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AMPLIFIER
An amplifier is an electronic
device that can
amplify(enlarge)the electronic
signal i.e voltage, current or
power.
 According
to the class of operation,
the amplifiers can be classified as:
Class A
Class B
Class AB
Class C
CLASS A

Class A: A class A amplifier is one in which the
operating point and the input signal are such
that the current in the output circuit flows at
full times.
The output signal of class A amplifiers varies for
a full 360o of the cycle
CLASS A OUTPUT STAGE
 Class
A output stage is a simple linear
current amplifier.
 It is also very inefficient, typical
maximum efficiency is 25%
 Only suitable for low power
applications.
 High power requires much better
efficiency.
CLASS A POWER AMPLIFIER WITH
CORRECT OUTPUT VOLTAGE SWING.
MAXIMUM CLASS A OUTPUT OCCURS
WHEN THE Q-POINT IS CENTERED ON THE
AC LOAD LINE.
Q-POINT CLOSER TO CUTOFF.
Q-POINT CLOSER TO SATURATION
CLASS B


Class B: A class B amplifier is one in which the
operating point is at an extreme end of its
characteristic, so that the quiescent power is very
small. If the signal voltage is sinusoidal,
amplification takes place for only one-half a
cycle.
A class B circuit provides an output signal
varying over one-half the input signal cycle, of for
180o of input signal.
Class B operation is provided when the dc bias
transistor biased just off, the transistor turning on
when the ac signal is applied.
 This is essentially no bias and the transistor
conducts current for only one-half of the signal cycle.

A diagram for push-pull operation.
To obtain output for the full cycle of signal, it is
necessary to use two transistors and have each conduct
on opposite half-cycles, the combined operation
providing a full cycle of output signal.
Since one part of the circuit pushes the signal high
during one-half cycle and the other part pulls the signal
low during the other half-cycle, the circuit is referred to
as a push-pull circuit.




When the signal on transistor Q1 is positive, the
signal on Q2 is negative by an equal amount.
During the first half-cycle of operation, transistor Q1
is driven into conduction, whereas transistor Q2 is
driven off. The current i1 through the transformer
results in the first half-cycle of signal to the load.
During the second half-cycle of operation, transistor
Q2 is driven into conduction, whereas transistor Q1
is driven off. The current i2 through the transformer
results in the second half-cycle of signal to the load.
The overall signal developed across the load then
varies over the full cycle of signal operation.
Class B amplifier operation
The transistor circuit of Fig. 18-8 operates class B
if R2=0 because a silicon transistor is essentially
at cutoff if the base is shorted to the emitter.
Advantages of Class B as compared with class
A operation
1. It is possible to obtain greater power output,
2. The efficiency is higher, and
3. There is negligible power loss at no signal.
Disadvantages of Class B as compared with
class A operation
1. The harmonic distortion is higher,
2. Self-bias cannot be used, and
3. The supply voltage must have good regulation.
Input power: The power supplied to the load by an amplifier is
drawn from the power supply which provides the input or dc
power.
The amount of this power can be calculated using
Pi (dc)  VCC I dc
(16.17)
In class B operation the current drawn from a single power
supply has the same form of a full-wave rectified signal. So,
I dc 
2

I ( p) (16.18)
and Pi (dc)  VCC
2

I (p) (16.19)
Output Power: The output across the load is given by
VL2 (rms )
Po (ac) 
(16.20)
RL
Po (ac) 
VL2 (p)
2 RL

VL2 (p - p)
8 RL
(16.21)
Efficiency: The efficiency of the class B amplifier can
be calculated using the basic equation
Po (ac)
% 
100%
Pi (dc)
% 
VL2 (p)

2 RL 2VCC I ( p )
100%
V ( p)
using, I (p)  L
RL
VL2 (p)

RL
% 
100%
2 RL 2VCC VL (p)
% 
 VL (p)
4 VCC
100% (16.22)
Maximum Efficiency:
Equation (16.22) shows that the
larger the peak voltage, the higher the circuit efficiency, up to a
maximum value when the , this maximum efficiency then being
VCC
Maximum, I (p) 
RL
Maximum,
2 VCC
Maximum, I dc 
 RL
2
2VCC
2 VCC
Pi (dc)  VCC

