Amplifier Basics
OBJECTIVES
After
completing this chapter, the student
should be able to:
•
•
•
•
Describe the purpose of an amplifier.
Identify the three basic configurations of transistor amplifier circuits.
Identify the classes of amplifiers.
Describe the operation of direct coupled amplifiers, audio amplifiers, video
amplifiers, RF amplifiers, IF amplifiers, and operational amplifiers.
• Draw and label schematic diagrams for the different types of amplifier circuits.
Amplifiers are electronic circuits that are used to increase the amplitude of an electronic signal. A circuit designed to convert a low voltage to a higher voltage is called a voltage amplifier. A circuit designed to convert a low current to a higher current is called a current amplifier.
See accompanying CD for Amplifier Basics examples in MultiSim,
Chapter 28.
AMPLIFIER
CONFIGURATIONS
In order for a transistor to provide amplification, it
must be able to accept an input signal and produce
an output signal that is greater than the input.
The input signal controls current flow in the
transistor. This, in turn, controls the voltage
through the load. The transistor circuit is designed to take voltage from an external power
source (Vee)and apply it to a load resistor (RL) in
the form of an output voltage. The output voltage
is controlled by a small input voltage.
The transistor is used primarily as an amplifying device. However, there is more than
one way of achieving this amplification. The
transistor can be connected in three different
circuit configurations.
The three circuit configurations are the
common-base circuit, the common-emitter circuit, and
the common-collector circuit. In each configuration,
one of the transistor's leads serves as a common
reference point and the other two leads serve
as input and output connections. Each configuration can be constructed using either NPN or PNP
transistors. In each, the transistor's emitter-base
267
SECTION 4 LINEAR ELECTRONIC
CIRCUITS
FIGURE 28-1
FIGURE 28-3
Common-base amplifier circuit.
Common-collector amplifier circuit.
Q
+
-=-
4rl
-=-
Vcc
FIGURE 28-2
VBB
the output leaves from the collector-base circuit.
The base is the element common to both the input and output circuits.
In the common-emitter circuit (Figure 28-2)
the input signal enters the base-emitter circuit,
and the output signal leaves from the collectoremitter circuit. The emitter is common to both the
input and output circuit. This method of connecting a transistor is the most widely used.
The third type of connection (Figure 28-3) is
the common-collector
circuit. In this configuration, the input signal enters the base-collector
circuit, and the output signal leaves from the
emitter-collector
circuit. Here, the collector is
common to both the input and output circuits.
This circuit is used as an impedance-matching
circuit.
Figure 28-4 charts the input-output
resistance and voltage, current, and power gains for the
Common-emitter amplifier circuit.
junction is forward biased, while the collectorbase junction is reverse biased. Each configuration offers advantages and disadvantages.
In the common-base circuit (Figure 28-1) the
input signal enters the emitter-base circuit, and
FIGURE 28-4
Amplifier circuit characteristics.
CIRCUIT
TYPE
INPUT
RESISTANCe
OUTPUT
RESISTANCE
VOLTAGE
GAIN
CURRENT
GAIN
POWER
GAIN
COMMON BASE
Low
High
High
Less than 1
Medium
COMMON EMITTER
Medium
Medium
Medium
Medium
High
COMMON COLLECTOR
High
Low
Less than 1
Medium
Medium
I
269
CHAPTER 28 AMPLIFIER BASICS
FIGURE 28-5
FIGURE 28-6
Amplifier circuit input-output phase relationships.
Common-emitter amplifier with single voltage source.
AMPLIfiER
TYPE
COMMON
BASE
COMMON
EMITTER
COMMON
COLLECTOR
INPUT
WAVEFORM
=.
OUTPUT
WAVEFORM
+
+ -i+ +
three circuit configurations. Figure 28-5 shows
the phase relationship of input and output waveforms for the three configurations. Note that the
common-emitter configuration provides a phase
reversal of the input-output signal.
28-1
QUESTIONS
1. Draw schematic diagrams of the three
basic configurations of transistor
amplifier circuits.
2. List the characteristics of the:
a. Common-base circuit
b. Common-emitter circuit
c. Common-collector circuit
3. Make a chart showing the input-output
phase relationship of the three
configurations.
4. Make a chart showing the input -output
resistances of the three configurations.
5. Make a chart showing the voltage,
current, and power gains of the three
configurations.
OUTPUT
INPUT
o--+------iH
+
Vcc
..=..
GROUND ':"
AMPLIFIER BIASING
The basic configurations of transistor amplifier circuits are the common base, the common emitter,
and the common collector. All require two voltages for proper biasing. The base-emitter junction
must be forward biased and the base-collector
junction must be reverse biased. However, both
bias voltages can be provided from a single source.
