Power Amplifiers- Chapter Notes, Electronics, Engineering, Semester Electronics and Communication Engineering (ECE) Notes | EduRev

 Page 1


1
POWER AMPLIFIERS
15.1 INTRODUCTION-DEFINITONS AND AMPLIFIER TYPES
An Amplifier receives a signal from some pickup transducer or other input source and provides a 
larger version of the signal to some output device or to another amplifier stage. An input transducer 
signal is generally small (a few millivolts from a cassette or CD input or a few microvolts from an 
antenna) and needs to be amplified sufficiently to operate an output device (speaker or other power-
handling device). In small signal amplifiers, the main factors are usually amplification linearity and 
magnitude of gain , since signal voltage and current are small in a small-signal amplifier, the amount 
of  power-handling capacity and power efficiency are of little concern. A voltage amplifier provides 
voltage amplification primarily to increase the voltage of the input signal. Large-signal or power 
amplifiers, on the other hand, primarily provide sufficient power to an output load to drive a speaker or 
other power device, typically a few watts to tens of watts. In the present chapter, we concentrate on 
those amplifier circuits used to handle large-voltage signals at moderate to high current levels. The 
main features of a large-signal amplifier are the circuit's power efficiency, the maximum amount of 
power that the circuit is capable of handling, and the impedance matching to the output device.
One method used to categorize amplifiers is by class. Basically, amplifier classes represent the
amount the output signal varies over one cycle of operation for a full cycle of input signal. A brief 
description of amplifier classes is provided next.
Class A: The output signal varies for full 360° of the cycle. Figure 15.1 a shows that this requires the
Fig 15.1 Amplifier operation classes
Page 2


1
POWER AMPLIFIERS
15.1 INTRODUCTION-DEFINITONS AND AMPLIFIER TYPES
An Amplifier receives a signal from some pickup transducer or other input source and provides a 
larger version of the signal to some output device or to another amplifier stage. An input transducer 
signal is generally small (a few millivolts from a cassette or CD input or a few microvolts from an 
antenna) and needs to be amplified sufficiently to operate an output device (speaker or other power-
handling device). In small signal amplifiers, the main factors are usually amplification linearity and 
magnitude of gain , since signal voltage and current are small in a small-signal amplifier, the amount 
of  power-handling capacity and power efficiency are of little concern. A voltage amplifier provides 
voltage amplification primarily to increase the voltage of the input signal. Large-signal or power 
amplifiers, on the other hand, primarily provide sufficient power to an output load to drive a speaker or 
other power device, typically a few watts to tens of watts. In the present chapter, we concentrate on 
those amplifier circuits used to handle large-voltage signals at moderate to high current levels. The 
main features of a large-signal amplifier are the circuit's power efficiency, the maximum amount of 
power that the circuit is capable of handling, and the impedance matching to the output device.
One method used to categorize amplifiers is by class. Basically, amplifier classes represent the
amount the output signal varies over one cycle of operation for a full cycle of input signal. A brief 
description of amplifier classes is provided next.
Class A: The output signal varies for full 360° of the cycle. Figure 15.1 a shows that this requires the
Fig 15.1 Amplifier operation classes
2
Q-point to be biased at a level so that at least half the signal swing of the output may vary up and 
down without going to a high-enough voltage to be limited by the supply voltage level or too low to 
approach the lower supply  level, or 0 V in this description
Class B: A class B circuit provides an output signal varying over one-half '.-: input signal cycle, or for 
180° of signal, as shown in Fig. 15.1 b. The dc bias point for class B is therefore at 0 V, with the 
output then varying from this bias point for a half-cycle. Obviously, the output is not a faithful 
reproduction of the input if only one half-cycle is present. Two class B operations-one to provide 
output on the positive output half-cycle and another to provide operation on the negative-output half-
cycle are necessary. The combined half-cycles then provide an output for a full 360° of operation. 
This type of connection is referred to as push-pull operation, which is discussed later in this chapter. 
Note that class B operation by itself creates a very distorted output signal since reproduction of the 
input takes place for only 180° of the output signal swing.