 RL
RL
2
VCC
Maximum, Po (ac) 
2 RL
2
VCC
RL 
Maximum, % 

 100%  78.5%
2
2 RL 2V
4
CC
The maximum efficiency of a class B amplifier is
thus seen to be 78.5%.
CROSS-OVER DISTORTION
A small base-emitter
voltage is needed to turn on
a transistor
 Q1 actually only conducts
when vin > 0.7 V
 Q2 actually only conducts
when vin < -0.7 V
 When 0.7 > vin > -0.7,
nothing conducts and the
output is zero.
 i.e. the input-output
relationship is not at all
linear.

ACTUAL INPUT-OUTPUT CURVE
vout
vout  vin  VBE
-VBE
+VBE
vin
vout  vin  VBE
Crossover Distortion
EFFECT OF CROSS-OVER DISTORTION
CLASS B
A class B output stage can be far more efficient than a class A
stage (78.5 % maximum efficiency compared with 25 %).
It also requires twice as many output transistors…
…and it isn’t very linear; cross-over distortion can be
significant. Class B amplifiers are used in low cost designs or
designs where sound quality is not that important.
Class B amplifiers are significantly more efficient than class
A amps.
They suffer from bad distortion when the signal level is low
(the distortion in this region of operation is called "crossover
distortion").
Class B is used most often where economy of design is
needed.
Before the advent of IC amplifiers, class B amplifiers were
common in clock radio circuits, pocket transistor radios, or
other applications where quality of sound is not that critical.
CLASS AB


Class AB: A class AB amplifier is one operating
between the two extremes defined for class A and
class B. Hence the output signal is zero for part
but less than one-half of an input sinusoidal
signal.
For class AB operation the output signal swing
occurs between 180o and 360o and is neither class
A nor class B operation.
A CLASS AB PUSH-PULL
OUTPUT VOLTAGE.
AMPLIFIER WITH CORRECT
A DARLINGTON CLASS AB PUSH-PULL AMPLIFIER.
LOAD LINES FOR A COMPLEMENTARY SYMMETRY PUSH-PULL
AMPLIFIER. ONLY THE LOAD LINES FOR THE NPN TRANSISTOR ARE
SHOWN.
CLASS AB

With such amplifiers, distortion is worst when the signal is
low, and generally lowest when the signal is just reaching
the point of clipping.
Class AB amps use pairs of transistors, both of them being
biased slightly ON so that the crossover distortion
(associated with Class B amps) is largely eliminated.
Class AB is probably the most common amplifier class
currently used in home stereo and similar amplifiers.

Class AB amps combine the good points of class A and B
amps.
They have the improved efficiency of class B amps and
distortion performance that is a lot closer to that of a class A
amp.
CLASS C
Class C: A class C amplifier is one in which the
operating point is chosen so that the output
current (or voltage) is zero for more than one-half
of the input sinusoidal signal cycle.
 The output of a class C amplifier is biased for
operation at less than 180o of the cycle and will
operate only with a tuned (resonant) circuit which
provides a full cycle of operation for the tuned or
resonant frequency.

CLASS C

Class C amps are never used for audio circuits.

They are commonly used in RF circuits.

Class C amplifiers operate the output transistor in a
state that results in tremendous distortion (it would
be totally unsuitable for audio reproduction).
FIGURE 9-22
BASIC CLASS C AMPLIFIER OPERATION (NON INVERTING).
FIGURE 9-23
BASIC CLASS C OPERATION.
FIGURE 9-24
CLASS C WAVEFORMS.
FIGURE 9-25
TUNED CLASS C AMPLIFIER.
CLASS C


However, the RF circuits where Class C amps
are used, employ filtering so that the final signal
is completely acceptable.
Class C amps are quite efficient.