Because the common-emitter circuit configuration is the most often used, it is described in detail here. The same principles apply to the
common-base and common-collector circuits.
Figure 28-6 shows a common-emitter transistor amplifier using a single voltage source. The circuit is schematically diagrammed in Figure 28-7.
The voltage source is identified as +Vcc The
ground symbol is the negative side of the voltage
source Vcc The single voltage source provides
proper biasing for the base-emitter and basecollector junctions. The two resistors (RB and RL)
are used to distribute the voltage for proper operation. Resistor RL'the collector load resistor, is in
series with the collector. When a collector current
flows, a voltage develops across resistor RL.The
voltage dropped across resistor RLand the voltage
SECTION 4 LINEAR ELECTRONIC
CIRCUITS
-
FIGURE 28-7
Schematic representation of common-emitter
amplifier with single voltage source.
....-----
FIGURE 28-8
Common-emitter amplifier with collector feedback.
-
..... +Vcc
OUTPUT
INPUT o-------4----H
OUTPUT
INPUT 0--+----+-1
dropped across the transistor's collector-to-emitter
junction must add up to the total voltage applied.
Resistor RB'connected between the base and
the voltage source, controls the amount of current flowing out of the base. The base current
flowing through resistor RB creates a voltage
across it. Most of the voltage from the source is
dropped across it. A small amount of voltage
drops across the transistor's base-to-emitter junction, providing the proper forward bias.
The single voltage source can provide the necessary forward-bias and reverse-bias voltages. For
an NPN transistor, the transistor's base and collector must be positive with respect to the emitter.
Therefore, the voltage source can be connected to
the base and the collector through resistors RBand
RL.This circuit is often called a base-biased circuit,
because the base current is controlled by resistor
RBand the voltage source.
The input signal is applied between the transistor's base and emitter or between the input terminal and ground. The input signal either aids or
opposes the forward bias across the emitter junction. This causes the collector current to vary,
which then causes the voltage across RLto vary.
The output signal is developed between the output terminal and ground.
The circuit shown in Figure 28-6 is unstable,
because it cannot compensate for changes in the
bias current with no signal applied. Temperature
changes cause the transistor's internal resistance
to vary, which causes the bias currents to change.
This shifts the transistor operating point, reducing
the gain of the transistor. This process is referred
to as thermal instability.
It is possible to compensate for temperature
changes in a transistor amplifier circuit. If a portion of the unwanted output signal is fed back to
the circuit input, the signal opposes the change.
This is referred to as degenerative or negative feedback (Figure 28-8). In a circuit using degenerative
feedback, the base resistor RBis connected directly
to the collector of the transistor. The current flowing through resistor RBis determined by the voltage at the collector. If the temperature increases,
the collector current increases, and the voltage
across RLincreases. The collector-to-emitter voltage decreases, reducing the voltage applied to RB.
This reduces the base current, which causes the
collector current to decrease. This is referred to as
a collectorfeedback circuit.
Figure 28-9 shows another type of feedback. The circuit is similar to the one shown in
Figure 28-7, except that a resistor RE is connected in series with the emitter lead. Resistors
RBand REand the transistor'S emitter-base junc-
CHAPTER 28 AMPLIFIER BASICS
FIGURE 28-5
FIGURE 28-6
Amplifier circuit input-output phase relationships.
Common-emitter amplifier with single voltage source.
AMPLIFIER
TYPE
COMMON
BASE
COMMON
EMITTER
COMMON
COLLECTOR
INPUT
WAVEFORM
-4J-4J{\
V
=.
OUTPUT
WAVEFORM
-i-4J-
three drcuit configurations. Figure 28-5 shows
the phase relationship of input and output waveforms for the three configurations. Note that the
common-emitter configuration provides a phase
reversal of the input-output signal.
28-1
QUESTIONS
1. Draw schematic diagrams of the three
basic configurations of transistor
amplifier circuits.
2. List the characteristics of the:
a. Common-base circuit
b. Common-emitter circuit
c. Common-collector circuit
3. Make a chart showing the input-output
phase relationship of the three
configurations.
4. Make a chart showing the input-output
resistances of the three configurations.
5. Make a chart showing the voltage,
current, and power gains of the three
configurations.
OUTPUT
INPUT O--- ..•...
---+-f
+
Vcc
GROUND
-=-
":'
AMPLIFIER BIASING
The basic configurations of transistor amplifier circuits are the common base, the common emitter,
and the common collector. All require two voltages for proper biasing. The base-emitter junction
must be forward biased and the base-collector
junction must be reverse biased. However, both
bias voltages can be provided from a single source.
