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Dr. Abdullah Almuhaisen
ElectricalEngineeringDepartment
College of Engineering
Analog & Digital Electronics EE 314
Lecture 4: Power Amplifiers

Power Amplifiers
o PA Classes
 Class A
 Series-Fed Class A Amplifier
 Transformer-Coupled Class A Amplifier
 Class B
 Transformer-Coupled Push-Pull
 Complementary-Symmetry Circuits
 Quasi-Complementary Push-Pull Amplifier
 Class C
 Class D
o Amplifier Distortion
Summary & References
Outline
EE314 Lecture 4 2

Definitions
In small-signal amplifiers the main factors are:
• Amplification
• Linearity
• Gain
Since large-signal, or power, amplifiers handle relatively large
voltage signals and current levels, the main factors are:
• Efficiency
• Maximum power capability
• Impedance matching to the output device
3Electronic Devices and Circuit Theory, 10/e
Robert L. Boylestad and Louis Nashelsky
Copyright ©2009 by Pearson Education, Inc.
Upper Saddle River, New Jersey 07458 • All rights reserved.

Amplifier Types
ClassA
The amplifier conducts through the full 360°of the input. The Q-point is
set near the middle of the load line.
Class B
The amplifier conducts through 180°of the input. The Q-point is set at
the cutoff point.
ClassAB
This is a compromise between the classAand B amplifiers. The
amplifier conducts somewhere between 180°and 360°. The Q-point is
located between the mid-point and cutoff.
more…

Amplifier Types
Class C
The amplifier conducts less than 180 of the input. The Q-point is located
below the cutoff level.
Class D
This is an amplifier that is biased especially for digital signals.

Class AAmplifier
The output of a classA
amplifier conducts for the full
360°of the cycle.
The Q-point is set at the
middle of the load line so that
the AC signal can swing a full
cycle.
Remember that the DC load line
indicates the maximum and
minimum limits set by the DC power
supply.

Class B Amplifier
Aclass B amplifier
output only conducts
for 180°or one-half of
theAC input signal.
The Q-point is at 0V on
the load line, so that the
AC signal can only
swing for one-half cycle.

Class AB Amplifier
This amplifier is a compromise between
the classAand class B amplifier—the
Q-point is above that of the Class B but
below the classA.
The output conducts between 180°
and 360°of theAC input signal.

Class C
The output of the class C
conducts for less than 180°of
the AC cycle. The Q-point is
below cutoff.

Amplifier Efficiency
Efficiency refers to the ratio of output to input power. The lower the
amount of conduction of the amplifier the higher the efficiency.
×100
Pdc
Po
%η=
PQ =Pdc −Po
Note: The larger the input and output signal,
the lower the heat dissipation.
Power dissipated as heat across the
transistor:

The transistor used is a power transistor that is capable of
operating in the range of a few to tens of watts
DC Operation
𝐼 𝐵 =
𝑉 𝐶𝐶−0.7𝑉
𝑅 𝐵
𝐼 𝐶 = 𝛽𝐼 𝐵
𝑉𝐶𝐸 = 𝑉𝐶𝐶 − 𝐼 𝐶 𝑅 𝐶
Q-Point (Quiescent Point, Operating Point)?
o Defined by the biasing circuit
 Sets a fixed level of I & V.
o Affects both:
 𝑷 𝑫𝑪 : a fixed level of I & V
 𝑷 𝒐: sets the maximum swing of output AC signal
Example:
 Increasing 𝑅 𝐶 𝐼 𝐶 𝑃 𝐷𝐶 and 𝑃𝑜
 decreasing 𝑉𝐶𝐶 𝑃 𝐷𝐶 and 𝑃𝑜
Series-Fed Class A Amplifier
EE314 Lecture 4 11

AC Operation
Asmall input AC signal causes:
 The output voltage swing between 0V to Vcc
 The output current swing between 0 mAto ICSAT (VCC/RC)
EE314 Lecture 4 12

