Power Amplifiers
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
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Amplifier Types
Class A
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.
Class AB
This is a compromise between the class A and B
amplifiers. The amplifier conducts somewhere
more…
between 180 and 360 . The Q-point is located
between the mid-point and cutoff.
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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.
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Class A Amplifier
The output of a class A
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.
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Class A Amplifier
The output power delivered The input power delivered
to the load: to the load:
POWER GAIN:
Power Considerations:
Output Power: (using RMS signals)
Efficiency:
The efficiency of any amplifier is the ratio of the output signal
power supplied to a load to the total power from the dc supply.
The average power supply current, ICC, is equal to ICQ and the
supply voltage is at least 2VCEQ. Therefore, the total dc power
is:
The maximum efficiency, of a capacitively coupled class A
amplifier is:
Series-Fed Class A Amplifier
This is similar to the
small-signal amplifier
except that it will handle
higher voltages.
The transistor used is a
high-power transistor.
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Transformer-Coupled Class A Amplifier
This circuit uses a
transformer to
couple to the load.
This improves the
efficiency of the
Class A to 50%.
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Transformer Action
A transformer improves the efficiency because it is able to transform the
voltage, current, and impedance
Voltage Ratio
V2 N 2
V1 N 1
Current Ratio
I 2 N1
I1 N 2
Impedance Ratio
2
R L R 1 N 1
a 2
RL R2 N2 13
Transformer-Coupled Class A Amplifier
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Transformer-Coupled Class A Amplifier
Signal Swing and Output AC Power
The voltage swing:
VCE(p p ) VCE max VCE min
The current swing:
I C max I C min
The AC power:
(VCEmax VCEmin )(I Cmax I Cmin )
Po(ac)
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Transformer-Coupled Class A Amplifier
Efficiency
Power input from the DC source:
Pi(dc) VCC I CQ
Power dissipated as heat across the transistor:
PQ Pi(dc) Po(ac)
Maximum efficiency:
2
VCEmax VCEmin
%η 50
VCEmax VCEmin
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Class B Amplifier
A class B amplifier output
only conducts for 180 or
one-half of the AC input
signal.
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Class B Amplifier
The Q-point is at 0V on the load line, so that the AC
signal can only swing for one-half cycle.
Class B Amplifier
Class B Amplifier
Input (DC) power:
Output (DC) power:
Class B Amplifier: Efficiency
Po(ac )
% 100
Pi(dc)
The maximum efficiency of a class B is 78.5%..
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Maximum Power Considerations:
maximum transistor power dissipation:
Transformer-Coupled Push-Pull
Class B Amplifier
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Class B Amplifier Push-Pull Operation
• During the positive
half-cycle of the AC
input, transistor Q1
(npn) is conducting
and Q2 (pnp) is off.
• During the negative
half-cycle of the AC
input, transistor Q2
(pnp) is conducting
and Q1 (npn) is off.
Each transistor produces one-half of an AC cycle. The
transformer combines the two outputs to form a full AC cycle.
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Class AB Amplifier
This amplifier is a compromise between the class A and class B
amplifier—the Q-point is above that of the Class B but below the
class A.
The output conducts between 180 and 360 of the AC input
signal.
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Class AB Amplifier
Class AB: – The amplifiers two output
transistors conduct somewhere between
180o and 360o of the input waveform.
Class C
The output of the
class C conducts for
less than 180 of the
AC cycle. The Q-point
is below cutoff.
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Class C Amplifiers
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Class C Amplifiers
Power Dissipation
Class D Amplifier
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.
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Class D Amplifier
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.
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Amplifier Distortion
If the output of an amplifier is not a complete AC 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.
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Harmonics
Harmonics are integer multiples of a fundamental
frequency.
If the fundamental frequency is 5kHz:
1st harmonic 1 x 5kHz
2nd harmonic 2 x 5kHz
3rd harmonic 3 x 5kHz
4th harmonic 4 x 5kHz
etc.
Note that the 1st and 3rd harmonics are called odd
harmonics and the 2nd and 4th are called even
harmonics.
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Harmonic Distortion
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Harmonic Distortion Calculations
Harmonic distortion (D) can be calculated:
An
% nth harmonic distortion %D n 100
A1
where
An is the amplitude of the fundamental frequency
An is the amplitude of the highest harmonic
The total harmonic distortion (THD) is determined by:
% THD D 22 D 23 D 23 100
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