Because the common-emitter circuit configuration is the most often used, it is described in detail here. The same prindples apply to the
common-base and common-collector circuits.
Figure 28-6 shows a common-emitter transistor amplifier using a single voltage source. The circuit is schematically diagrammed in Figure 28-7.
The voltage source is identified as + Vcc The
ground symbol is the negative side of the voltage
source Vcc The single voltage source provides
proper biasing for the base-emitter and basecollector junctions. The two resistors (RB and RL)
are used to distribute the voltage for proper operation. Resistor RL'the collector load resistor, is in
series with the collector. When a collector current
flows, a voltage develops across resistor RL.The
voltage dropped across resistor RLand the voltage
SECTION 4 LINEAR ELECTRONIC
FIGURE 28-7
Schematic representation of common-emitter
amplifier with single voltage source.
CIRCUITS
-
FIGURE 28-8
Common-emitter amplifier with collector feedback.
-
.------ ...• +Vcc
OUTPUT
INPUT
o----~----H
OUTPUT
INPUT
o--4----H
dropped across the transistor's collector-to-emitter
junction must add up to the total voltage applied.
Resistor RB'connected between the base and
the voltage source, controls the amount of current flowing out of the base. The base current
flowing through resistor RB creates a voltage
across it. Most of the voltage from the source is
dropped across it. A small amount of voltage
drops across the transistor's base-to-emitter junction, providing the proper forward bias.
The single voltage source can provide the necessary forward-bias and reverse-bias voltages. For
an NPN transistor, the transistor's base and collector must be positive with respect to the emitter.
Therefore, the voltage source can be connected to
the base and the collector through resistors RBand
RL.This circuit is often called a base-biased circuit,
because the base current is controlled by resistor
RBand the voltage source.
The input signal is applied between the transistor's base and emitter or between the input terminal and ground. The input signal either aids or
opposes the forward bias across the emitter junction. This causes the collector current to vary,
which then causes the voltage across RLto vary.
The output signal is developed between the output terminal and ground.
The circuit shown in Figure 28-6 is unstable,
because it cannot compensate for changes in the
bias current with no signal applied. Temperature
changes cause the transistor's internal resistance
to vary, which causes the bias currents to change.
This shifts the transistor operating point, reducing
the gain of the transistor. This process is referred
to as thermal instability.
It is possible to compensate for temperature
changes in a transistor amplifier circuit. If a portion of the unwanted output signal is fed back to
the circuit input, the signal opposes the change.
This is referred to as degenerative or negative feedback (Figure 28-8). In a circuit using degenerative
feedback, the base resistor RBis connected directly
to the collector of the transistor. The current flowing through resistor RBis determined by the voltage at the collector. If the temperature increases,
the collector current increases, and the voltage
across RLincreases. The collector-to-emitter voltage decreases, reducing the voltage applied to Rw
This reduces the base current, which causes the
collector current to decrease. This is referred to as
a collectorfeedback circuit.
Figure 28-9 shows another type of feedback. The circuit is similar to the one shown in
Figure 28-7, except that a resistor RE is connected in series with the emitter lead. Resistors
RBand REand the transistor'S emitter-base junc-
CHAPTER 28 AMPLIFIER BASICS
FIGURE 28-9
Common-emitter amplifier with emitter feedback.
.------- ....
+Vcc
OUTPUT
INPUT 0----
.•....
---+-1
tion are connected in series with the source
voltage VccAn increase in temperature causes the collector current to increase. The emitter current then
also increases, causing the voltage drop across resistor REto increase and the voltage across resistor
RB to decrease. The base current then decreases,
which reduces both the collector current and the
emitter current. Because the feedback is generated at the transistor's emitter, the circuit is called
an emitter feedback circuit.
There is a problem with this type of circuit because an AC input signal develops across resistor
RE as well as across the load resistor RL and the
transistor. This reduces the overall gain of the cir-
-
FIGURE 28-10
Emitter feedback with bypass capacitor.
cuit. With the addition of a capacitor across the
emitter resistor RE (Figure 28-10), the AC signal
bypasses resistor RE. The capacitor is often referred to as a bypass capacitor.
The bypass capacitor prevents any sudden
voltage changes from appearing across resistor RE
by offering a lower impedance to the AC signal.
The bypass capacitor holds the voltage across resistor REsteady, while at the same time not interfering with the feedback action provided by
resistor RE"
A voltage-divider feedback circuit (Figure 28--11)
offers even more stability.This circuit is the one most
widely used. Resistor RBis replaced with two resistors, Rl and R2·The two resistors are connected in series across the source voltage V cc- The resistors divide
the source voltage into two voltages, forming a voltage divider.