EE314 Lecture 4 13
DC Power
The power into the amplifier is from the DC supply. With no
input signal, the DC current drawn is the collector bias current,
ICQ.
Output Power
Efficiency
×100
Pdc
Po
%η=
𝑷 𝒐 =
𝑽 𝒑 𝑰 𝒑
𝟐
=
𝑽 𝒑
𝟐
𝟐𝑹 𝑳
𝑷 𝒅𝒄 = 𝑽 𝒅𝒄 𝑰 𝒅𝒄 = 𝑽 𝒄𝒄
𝑰 𝒎𝒂𝒙
𝟐
= 𝑽 𝒄𝒄 𝑰 𝑪𝑸
%

Maximum Efficiency:
𝑷 𝑫𝑪 = 𝑽 𝑫𝑪 ∙ 𝑰 𝑫𝑪 = 𝑽 𝑪𝑪 ∙ 𝑰 𝑪𝑸
𝑷 𝒐 =
𝑽 𝑪𝑬 𝒑
𝟐
∙
𝑰 𝑪 𝒑
𝟐
= 0.5 𝑽 𝑪𝑬 𝒑 ∙ 𝑰 𝑪 𝒑
𝜼 𝒎𝒂𝒙𝒊𝒎𝒖𝒎 =
𝑷 𝒐
𝑷 𝑫𝑪
× 𝟏𝟎𝟎%
= 0.5 𝑽 𝑪𝑬 𝒑 ∙ 𝑰 𝑪 𝒑 ×
𝟏
𝑽 𝑪𝑪∙𝑰 𝑪𝑸
× 𝟏𝟎𝟎%
= 0.5
𝑽 𝑪𝑪
𝟐
∙ 𝑰 𝑪𝑸 ×
𝟏
𝑽 𝑪𝑪∙𝑰 𝑪𝑸
× 𝟏𝟎𝟎%
= 𝟐𝟓%
EE314 Lecture 4 14

Transformer-Coupled Class AAmplifier
This circuit uses a
transformer to couple to
the load. This improves
the efficiency of the Class
Ato 50%.

Atransformer improves the efficiency because it is able to transform the
voltage, current, and impedance.
Voltage Transformation
𝑽 𝟐
𝑽 𝟏
=
𝑵 𝟐
𝑵 𝟏
Current Transformation
𝑰 𝟐
𝑰 𝟏
=
𝑵 𝟏
𝑵 𝟐
Impedance Transformation
𝑹 𝑳
′
𝑹 𝑳
=
𝑵 𝟏
𝑵 𝟐
𝟐
Changing the turn ratio can step up or step down the current, voltage or
impedance.
Transformer Action
EE314 Lecture 4 16

Transformer-Coupled Class AAmplifier
Signal Swing
The voltage swing: VCE min to VCE max
The current swing: ICmin to ICmax
DC Load Line
As in all classAamplifiers the Q-point is
established close to the midpoint of the
DC load line.
AC Load Line
The saturation point (ICmax) is at Vcc/R L
and the cutoff point is at V2 (the secondary
voltage of the transformer). This increases
the maximum output swing because the
minimum and maximum values of IC and
VCE are spread further apart.

Transformer-Coupled Class A Amplifier
EE314 Lecture 4 *this the maximum output power, 𝑉𝐶𝐶 :DC Source, 𝐼 𝑚𝑎𝑥: max. Current, 𝑉0:
output fundamental power, 𝐼0: output fundamental power
18
DC Power
Output Power
𝑃𝑜 =
𝑉𝑝 𝐼 𝑝
2
=
𝑉𝑝
2
2𝑅 𝐿
Dissipated Power
PEP (Peek Envelop Power)*:
PEP Efficiency
𝑃𝑜 =
𝑉𝑝 𝐼 𝑝
2
=
1
2
𝑉𝐶𝐶
𝐼 𝑚𝑎𝑥
2
𝜂 =
𝑃0
𝑃𝑑𝑐
× 100% =
1
2
𝑉𝐶𝐶
𝐼 𝑚𝑎𝑥
2
1
𝑉𝐶𝐶
𝐼 𝑚𝑎𝑥
2
× 100% = 50%
𝑃𝑑𝑐 = 𝑉𝑑𝑐 𝐼 𝑑𝑐 = 𝑉𝑐𝑐
𝐼 𝑚𝑎𝑥
2
𝑃𝑄 = 𝑃𝑑𝑐 − 𝑃𝑜