Class AB: An amplifier may be biased at a dc level above the zero base current level of class B and 
above one-half the supply voltage level of class A; this bias condition is class AB. Class AB operation 
still requires a push-pull connection to achieve a full output cycle, but the dc bias level is usually 
closer to the zero base current level for better power efficiency, as described shortly. For class AB 
operation, the output signal swing occurs between 1800 and 3600 and is neither class A nor class B
operation.
Class C: The output of a class C amplifier is biased for operation at Iess than 180 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. This operating class is therefore used in special areas of tuned circuits, such 
as radio or communication.
Class D: This operating class is a form of amplifier operation using pulse (digital) signals, which are 
on for a short interval and off for a longer interval. Using digital techniques makes it possible to obtain 
a signal that varies over the full cycle (using sample-and-hold circuitry) to recreate the output from 
many pieces of input signal. The major advantage of class D operation is that the amplifier is on 
(using power) only for short intervals and the overall efficiency can practically be very high, as 
described next.
Page 3


1
POWER AMPLIFIERS
15.1 INTRODUCTION-DEFINITONS AND AMPLIFIER TYPES
An Amplifier receives a signal from some pickup transducer or other input source and provides a 
larger version of the signal to some output device or to another amplifier stage. An input transducer 
signal is generally small (a few millivolts from a cassette or CD input or a few microvolts from an 
antenna) and needs to be amplified sufficiently to operate an output device (speaker or other power-
handling device). In small signal amplifiers, the main factors are usually amplification linearity and 
magnitude of gain , since signal voltage and current are small in a small-signal amplifier, the amount 
of  power-handling capacity and power efficiency are of little concern. A voltage amplifier provides 
voltage amplification primarily to increase the voltage of the input signal. Large-signal or power 
amplifiers, on the other hand, primarily provide sufficient power to an output load to drive a speaker or 
other power device, typically a few watts to tens of watts. In the present chapter, we concentrate on 
those amplifier circuits used to handle large-voltage signals at moderate to high current levels. The 
main features of a large-signal amplifier are the circuit's power efficiency, the maximum amount of 
power that the circuit is capable of handling, and the impedance matching to the output device.
One method used to categorize amplifiers is by class. Basically, amplifier classes represent the
amount the output signal varies over one cycle of operation for a full cycle of input signal. A brief 
description of amplifier classes is provided next.
Class A: The output signal varies for full 360° of the cycle. Figure 15.1 a shows that this requires the
Fig 15.1 Amplifier operation classes
2
Q-point to be biased at a level so that at least half the signal swing of the output may vary up and 
down without going to a high-enough voltage to be limited by the supply voltage level or too low to 
approach the lower supply  level, or 0 V in this description
Class B: A class B circuit provides an output signal varying over one-half '.-: input signal cycle, or for 
180° of signal, as shown in Fig. 15.1 b. The dc bias point for class B is therefore at 0 V, with the 
output then varying from this bias point for a half-cycle. Obviously, the output is not a faithful 
reproduction of the input if only one half-cycle is present. Two class B operations-one to provide 
output on the positive output half-cycle and another to provide operation on the negative-output half-
cycle are necessary. The combined half-cycles then provide an output for a full 360° of operation. 
This type of connection is referred to as push-pull operation, which is discussed later in this chapter. 
Note that class B operation by itself creates a very distorted output signal since reproduction of the 
input takes place for only 180° of the output signal swing.
Class AB: An amplifier may be biased at a dc level above the zero base current level of class B and 
above one-half the supply voltage level of class A; this bias condition is class AB. Class AB operation 
still requires a push-pull connection to achieve a full output cycle, but the dc bias level is usually 
closer to the zero base current level for better power efficiency, as described shortly. For class AB 
operation, the output signal swing occurs between 1800 and 3600 and is neither class A nor class B
operation.
Class C: The output of a class C amplifier is biased for operation at Iess than 180 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. This operating class is therefore used in special areas of tuned circuits, such 
as radio or communication.