Resistor R2 drops less voltage than resistor Rl.
The voltage at the base, with respect to ground, is
equal to the voltage developed across resistor R2.
The purpose of the voltage divider is to establish a
constant voltage from the base of the transistor to
ground. The current flow through resistor R2 is toward the base. Therefore, the end of resistor R2 attached to the base is positive with respect to ground.
Because the emitter current flows up through
resistor RE' the voltage dropped across resistor RE
Common-emitter amplifier with voltage-divider
feedback.
.----OUTPUT
INPUT
-
FIGURE 28-11
..
+Vcc
OUTPUT
INPUT
SECTION
4 LINEAR ELECTRONIC
CIRCUITS
is more positive at the end that is attached to the
emitter. The voltage developed across the emitterbase junction is the difference between the two
positive voltages developed across resistor R2 and
resistor RE. For the proper forward bias to occur,
the base must be slightly more positive than the
emitter.
When the temperature increases, the collector and emitter currents also increase. The increase in the emitter current causes an increase in
the voltage drop across the emitter resistor RE.
This results in the emitter becoming more positive
with respect to ground. The forward bias across
the emitter-base junction is then reduced, causing
the base current to decrease. The reduction in the
base current reduces the collector and emitter
currents. The opposite action takes place if the
temperature decreases: The base current increases, causing the collector and emitter currents
to increase.
The amplifier circuits discussed so far have
been biased so that all of the applied AC input signal appears at the output. Except for a higher voltage, the output signal is the same as the input
signal. An amplifier that is biased so that the current flows throughout the entire cycle is operating
as a class A amplifier (Figure 28-12).
An amplifier that is biased so that the output current flows for less than a full cycle but
more than a half cycle is operating as a class AB
amplifier. More than half but less than the full AC
input signal is amplified in the class AB mode
(Figure 28-13).
An amplifier that is biased so that the output
current flows for only half of the input cycle is operating as a class B amplifier. Only one-half of the
AC input signal is amplified in the class B mode
(Figure 28-14).
An amplifier that is biased so that the output
current flows for less than half of the AC input cycle is operating as a class C amplifier. Less than one
alternation is amplified in the class C mode (Figure 28-15).
Class A amplifiers are the most linear of the
types mentioned. They produce the least amount
of distortion. They also have lower output ratings
and are the least efficient. They find wide application where the full signal must be maintained, as
FIGURE 28-13
Class AB amplifier output.
INPUT
OUTPUT
FIGURE 28-14
Class B amplifier output.
INPUT
OUTPUT
FIGURE 28-12
FIGURE 28-15
Class A amplifier output.
Class C amplifier output.
06
L----~----r\
\
OL--------------INPUT
OUTPUT
INPUT
I
'
OUTPUT
I
/
CHAPTER 28 AMPLIFIER BASICS
FIGURE 28 16
Push-pull amplifier configuration.
o--ll--~---iH
%INPUT
1
C1
in the amplification of audio signals in radios and
televisions. However, because of the high powerhandling capabilities required for class A operation, transistors are usually operated in the class
AB or class B mode.
Class AB, B, and C amplifiers produce a substantial amount of distortion. This is because they
amplify only a portion of the input signal. To amplify the full AC input signal, two transistors are
needed, connected in a push-pull configuration
(Figure 28-16). Class B amplifiers are used as output stages of stereo systems and public address
amplifiers, and in many industrial controls. Class
C amplifiers are used for high-power amplifiers
and transmitters where only a single frequency is
amplified, such as the RF (radio frequency) carrier
used for radio and television transmission.
28-2
QUESTIONS
1. Draw a schematic diagram of a commonemitter transistor amplifier using a single
voltage source.
2. How are temperature changes
compensated for in a transistor amplifier?
3. Draw a schematic diagram of a voltagedivider feedback circuit.
4. List the classes of amplifiers and identify
their outputs.
5. List the applications
of each class of
amplifier.
AMPLIFIER COUPLING
To obtain greater amplification, transistor amplifiers may be connected together. However, to
prevent one amplifier's bias voltage from affecting the operation of the second amplifier, a coupling technique must be used. The coupling
method used must not disrupt the operation of
either circuit. Coupling methods used include
resistance-capacitance coupling, impedance coupling, transformer coupling, and direct coupling.