Class B Amplifier
In class B, the transistor is
biased just off. TheAC signal
turns the transistor on.
The transistor only conducts
when it is turned on by one
half of theAC cycle.
In order to get a full AC cycle
out of a class B amplifier, you
need two transistors:
• An npn transistor that provides the
negative half of theAC cycle
• Apnp transistor that provides the
positive half.

Class B Amplifier
EE314 Lecture 4 *this the maximum output power, 𝑉𝐶𝐶 :DC Source, 𝐼 𝑚𝑎𝑥: max. Current, 𝑉0:
output fundamental voltage, 𝐼0: output fundamental current
20
DC Power
Output Power
𝑃𝑜 =
𝑉𝑝 𝐼 𝑝
2
=
𝑉𝑝
2
2𝑅 𝐿
At PEP (Peek Envelop Power)*:
PEP Efficiency
𝑃𝑜 =
𝑉𝑐𝑐
2
2𝑅 𝐿
𝜂 =
𝑃0
𝑃𝑑𝑐
× 100% =
𝑉𝑐𝑐
2
2𝑅 𝐿
1
2
𝜋
𝑉𝑐𝑐
2
𝑅 𝐿
× 100% =
𝜋
4
× 100% = 78.5%
𝑃𝑑𝑐 = 𝑉𝑑𝑐 𝐼 𝑑𝑐 = 𝑉𝑑𝑐
2
𝜋
𝐼 𝑝
𝑃𝑑𝑐 = 𝑉𝑑𝑐 𝐼 𝑑𝑐 = 𝑉𝑐𝑐
2
𝜋
𝐼 𝑚𝑎𝑥 =𝑉𝑐𝑐
2
𝜋
𝑉𝑐𝑐
𝑅 𝐿
=
2
𝜋
𝑉𝑐𝑐
2
𝑅 𝐿
𝐼 𝑑𝑐: is not fixed as in Class A, it depends
on the magnitude of the output Current,
These waveforms
are for a single
transistor ,
𝐼 𝑑𝑐 =
1
𝜋
𝐼 𝑝, for a full
wave rectified
signal 𝐼 𝑑𝑐 =
2
𝜋
𝐼 𝑝

Transformer-Coupled Push-Pull
Class B Amplifier
The center-tapped
transformer on the input
produces opposite
polarity signals to the
two transistor inputs.
The center-tapped
transformer on the
output combines the two
halves of theAC
waveform together.

Class B Amplifier Push-Pull Operation
22
Centre tapped input transformer:
• To produce opposite-polarity signals
Operation:
• During the first half cycle, Q1 is driven
into conduction and Q2 is Off.
• I1 results in first half cycle to load.
• During the second half cycle, Q2 is driven
into conduction and Q1 is off.
• I2 results in second half cycle to
load.
*
*
*

Another way to produce push-pull input signal, i.e., to produce
opposite polarity signals.
Push-Pull Input Signal
EE314 Lecture 4 23

- Using complementary transistors (npn & pnp)
- Transistors are biased by the input signal
Complementary-Symmetry Circuits
EE314 Lecture 4 24
During the positive
half-cycle of theAC
input, transistor Q1
(npn) is conducting
and Q2 (pnp) is off.
During the negative
half-cycle of theAC
input, transistor Q2
(pnp) is conducting
and Q1 (npn) is off.
Each transistor produces one-half of anAC cycle. The transformer
combines the two outputs to form a fullAC cycle.