Class D: This operating class is a form of amplifier operation using pulse (digital) signals, which are 
on for a short interval and off for a longer interval. Using digital techniques makes it possible to obtain 
a signal that varies over the full cycle (using sample-and-hold circuitry) to recreate the output from 
many pieces of input signal. The major advantage of class D operation is that the amplifier is on 
(using power) only for short intervals and the overall efficiency can practically be very high, as 
described next.
3
Amplifier Efficiency
The power efficiency of an amplifier, defined as the ratio of power output to power input, improves 
(gets higher) going from class A to class D. In general terms, we see that a class A amplifier, with dc 
bias at one-half the supply voltage level, uses a good amount of power to maintain bias, even with no 
input signal applied. This results in very poor efficiency, especially with small input signals, when very 
little ac power is delivered to the load, In fact, the maximum efficiency of a class A circuit, occurring 
for the largest output voltage and current swing, is only 25% with a direct or series-fed load 
connection and 50% with a transformer connection to the load. Class B operation, with no dc bias 
power for no input signal, can be shown to provide a maximum efficiency that reaches 78.5%. Class 
D operation can achieve power efficiency over 90% and provides the most efficient operation of all 
the operating classes. Since class AB falls between class A and class B in bias, it also falls between 
their efficiency ratings-between 25% (or 50%) and 78.5%. Table 15.1 summarizes the operation of 
the various amplifier classes. 
This table provides a relative comparison of the output cycle operation and power efficiency for the 
various class types. In class B operation, a push-pull connection is obtained using either a 
transformer coupling or by using complementary (or quasi-complementary) operation with npn and
pnp transistors to provide operation on opposite polarity cycles. While transformer operation can 
provide opposite cycle signals, the transformer itself is quite large in many application. A transformer
less circuit using complementary transistors provides the same operation in a much smaller package. 
Circuits and examples are provided later in this chapter.
15.2 SERIES-FED CLASS A AMPLIFIER
This simple fixed-bias circuit connection shown in Fig. 15.2 can be used to discuss the main features 
of a class A series-fed amplifier. The only differences between this circuit and the small-signal version 
considered previously is that the signals handled by the large-signal circuit are in the range of volts 
and the transistor used is a power transistor  that is capable of operating in the range of a few to tens 
of watts. As will be shown in this section, this circuit is not the best to use as a large-signal amplifier
because of its poor power efficiency. The beta of a power transistor is generally less than 100, the 
overall amplifier circuit using power transistors that are capable of handling large power or current 
while not providing much voltage gain.
Page 4


1
POWER AMPLIFIERS
15.1 INTRODUCTION-DEFINITONS AND AMPLIFIER TYPES
An Amplifier receives a signal from some pickup transducer or other input source and provides a 
larger version of the signal to some output device or to another amplifier stage. An input transducer 
signal is generally small (a few millivolts from a cassette or CD input or a few microvolts from an 
antenna) and needs to be amplified sufficiently to operate an output device (speaker or other power-
handling device). In small signal amplifiers, the main factors are usually amplification linearity and 
magnitude of gain , since signal voltage and current are small in a small-signal amplifier, the amount 
of  power-handling capacity and power efficiency are of little concern. A voltage amplifier provides 
voltage amplification primarily to increase the voltage of the input signal. Large-signal or power 
amplifiers, on the other hand, primarily provide sufficient power to an output load to drive a speaker or 
other power device, typically a few watts to tens of watts. In the present chapter, we concentrate on 
those amplifier circuits used to handle large-voltage signals at moderate to high current levels. The 
main features of a large-signal amplifier are the circuit's power efficiency, the maximum amount of 
power that the circuit is capable of handling, and the impedance matching to the output device.
One method used to categorize amplifiers is by class. Basically, amplifier classes represent the
amount the output signal varies over one cycle of operation for a full cycle of input signal. A brief 
description of amplifier classes is provided next.
