Electronic Circuits 10CS32

Unit – 5: Large Signal Amplifiers

Large signal or power amplifiers provide power amplification and are used in applications to provide
sufficient power to the load or the power device. The output power delivered by these amplifiers is of
the order of few watts to few tens of watts. They handle moderate-to-high levels of current and
voltage signals as against small levels of current and voltage signals in the case of small signal
amplifiers.

5.1 Classification of Large Signal Amplifiers

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On the basis of their circuit configurations and principle of operation, amplifiers are classified into
different classes.

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Class A Amplifiers

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A class A power amplifier is defined as a power amplifier in which output current flows for the full-
cycle (360°) of the input signal. In other words, the transistor remains forward biased throughout the
input cycle. The active device in a class A amplifier operates during the whole during the whole of the

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input cycle and the output signal is an amplified replica of the input signal with no clipping. The
amplifying element is so biased that it operates over the linear region of its output characteristics
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during full period of the input cycle and is always conducting to some extent.
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Class A amplifiers offer very poor efficiency and a maximum of 50% efficiency is possible in these
amplifiers
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Class B Amplifier
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Class B amplifiers operate only during the half of the input cycle. Class B amplifiers offer much
improved efficiency over class A amplifiers with a possible maximum of 78.5%. They also create a
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large amount of distortion.

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Class AB Amplifier

In a class AB amplifier, the amplifying device conducts for a little more than half of the input
waveform. They sacrifice some efficiency over class B amplifiers but they offer better linearity than
class B amplifier. They offer much more efficiency than class A amplifiers.

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Class C Amplifiers

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Class C amplifiers conducts for less than half cycle of the input signal resulting in a very high
efficiencies up to 90%. But they are associated with very high level of distortion at the output. Class C
amplifiers operate in two modes, namely, the tuned mode and the unturned mode.
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Class D Amplifiers
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Class D amplifiers use the active device in switching mode to regulate the output power. These
amplifiers offer high efficiencies and do not require heat sinks and transformers. These amplifiers use
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pulse width modulation (PWM), pulse density modulation or sigma delta modulation to convert the
input signal into a string of pulses. The pulse width of the PWM output waveform at any time instant
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is directly proportional to the amplitude of the input signal.

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Electronic Circuits 10CS32

5.2 Large signal Amplifier Characteristics

The main characteristics that define the performance of a power amplifier are efficiency, distortion
level and output power.

Efficiency

Efficiency of an amplifier is defined as the ability of the amplifier to convert the DC input power of
the supply into an AC output power that can be delivered to the load. The expression for efficiency is

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given by

Po
´ 100%

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h =
Pi

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Where, Po is the AC output power delivered to the load and

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Pi is the DC input power.

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Harmonic Distortion
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Distortion in large signal amplifiers is mainly caused due to harmonic distortion. Harmonic distortion
refers to the distortion in the amplitude of the output signal of an amplifier caused due to the non-
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linearity in the characteristics of the active device used for amplification. In other words, the active
device does not equally amplify all portions of the input signal over its positive and negative
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excursions. The distortion is more in the case of a large input signal level.
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The total current of an amplifier is given by Iot = Io + IQ

Where Io is the alternating portion of the output current and

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IQ is the output DC current under quiescent condition.

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Where Io = A0 + A1cosωt + A2cos2ωt

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\ Iot = IQ + A0 + A1cosωt + A2cos2ωt

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Where A0 is the extra DC component due to rectification of the signal;

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A1 is the amplitude of the desired signal at the fundamental input signal frequency ω;
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A2 is the amplitude of the desired signal at the fundamental input signal frequency 2ω;

Second harmonic distortion is a measure of the relative amount of second harmonic

A2
component to the fundamental frequency component and is expressed as D 2 = ´ 100%
A1

A3
IIIly the third harmonic distortion component (D3) is given by D3 = ´ 100%
A1

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Electronic Circuits 10CS32

A4
The fourth harmonic distortion component (D3) is given by D 4 = ´ 100%
A1

The total harmonic distortion (D) is given by the square root of the mean square of the individual
harmonic components.

D = D 22 + D32 + D 42 + ...

