
 
 
 
 
THE FEEDBACK CONCEPT: 
In each of the above discussed amplifiers, we can sample the output voltage or current by means of 
a suitable sampling network & this sampled portion is feedback to the input through a feedback 
network as shown below. 
 
All the input of the amplifier, the feedback signal is combined with the source signal through a unit 
called mixer. The signal source shown in the above figure can be either a voltage source VS or a 
current source. The feedback connection has three networks. 
1. Sampling network 
2. Feedback network 
3. Mixer network 
SAMPLING NETWORK: 
There are two ways to sample the output, depending on the required feedback parameter. The 
output voltage is sampled by connecting the feedback network in shunt with the output. This is 
called as voltage sampling.  
 
 
R L 
FEEDBACK NETWORK: 
This is usually a passive two-port network consisting of resistors, capacitors and inductors. In case 
of a voltage shunt feedback, it provides a fraction of the output voltage as feedback signal Vf to the 
input of the mixer.  
MIXER: 
There are two ways of mixing the feedback signal with the input signal with the input signal as 
shown in figure . below. 
 
When the feedback voltage is applied in series with the input voltage through the feedback network 
as shown in figure 6.7 (a) above, it is called series mixing. Otherwise, when the feedback voltage is 
applied in parallel to the input of the amplifier as shown in figure (b) above, it is called shunt 
feedback. 
GAIN OR TRANSFER RATIO: 
The ratio of the output signal to the input signal of the basic amplifier is represented by the symbol 
A , with proper suffix representing the different quantities. 
TYPES OF FEEDBACK: 
Feedback amplifiers can be classified as positive or negative feedback depending on how the 
feedback signal gets added to the incoming signal. If the feedback signal is of the same sign as the 
incoming signal, they get added & this is called as positive feedback. On the other hand, if the 
feedback signal is in phase inverse with the incoming signal, they get subtracted from each other; it 
will be called as negative feedback amplifier. Positive feedback is employed in oscillators whereas 
negative feedback is used in amplifiers. 
FEATURE OF NEGATIVE FEEDBACK AMPLIFIERS: 
? Overall gain is reduced 
? Bandwidth is improved 
? Distortion is reduced 
? Stability is improved 
? Noise is reduced 
ANALYSIS OF FEEDBACK AMPLIFIER: 
The analysis of the feedback amplifier can be carried out by replacing each active element (BJT, 
FET) by its small signal model and by writing Kirchoff’s loop or nodal equations. Consider the 
schematic representation of the feedback amplifier as shown below.