Class A: The output signal varies for full 360° of the cycle. Figure 15.1 a shows that this requires the
Fig 15.1 Amplifier operation classes
2
Q-point to be biased at a level so that at least half the signal swing of the output may vary up and 
down without going to a high-enough voltage to be limited by the supply voltage level or too low to 
approach the lower supply  level, or 0 V in this description
Class B: A class B circuit provides an output signal varying over one-half '.-: input signal cycle, or for 
180° of signal, as shown in Fig. 15.1 b. The dc bias point for class B is therefore at 0 V, with the 
output then varying from this bias point for a half-cycle. Obviously, the output is not a faithful 
reproduction of the input if only one half-cycle is present. Two class B operations-one to provide 
output on the positive output half-cycle and another to provide operation on the negative-output half-
cycle are necessary. The combined half-cycles then provide an output for a full 360° of operation. 
This type of connection is referred to as push-pull operation, which is discussed later in this chapter. 
Note that class B operation by itself creates a very distorted output signal since reproduction of the 
input takes place for only 180° of the output signal swing.
Class AB: An amplifier may be biased at a dc level above the zero base current level of class B and 
above one-half the supply voltage level of class A; this bias condition is class AB. Class AB operation 
still requires a push-pull connection to achieve a full output cycle, but the dc bias level is usually 
closer to the zero base current level for better power efficiency, as described shortly. For class AB 
operation, the output signal swing occurs between 1800 and 3600 and is neither class A nor class B
operation.
Class C: The output of a class C amplifier is biased for operation at Iess than 180 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. This operating class is therefore used in special areas of tuned circuits, such 
as radio or communication.
Class D: This operating class is a form of amplifier operation using pulse (digital) signals, which are 
on for a short interval and off for a longer interval. Using digital techniques makes it possible to obtain 
a signal that varies over the full cycle (using sample-and-hold circuitry) to recreate the output from 
many pieces of input signal. The major advantage of class D operation is that the amplifier is on 
(using power) only for short intervals and the overall efficiency can practically be very high, as 
described next.
3
Amplifier Efficiency
The power efficiency of an amplifier, defined as the ratio of power output to power input, improves 
(gets higher) going from class A to class D. In general terms, we see that a class A amplifier, with dc 
bias at one-half the supply voltage level, uses a good amount of power to maintain bias, even with no 
input signal applied. This results in very poor efficiency, especially with small input signals, when very 
little ac power is delivered to the load, In fact, the maximum efficiency of a class A circuit, occurring 
for the largest output voltage and current swing, is only 25% with a direct or series-fed load 
connection and 50% with a transformer connection to the load. Class B operation, with no dc bias 
power for no input signal, can be shown to provide a maximum efficiency that reaches 78.5%. Class 
D operation can achieve power efficiency over 90% and provides the most efficient operation of all 
the operating classes. Since class AB falls between class A and class B in bias, it also falls between 
their efficiency ratings-between 25% (or 50%) and 78.5%. Table 15.1 summarizes the operation of 
the various amplifier classes. 
This table provides a relative comparison of the output cycle operation and power efficiency for the 
various class types. In class B operation, a push-pull connection is obtained using either a 
transformer coupling or by using complementary (or quasi-complementary) operation with npn and
pnp transistors to provide operation on opposite polarity cycles. While transformer operation can 
provide opposite cycle signals, the transformer itself is quite large in many application. A transformer
less circuit using complementary transistors provides the same operation in a much smaller package. 
Circuits and examples are provided later in this chapter.
15.2 SERIES-FED CLASS A AMPLIFIER
This simple fixed-bias circuit connection shown in Fig. 15.2 can be used to discuss the main features 
of a class A series-fed amplifier. The only differences between this circuit and the small-signal version 
considered previously is that the signals handled by the large-signal circuit are in the range of volts 
and the transistor used is a power transistor  that is capable of operating in the range of a few to tens 
of watts. As will be shown in this section, this circuit is not the best to use as a large-signal amplifier
because of its poor power efficiency. The beta of a power transistor is generally less than 100, the 
overall amplifier circuit using power transistors that are capable of handling large power or current 
while not providing much voltage gain.