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The power delivered at the fundamental frequency is P1 =
2

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RL

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The total power output is given by P = (A12 + A22 + A32 + ...)´
2

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Therefore, P = (1 + D 22 + D32 + ...)´ P1

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\ P = (1 + D 2 ) ´ P1 or
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5.3 Feedback Amplifiers

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Classification of Amplifiers
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On the basis of the input and output parameters of interest, amplifiers are classified as voltage
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amplifiers, current amplifiers, transresistance and transconductance amplifiers.

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In the case of a voltage amplifier, a small change in the input voltage produces a large change in the
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output voltage. Voltage gain, which is the ratio of the change in the output voltage to change in the
input voltage, is the gain parameter.
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In the case of a current amplifier, a small change in the input current produces a large change in the
output current. Current gain, which is the ratio of change in the output current to the change in the
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input current, is the gain parameter.

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In the case of a transresistance amplifier, a small change in the input current produces a large change
in the output voltage. Ratio of the change in the output voltage to change in the input current is the
gain parameter. The gain parameter has the units of resistance.
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In the case of a transconductance amplifier, a small change in the input voltage produces a large
change in the output current. Ratio of the change in the output current to the change in the input
voltage is the gain parameter. The gain parameter has the units of conductance.

Amplifier with Negative Feedback

In a negative feedback amplifier, a sample of the output signal is fed back to the input and the
feedback signal is combined with the externally applied input signal in a subtractor circuit as shown in
the figure below.

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Electronic Circuits 10CS32

Xi
+ Basic amplifier
XS Σ (A)
Xo

-
Xf = βXo

Feedback network
(β)

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The actual signal Xi applied to the basic amplifier is the difference of the externally applied input
signal Xs and the feedback signal Xf. The generalized representation of input and output signals is
intended to indicate that the signal could either be a current or a voltage signal.

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Is Ii Io
Signal + Ii + Basic Sampling
+
Vs Subtractor Vo RL

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Source - - Amplifier (A) Network -

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If

+
or
Feedback
Vf
Network (β)
-
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The gain parameters could be a voltage gain, current gain, transresistance or transconductance. The
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gain has two values namely gain of the basic amplifier without feedback and the gain of the amplifier
with feedback.
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From the above circuit, the actual signal applied to the amplifier input X i is the difference of
externally applied input signal Xs and the feedback signal Xf. It is given by
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Xi = Xs – Xf --- (1)
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Also, Xf = βXo, Xo = AXi and Xo = AfXs

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Substituting for Xi and Xf in eq (1) we get

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Xo
= X s - βXo
A
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Simplifying the equation we get

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Xo A
Af = =
Xs 1 + βA

The feedback is expressed in decibels. It is given by

Af 1
dB of feedback = 20log = 20log
A 1 + βA

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The three important assumptions to be satisfied by a feedback network is,

1. The input signal is transmitted to the output through the amplifier only and not through the
feedback network. That is, forward transmission through feedback network is zero. This
further implies that if the gain of the amplifier were reduced to zero, the output must drop to
zero.
2. The feedback signal is transmitted from output to input through feedback network only. That
is, reverse transmission through amplifier is zero.
3. The feedback factor is independent of load and source resistance.

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Advantages of Negative Feedback

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1. Gain parameter is independent of variation in values of components used in building the basic
amplifier.

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2. Bandwidth is increased.
3. Distortion is reduced and linearity is increased.
4. Noise is reduced.

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5. Input resistance can be reduced or decreased depending on the feedback topology.
6. Output resistance can be reduced or decreased depending on the feedback topology.

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5.4 Stability of Gain Factor or
The introduction of negative feedback makes the amplifier insensitive to variations in the values of
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the components and parameters of the active devices used in building the amplifier. The expression
that relates percentage variation in the gain parameter of the amplifier with feedback to the percentage
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variation in the same without feedback can be derived as follows.

