Transcript Class A Amplifier

TOPIC NAME
POWER AMPLIFIERS
PREPARED BY :
TIRTH PANDYA
ENROLLMENT NO : 140950111006
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What is amplifier?

An amplifier is an electronic device that
increases the voltage, current, or power of
a signal. Amplifiers are used in wireless
communication and broadcasting and in audio
equipments of all kinds. They can be
categorized as either weak signal amplifiers or
power amplifiers.
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Power Amplifier

A power amplifier is an electronic device
that receives an electrical signal and
reprocesses it to amplify, or increase, its
power. The boost in power is achieved by
significantly increasing the input signal’s
voltage. A power amplifier is used to power an
output source, such as a stereo speaker, a
relay or a motor.
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Classification of Power Amplifiers
Class A Amplifiers
Class B Amplifiers
Class AB Amplifiers
Class C Amplifiers
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Class A Amplifiers
The VDB amplifier of the fig is a class A amplifier
as long as the output signal is not clipped. With this
kind of amplifier, collector current flows throughout
the cycle.
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• The most commonly used type of power amplifier
configuration is the Class A Amplifier. It uses the switching
transistor in the standard common emitter circuit
configuration. The transistor is always biased “ON” so that it
conducts during one complete cycle of the input signal
waveform producing minimum distortion and maximum
amplitude to the output.
• Class A Amplifier configuration is the ideal operating mode,
because there can be no crossover or switch-off distortion to
the output waveform even during the negative half of the
cycle. Its output stages may use a single power transistor or
pairs of transistors connected together to share the high load
current.
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Class B Amplifiers
Class B operation means that the collector
current flows for only 180 degree of the ac cycle. For
this to occur, the Q point is located at cutoff on both
the dc and the ac load lines.
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• To improve the full power efficiency of Class A amplifier
by reducing the wasted power in the form of heat, it is
possible to design the power amplifier circuit with two
transistors in its output stage producing what is
commonly termed as a Class B Amplifier, also known as
a push-pull amplifier configuration.
• The conduction angle for this type of amplifier circuit is
only 180o or 50% of the input signal. This pushing and
pulling effect of the alternating half cycles by the
transistors.
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Class AB Amplifier
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• The class AB push-pull output circuit is slightly less
efficient than class B because it uses a small
quiescent current flowing, to bias the transistors just
above cut off, but the crossover distortion created by
the non-linear section of the transistor’s input
characteristic curve, near to cut off in class B is
overcome. In class AB each of the push-pull
transistors is conducting for slightly more than the
half cycle of conduction in class B, but much less
than the full cycle of conduction of class A.
• As each cycle of the waveform crosses zero volts,
both transistors are conducting momentarily and the
bend in the characteristic of each one cancels out. 10
Class C Amplifier
Class-C amplifiers conduct less than 50% of the
input signal and the distortion at the output is high, but
high efficiencies (up to 90%) are possible.
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• The Class C Amplifier design has the greatest efficiency but
the poorest linearity of the classes of amplifier. The A,
B and AB are considered linear amplifiers, as the output
signals amplitude and phase are linearly related to the input
signals amplitude and phase.
• However, the class C amplifier is heavily biased so that the
output current is zero for more than one half of an input
sinusoidal signal cycle with the transistor idling at its cut-off
point. In other words, the conduction angle for the transistor
is significantly less than 180 degrees, and is generally around
the 90 degrees area.
• While this form of transistor biasing gives a much improved
efficiency of around 80% to the amplifier, it introduces a very
heavy distortion of the output signal. Therefore, class C
amplifiers are not suitable for use as audio amplifiers.
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Push-Pull Amplifier
When a transistor operates as class B, it clips off
half a cycle.
Push-pull means that one transistor conducts
when the other is off, & vice versa.
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On the positive half cycle of input voltage, the
secondary winding of T1 has voltage V1 and V2 as shown
in the fig. Therefore the upper transistor conducts and
the lower one cuts off. The collector current through Q1
flows through the upper half of the output primary
winding. This produces an amplified and inverted
voltage, which is transformer coupled to the
loudspeaker.
On the next half cycle, the polarities reverse. The
lower transistor turns on and the upper transistor turns
off. The lower transistor amplifies the signal, and the
alternate half cycle appears across the loudspeaker.
Since each transistor amplifies one-half of the input
cycle, the loudspeaker receives a complete cycle of the
amplified signal.
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Class B as Push-Pull Amplifier
Advantages
- Possible to obtain greater power output
- efficiency is higher
- Negligible power loss at no signal
Disadvantages
- Harmonic distortion in higher
- Supply voltage must have good regulation
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Cross Over Distortion
The figure shows the ac equivalent circuit of a class B
push-pull emitter follower.
Suppose that no bias is applied to the emitter
diodes. Then, the incoming ac voltage has to rise to
about 0.7 V to overcome barrier potential of the
emitter diodes. Because of this, no current flows
through Q1 when signal is less than 0.7 V.
No current flows through Q2 until the ac input
voltage is more negative than -0.7 V. For this reason, if
no bias is applied to the emitter diodes, the output
looks like the figure shown below.
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Because of clipping between half cycles, the output
is distorted. Since the clipping occurs between the time,
one transistor cuts off and the other one comes on, it is
called cross over distortion.
To eliminate cross over distortion, we need to apply
a slide forward bias to each emitter diode.
This means locating the Q point slightly above cut -off.
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Parameters
Class A
Class B
Class C
Class AB
Collector current
waveform
Conduction
angle of
collector current
360⁰ or full cycle 180⁰ or half
cycle
Less than 180⁰
Between 180⁰ &
360⁰
Position of Q
point on the
load line
At the centre
Below the axis
Above the X axis
but below the
mid-point
On the axis
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Parameters
Class A
Class B
Class C
Class AB
Distortion in
output voltage
No distortion
More than class
A (Cross over)
More than A, B
and AB
Low
Efficiency
Lowest
25% to 50%
Higher
78.5%
Very high
95%
Between 50% to
78.5%
Power
dissipation in
transistor
Very high
Low
Very low
Moderate
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