4
Fig 15.2 Series-fed class A large-signal amplifier
DC Bias Operation
The dc bias set by VCC and RB fixes the dc base-bias current at
With the collector current then being
With the collector-emitter voltage then
To appreciate the importance of the dc bias on the operation of the power amplifier, consider the 
collector characteristic shown in Fig. 15.3. An ac load line is drawn using the values of V
CC
and R
C
.. 
The intersection of the dc bias value of I
B
with the dc load line then determines the operating point (Q-
point) for the circuit. The quiescent point values are those calculated using Eqs. (15.1) through (15.3),
If the dc bias collector current is set at one-half the possible signal swing (between 0 and V
CC
/R
C
 ), 
the largest collector current swing will be possible. Additionally, if the quiescent collector-emitter 
voltage is set at one-half the supply voltage, the largest voltage swing will be possible. With the Q-
point set at this optimum bias point, the power considerations for the circuit of Fig. 15.2 are 
determined as described below.
Page 5


1
POWER AMPLIFIERS
15.1 INTRODUCTION-DEFINITONS AND AMPLIFIER TYPES
An Amplifier receives a signal from some pickup transducer or other input source and provides a 
larger version of the signal to some output device or to another amplifier stage. An input transducer 
signal is generally small (a few millivolts from a cassette or CD input or a few microvolts from an 
antenna) and needs to be amplified sufficiently to operate an output device (speaker or other power-
handling device). In small signal amplifiers, the main factors are usually amplification linearity and 
magnitude of gain , since signal voltage and current are small in a small-signal amplifier, the amount 
of  power-handling capacity and power efficiency are of little concern. A voltage amplifier provides 
voltage amplification primarily to increase the voltage of the input signal. Large-signal or power 
amplifiers, on the other hand, primarily provide sufficient power to an output load to drive a speaker or 
other power device, typically a few watts to tens of watts. In the present chapter, we concentrate on 
those amplifier circuits used to handle large-voltage signals at moderate to high current levels. The 
main features of a large-signal amplifier are the circuit's power efficiency, the maximum amount of 
power that the circuit is capable of handling, and the impedance matching to the output device.
One method used to categorize amplifiers is by class. Basically, amplifier classes represent the
amount the output signal varies over one cycle of operation for a full cycle of input signal. A brief 
description of amplifier classes is provided next.
Class A: The output signal varies for full 360° of the cycle. Figure 15.1 a shows that this requires the
Fig 15.1 Amplifier operation classes
2
Q-point to be biased at a level so that at least half the signal swing of the output may vary up and 
down without going to a high-enough voltage to be limited by the supply voltage level or too low to 
approach the lower supply  level, or 0 V in this description
Class B: A class B circuit provides an output signal varying over one-half '.-: input signal cycle, or for 
180° of signal, as shown in Fig. 15.1 b. The dc bias point for class B is therefore at 0 V, with the 
output then varying from this bias point for a half-cycle. Obviously, the output is not a faithful 
reproduction of the input if only one half-cycle is present. Two class B operations-one to provide 
output on the positive output half-cycle and another to provide operation on the negative-output half-
cycle are necessary. The combined half-cycles then provide an output for a full 360° of operation. 
This type of connection is referred to as push-pull operation, which is discussed later in this chapter. 
Note that class B operation by itself creates a very distorted output signal since reproduction of the 
input takes place for only 180° of the output signal swing.
Class AB: An amplifier may be biased at a dc level above the zero base current level of class B and 
above one-half the supply voltage level of class A; this bias condition is class AB. Class AB operation 
still requires a push-pull connection to achieve a full output cycle, but the dc bias level is usually 
closer to the zero base current level for better power efficiency, as described shortly. For class AB 
operation, the output signal swing occurs between 1800 and 3600 and is neither class A nor class B
operation.
Class C: The output of a class C amplifier is biased for operation at Iess than 180 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. This operating class is therefore used in special areas of tuned circuits, such 
as radio or communication.