A
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Af =
1 + βA
s

Differentiating the above equation with respect to A we get,

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dAf
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1 dA
= X
Af 1 + βA A
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|(1+βA)| is called the desensitivity parameter D. thus, percentage variation in gain with
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feedback is equal to the percentage variation in gain without feedback divided by the desensitivity
parameter D.
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Effect on bandwidth
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Bandwidth increases with introduction of negative feedback. Increase in bandwidth results from the
fact that amplifier exhibit a constant gain-bandwidth product. Reduction in gain is, therefore,
accompanied by increase in bandwidth. Bandwidth increases by the same desensitivity factor D =
1+βA by which the gain reduces. Bandwidth of amplifier with feedback is given by

(BW)f = BW X (1+βA)

Effect on Non-Linear Distortion

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Electronic Circuits 10CS32

It can be proved that non-linear distortion decreases by the desensitivity factor D = 1+βA. Let us
assume that the distortion levels without and with negative feedback are D2 and D2f, respectively. D 2
is the distortion contributed by the active device. In the presence of feedback, D 2f appears as –βAD2f
at the input of the amplifier. The following gives the expression for D2f.

D2
D2-βAD2f = D2f =
1 + βA

Effect on Noise

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Introduction of negative feedback acts on the noise generated in the amplifier in the same manner as it
does on the non-linear distortion. Let us assume Nf and N are the noise levels with and without

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negative feedback respectively. Reduction in noise is governed by

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N
Nf =
1 + βA

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Effect on Input Resistance

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Input resistance in the case of an amplifier with negative feedback is affected depending upon how the
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feedback signal is connected to the source of external input signal. The input resistance increases if
the feedback signal is connected in series with the source of input and decreases if the feedback signal
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is connected across it in shunt.
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Thus in the case of voltage-series and current-series feedback, Ri is the input resistance without
feedback and the input resistance without feedback Rif is given by
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Rif = Ri X (1+βA)
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Input resistance in the case of voltage-shunt and current-shunt feedback is given by

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Ri
Rif =
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1 + βA
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Effect on Output Resistance

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Output resistance in the case of an amplifier with negative feedback is affected depending upon how
the feedback signal is connected to the source of external input signal. The output resistance increases
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in the case of output is current and decreases in the case of output is voltage.

Thus in the case of current-series and current-shunt feedback, Ri is the input resistance without
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feedback and the input resistance without feedback Rif is given by

Rof = Ro X (1+βA)

Input resistance in the case of voltage-series and voltage-shunt feedback is given by

Ro
R of =
1 + βA

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5.5 Feedback Topologies

On the basis of the nature of sampled signal and the mode in which it is fed back to the input, there
are four feedback topologies. They are

1. Voltage-series feedback topology (series-shunt)

2. Voltage-shunt feedback topology (shunt-shunt)
3. Current-series feedback topology (series-series)
4. Current-shunt feedback topology (shunt-series)

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Voltage-Series Feedback (Series-Shunt)

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RS
+ +

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+ Basic Voltage
V sVi Vi Amplifier Vo RL
_ _ _

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(Av)
- Vf +

Feedback

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or
network
(β)
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Schematic arrangement of voltage-series (series-shunt) feedback

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RS Ii Ro Io
+ + + +
Is VsVi Vi RL Vo RL
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A vV i
_ _ _ _
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- Vf +
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+ +
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βVo Vo
_ _
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Equivalent circuit of voltage-series feedback

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Electronic Circuits 10CS32

5.6 Recommended Questions

1. List the important features of MOSFET.

2. With the help of neat diagrams explain the operation and drain characteristics of n-
channel depletion type MOSFET. Explain clearly the mechanism of “Pinch-off
Condition”. Sketch the drain characteristics and define rd.
3. Explain the constructional features of a depletion mode MOSFET and explain its
basic operation.
4. With the help of neat diagram explain the operation of an n-channel enhancement

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type MOSFET.
5. Sketch the drain characteristics of MOSFET for different values of VGS and mark
different region of operation.

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6. Draw and explain the drain characteristics of n-channel enhancement type MOSFET.

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7. Sketch the graphic symbols for: n-channel JFET, p-channel JFET, n-channel
enhancement type MOSFET, p-channel enhancement type MOSFET, n-channel

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depletion type MOSFET and p-channel depletion type MOSFET.
8. Explain the structure of the depletion mode MOSFET. And the D-MOSFET curves.

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(July-2008)
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