This document summarizes a lecture on power amplifiers, including:
- Different classes of power amplifiers like Class A, B, AB, C, and D based on conduction angle.
- Circuit designs for series-fed and transformer-coupled Class A amplifiers.
- Circuit designs for Class B amplifier using complementary pairs or a Darlington pair to achieve push-pull operation.
- Considerations for efficiency and maximum output power of different classes.
In the case of class A amplifier, we have observed that the transistor conducts for
the full cycle of the input signal i.e. the conduction angle is 180◦. Although
the transistor conducts for the full cycle of the input signal, the power conversion
efficiency is poor in class A amplifier. In addition to that, a great deal of
distortion is introduced by the nonlinearity in the dynamic transfer characteristic
of the transistor. The power conversion efficiency can be improved by biasing
the transistor at cut off point on VCE axis and a great deal of the distortion
due to nonlinearity in dynamic transfer characteristic may be eliminated by
the push-pull configuration of the transistor as discussed in next section
In the case of class A amplifier, we have observed that the transistor conducts for
the full cycle of the input signal i.e. the conduction angle is 180◦. Although
the transistor conducts for the full cycle of the input signal, the power conversion
efficiency is poor in class A amplifier. In addition to that, a great deal of
distortion is introduced by the nonlinearity in the dynamic transfer characteristic
of the transistor. The power conversion efficiency can be improved by biasing
the transistor at cut off point on VCE axis and a great deal of the distortion
due to nonlinearity in dynamic transfer characteristic may be eliminated by
the push-pull configuration of the transistor as discussed in next section
Power Amplifiers: Classification of amplifiers, Class A power Amplifiers and their analysis, Harmonic Distortions,
Class B Push-pull amplifiers and their analysis, Complementary symmetry push pull amplifier, Class AB power
amplifier, Class-C power amplifier, Thermal stability and Heat sinks, Distortion in amplifiers.
Power Amplifier circuits.
Output stages of types of power amplifier (class A, class B, class AB, class C, class D)
Distortions( Harmonic and Crossover).
Push-pull amplifier with and without transformer.
Complimentary symmetry and Quasi- complimentary symmetry push pull amplifier.
Conventional power amplifier systems trade-off efficiency for linearity due to transistor operation limitations. The Smart Power Amplifier architecture improves RF Power Amplifier linearity without compromising efficiency. This novel invention generates and utilizes specialized outphased signals that can be used in outphasing RF power amplifiers in order to amplify the input signal without suffering from the nonlinearities and inefficiencies associated with conventional power amplifiers. Using this proposed technique, designers and developers can generate variable number of outphased signals on the fly with minimal processing and memory footprints. The proposed architecture for constructing and combining a variable number of constant amplitude outphased signals results in both highly linear and significantly efficient power amplification system. The complexities of both constructing and combining the decomposed signals are significantly reduced due to the fact that all of the decomposed signals except only two of them are actually scaled versions of the input signal.
This Presentation is related to multistage amplifiers. different topics related to multistage amplifiers like two stage amplifiers. Two stage RC coupled amplifiers, cascading techniques, CE-CB cascod amplifiers, darlington pair, fet analysis
Introduction
Band Pass Amplifiers
Series & Parallel Resonant Circuits & their Bandwidth
Analysis of Single Tuned Amplifiers
Analysis of Double Tuned Amplifiers
Primary & Secondary Tuned Amplifiers with BJT & FET
Merits and de-merits of Tuned Amplifiers
Power Amplifiers: Classification of amplifiers, Class A power Amplifiers and their analysis, Harmonic Distortions,
Class B Push-pull amplifiers and their analysis, Complementary symmetry push pull amplifier, Class AB power
amplifier, Class-C power amplifier, Thermal stability and Heat sinks, Distortion in amplifiers.
Power Amplifier circuits.
Output stages of types of power amplifier (class A, class B, class AB, class C, class D)
Distortions( Harmonic and Crossover).
Push-pull amplifier with and without transformer.
Complimentary symmetry and Quasi- complimentary symmetry push pull amplifier.
Conventional power amplifier systems trade-off efficiency for linearity due to transistor operation limitations. The Smart Power Amplifier architecture improves RF Power Amplifier linearity without compromising efficiency. This novel invention generates and utilizes specialized outphased signals that can be used in outphasing RF power amplifiers in order to amplify the input signal without suffering from the nonlinearities and inefficiencies associated with conventional power amplifiers. Using this proposed technique, designers and developers can generate variable number of outphased signals on the fly with minimal processing and memory footprints. The proposed architecture for constructing and combining a variable number of constant amplitude outphased signals results in both highly linear and significantly efficient power amplification system. The complexities of both constructing and combining the decomposed signals are significantly reduced due to the fact that all of the decomposed signals except only two of them are actually scaled versions of the input signal.
This Presentation is related to multistage amplifiers. different topics related to multistage amplifiers like two stage amplifiers. Two stage RC coupled amplifiers, cascading techniques, CE-CB cascod amplifiers, darlington pair, fet analysis
Introduction
Band Pass Amplifiers
Series & Parallel Resonant Circuits & their Bandwidth
Analysis of Single Tuned Amplifiers
Analysis of Double Tuned Amplifiers
Primary & Secondary Tuned Amplifiers with BJT & FET
Merits and de-merits of Tuned Amplifiers
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2. 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
11. 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
12. 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)
Series-Fed Class A Amplifier
EE314 Lecture 4 12
13. Series-Fed Class A Amplifier
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
%
16. 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
20. 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
22. 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.
*
*
*
23. Another way to produce push-pull input signal, i.e., to produce
opposite polarity signals.
Push-Pull Input Signal
EE314 Lecture 4 23
24. - 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.
25. 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
26. 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)
31. 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.
33. 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)
34. 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