Resistive-capacitive coupling, or RCcoupling, consists of two resistors and a capacitor connected as
shown in Figure 28-17. Resistor R3is the collector
load resistor of the first stage. Capacitor C1 is the
DC-blocking and AC-coupling capacitor. Resistors
R5and R6are the input load resistor and DC return
resistor for the base-emitter junction of the second
stage. Resistance-capacitive coupling is used primarily in audio amplifiers.
SECTION 4 LINEAR ELECTRONIC
FIGURE 28-17
RC coupling.
CIRCUITS
-
FIGURE 28-18
Impedance coupling.
-
INPUT
INPUT
Coupling capacitor C1 must have a low reactance to minimize low-frequency attenuation.
Typically, a high-capacitance value of 10 to 100
microfarads is used. The coupling capacitor is generally an electrolytic type.
The reactance of the coupling capacitor increases as the frequency decreases. The lowfrequency limit is determined by the size of the
coupling capacitor. The high-frequency limit is
determined by the type of transistor used.
The impedance-coupling method is similar to
the RC-coupling method, but an inductor is used
in place of the collector load resistor for the first
stage of amplification (Figure 28-18).
Impedance coupling works just like RC coupling. The advantage is that the inductor has a
very low DC resistance across its windings. The
AC output signal is developed across the inductor
just as across the load resistor. However, the inductor consumes less power than the resistor, increasing the overall efficiency of the circuit.
The disadvantage of impedance coupling is
that the inductance reactance increases with the
frequency. The voltage gain varies with the frequency. This type of coupling is ideal for singlefrequency amplification when a very narrow
band of frequencies must be amplified.
In a transformer-coupled circuit, the two amplifier stages are coupled through the transformer
(Figure 28-19). The transformer can effectively
match a high-impedance source to a low-impedance load. The disadvantage is that transformers
are large and expensive. Also, like the inductorcoupled amplifier, the transformer-coupled amplifier is useful only for a narrow band of frequencies.
CHAPTER
FIGURE 28-20
Direct coupling.
28 AMPLIFIER BASICS
4. What is the disadvantage of transformer
coupling?
5. What coupling method is used for DC
and low-frequency signals?
SUMMARY
INPUT
When very low frequencies or a DC signal
must be amplified, the direct-coupling technique
must be used (Figure 28-20). Direct-coupled amplifiers provide a uniform current or voltage gain
over a wide range of frequencies. This type of
amplifier can amplify frequencies from zero (DC)
hertz to many thousands of hertz. However,
direct-coupled amplifiers find their best applications with low frequencies.
A drawback of direct-coupled amplifiers is
that they are not stable. Any change in the output
current of the first stage is amplified by the second
stage. This occurs because the second stage is essentially biased by the first stage. To improve the
stability requires the use of expensive precision
components.
28-3
QUESTIONS
1. What are the four main methods of
coupling transistor amplifiers?
2. Where is resistance-capacitive coupling
primarily used?
3. What is the difference between
resistance-capacitive coupling and
impedance coupling?
• Amplifiers are electronic circuits used to
increase the amplitude of an electronic
signal.
• The transistor is used primarily as an
amplifying device.
• The three transistor amplifier
configurations are common base, common
collector, and common emitter.
• Common-collector amplifiers are used for
impedance matching.
• Common-emitter amplifiers provide phase
reversal of the input-output signal.
• All transistor amplifiers require two
voltages for proper biasing.
• A single voltage source can provide the
necessary forward-bias and reverse-bias
voltages using a voltage-divider
arrangement.
• A voltage-divider feedback arrangement is
the most commonly used biasing
arrangement.
• A transistor amplifier can be biased so that
all or part of the input signal is present at
the output.
• Class A amplifiers are biased so that the
output current flows throughout the cycle.
• Class AB amplifiers are biased so that the
output current flows for less than the full
but more than half of the input cycle.
• Class B amplifiers are biased so that the
output current flows for only half of the
input cycle.
• Class C amplifiers are biased so that the
output current flows for less than half of
the input cycle.
SECTION 4 LINEAR ELECTRONIC
CIRCUITS
• Coupling methods used to connect one
transistor to another include resistancecapacitance coupling, impedance coupling,
transformer coupling, and direct coupling.
• Direct-coupled amplifiers are used for
high gain at low frequencies or
amplification of a DC signal.
1. Briefly describe how a transistor is used to provide amplification.
2. Why is the common-emitter amplifier the most widely used transistor amplifier
configuration?
3. What factor affects the gain of the transistor, and what must be done to compensate for it?
4. How does an amplifier's class of operation affect the biasing of the amplifier?
5. What factor must be taken into consideration when connecting one amplifier to another?
6. How does the frequency of operation of an amplifier affect the coupling method used in
connecting amplifiers together?
r