Class D: This operating class is a form of amplifier operation using pulse (digital) signals, which are 
on for a short interval and off for a longer interval. Using digital techniques makes it possible to obtain 
a signal that varies over the full cycle (using sample-and-hold circuitry) to recreate the output from 
many pieces of input signal. The major advantage of class D operation is that the amplifier is on 
(using power) only for short intervals and the overall efficiency can practically be very high, as 
described next.
3
Amplifier Efficiency
The power efficiency of an amplifier, defined as the ratio of power output to power input, improves 
(gets higher) going from class A to class D. In general terms, we see that a class A amplifier, with dc 
bias at one-half the supply voltage level, uses a good amount of power to maintain bias, even with no 
input signal applied. This results in very poor efficiency, especially with small input signals, when very 
little ac power is delivered to the load, In fact, the maximum efficiency of a class A circuit, occurring 
for the largest output voltage and current swing, is only 25% with a direct or series-fed load 
connection and 50% with a transformer connection to the load. Class B operation, with no dc bias 
power for no input signal, can be shown to provide a maximum efficiency that reaches 78.5%. Class 
D operation can achieve power efficiency over 90% and provides the most efficient operation of all 
the operating classes. Since class AB falls between class A and class B in bias, it also falls between 
their efficiency ratings-between 25% (or 50%) and 78.5%. Table 15.1 summarizes the operation of 
the various amplifier classes. 
This table provides a relative comparison of the output cycle operation and power efficiency for the 
various class types. In class B operation, a push-pull connection is obtained using either a 
transformer coupling or by using complementary (or quasi-complementary) operation with npn and
pnp transistors to provide operation on opposite polarity cycles. While transformer operation can 
provide opposite cycle signals, the transformer itself is quite large in many application. A transformer
less circuit using complementary transistors provides the same operation in a much smaller package. 
Circuits and examples are provided later in this chapter.
15.2 SERIES-FED CLASS A AMPLIFIER
This simple fixed-bias circuit connection shown in Fig. 15.2 can be used to discuss the main features 
of a class A series-fed amplifier. The only differences between this circuit and the small-signal version 
considered previously is that the signals handled by the large-signal circuit are in the range of volts 
and the transistor used is a power transistor  that is capable of operating in the range of a few to tens 
of watts. As will be shown in this section, this circuit is not the best to use as a large-signal amplifier
because of its poor power efficiency. The beta of a power transistor is generally less than 100, the 
overall amplifier circuit using power transistors that are capable of handling large power or current 
while not providing much voltage gain.
4
Fig 15.2 Series-fed class A large-signal amplifier
DC Bias Operation
The dc bias set by VCC and RB fixes the dc base-bias current at
With the collector current then being
With the collector-emitter voltage then
To appreciate the importance of the dc bias on the operation of the power amplifier, consider the 
collector characteristic shown in Fig. 15.3. An ac load line is drawn using the values of V
CC
and R
C
.. 
The intersection of the dc bias value of I
B
with the dc load line then determines the operating point (Q-
point) for the circuit. The quiescent point values are those calculated using Eqs. (15.1) through (15.3),
If the dc bias collector current is set at one-half the possible signal swing (between 0 and V
CC
/R
C
 ), 
the largest collector current swing will be possible. Additionally, if the quiescent collector-emitter 
voltage is set at one-half the supply voltage, the largest voltage swing will be possible. With the Q-
point set at this optimum bias point, the power considerations for the circuit of Fig. 15.2 are 
determined as described below.
5
Fig 15.3Transistor characteristic   showing load line and Q-point
AC Operation
When an input ac signal is applied to the amplifier of Fig. 15.2, the output will vary from its dc bias 
operating voltage and current. A small input signal, as shown in Fig. 15.4, will cause the base current 
to vary above and below the dc bias point, which will then cause the collector current (output) to vary 
from the dc bias point set as well as the collector-emitter voltage to vary around its dc bias value. 
Fig 15.4 Amplifier input and output signal variation
As the input signal is made larger, the output will vary further around the established dc bias point 
until either the current or the voltage reaches a limiting condition. For the current this limiting condition