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.
Page 2
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.
Page 3
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
Page 4
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.
Page 5
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.
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