Crossover Distortion
If the transistors Q1 and Q2 do
not turn on and off at exactly
the same time, then there is a
gap in the output voltage.
Note that, usually the
transistors are biased above
0.7 V, or in class AB.
25
Disadvantages of this circuit are:
1. Two DC supplies
2. Resulting Crossover distortion

Quasi-Complementary Push-PullAmplifier
ADarlington pair and a feedback
pair combination perform the
push-pull operation. This
increases the output power
capability.
The adjustable transistor R2 is
used to bias the transistors and to
minimize the crossover distortion.
26
it is preferable to use npn transistors for both
high-current-output devices.
Push-Pull operation by means of Q1 and Q2
High power
npn is
preferable for
high current PA
(carrier =
electrons)

Class C Amplifiers
Aclass C amplifier conducts for less
than 180°. In order to produce a full
sine wave output, the class C uses a
tuned circuit (LC tank) to provide
the fullAC sine wave.
Class C amplifiers are used
extensively in radio communications
circuits.

Class D Amplifier
A class D amplifier amplifies
pulses, and requires a pulsed
input.
There are many circuits that
can convert a sinusoidal
waveform to a pulse, as well
as circuits that convert a
pulse to a sine wave. This
circuit has applications in
digital circuitry.

Amplifier Distortion
If the output of an amplifier is not a completeAC sine wave,
then it is distorting the output. The amplifier is non-linear.
This distortion can be analyzed using Fourier analysis. In
Fourier analysis, any distorted periodic waveform can be
broken down into frequency components. These
components are harmonics of the fundamental frequency.

Harmonics
Harmonics are integer multiples of a fundamental frequency.
If the fundamental frequency is 5kHz:
1st harmonic
2nd harmonic
3rd harmonic
4th harmonic
etc.
1 x 5kHz
2 x 5kHz
3 x 5kHz
4 x 5kHz
Note that the 1st and 3rd harmonics are called odd harmonics and the
2nd and 4th are called even harmonics.

Harmonic Distortion
According to Fourier
analysis, if a signal is not
purely sinusoidal, then it
contains harmonics.
31
In the lab, you can use
Spectrum Analyser to
measure the harmonics
contained in the signal.

Harmonic Distortion Calculations
Harmonic distortion (D) can be
calculated:
×100
A1
An%nth harmonic distortion =%Dn=
where A1 is the amplitude of the fundamental
frequency An is the amplitude of the highest
harmonic
The total harmonic distortion (THD) is determined
by:
%THD= D2 +D2 +D 2+ .. ×100 %
2 3 3

Power Transistor Heat Sinking
PowerTransistor Derating Curve:
Power transistors dissipate a lot of
power in heat. This can be
destructive to the amplifier as well
as to surrounding components.
Heat Sink:
The heat produced by the transistor will
be dissipated on a larger area. Thus,
holding the case temperature to a much
lower value.
33
Power dissipation is allowed only up to a maximum temperature, maximum junction
temperature:
• Silicon: 150–200°C
• Germanium: 100–110°C
Above this temperature, the device power dissipation capacity must be reduced (or derated)

H/W Chapter 12: (Ninth Edition):
Problems: 1, 8, 9, 10, 11, 12, 13, 16, 19, 20
Summary
o PA Classes
 Class A
 Series-Fed Class A Amplifier
 Transformer-Coupled Class A Amplifier
 Class B
 Transformer-Coupled Push-Pull
 Complementary-Symmetry Circuits
 Quasi-Complementary Push-Pull Amplifier
 Class C
 Class D
o Amplifier Distortion
References
o Text Book: R. BOYLESTAD, L. NASHELSKY " ELECTRONIC DEVICES AND
CIRCUIT THEORY“.
o A. Sedra and K. Smith, “Microelectronic Circuits”.
Summary & References
EE314 Lecture 4 34

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