AEC Module4 October12th

Power amplifiers , Millers Theorem and High frequency model of transistors
• Power amplifiers Chapter , Boylestad :

• classification of power amplifiers and performance
• circuit diagram of a Series fed Class A amplifier and write the expression for power
efficiency
• circuit diagram of a Transformer coupled Class B push pull amplifier and write the
expression for power efficiency
• crossover distortion in class B amplifier? How is it overcome.
• Millers Theorem : examples Page No:553 (Razavi Text book)

• High frequency model of transistors Page No.559 (Razavi Text book)
(Mode of operation)
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 In class B transistor conducts current for only one-half of the signal cycle. To
obtain output for the full cycle of signal, it is necessary to use two transistors
and have each conduct on opposite half cycles, the combined operation
providing a full cycle of output signal.
 Since one part of the circuit pushes the signal high during one-half cycle and
the other part pulls the signal low during the other half cycle, the circuit is
referred to as a push-pull circuit.
 the power transistor used in the push pull circuit can deliver the desired power
to the load, and the class B operation of these transistors provides greater
efficiency than single transistor in class A operation.
 Class B amplifier: When an amplifier is biased at cutoff so that it operates in the
linear region for 180o of the input cycle and is in cutoff for 180o
 Class AB amplifiers: are biased to conduct for slightly more than 180o
 Both are more efficient than a class A amplifier;
 A disadvantage of class B or class AB is that it is more difficult to implement the

circuit in order to get a linear reproduction of the input waveform.
 The term push-pull refers to a common type of class B or class AB amplifier
circuit in which two transistors are used on alternating half-cycles to reproduce
EEDP - Basem ElHalawany
the input waveform at the output. 7
 The class B amplifier is biased at the cutoff point so that
 It is brought out of cutoff and operates in its linear region when the input
signal drives the transistor into conduction.
 The Circuit only conducts for the

positive half of the cycle.
 Can not amplify the entire cycle
emitter-follower circuit

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 To amplify the entire cycle, it is necessary to add a second class B amplifier
that operates on the negative half of the cycle.
 The combination of two class B amplifiers working together is called push-pull
operation
 There are two common approaches for using push-pull amplifiers to
reproduce the entire waveform.
1. Transformer Coupling
 The input transformer thus

converts the input signal to two
out-of-phase signals for the two
npn transistors.
 The output transformer combines the signals by permitting current in both

directions, even though one transistor is always cut off.

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2. Complementary Symmetry Transistors
 The figure shows one of the most popular types of push-pull class B amplifiers
using two emitter-followers and both positive and negative power supplies.
 This is a complementary amplifier because one emitter-follower uses an npn
transistor and the other a pnp, which conduct on opposite alternations of the
input cycle.

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Crossover Distortion
 When the dc base voltage is zero, both transistors are off and the input signal
voltage must exceed VBE before a transistor conducts.
 Because of this, there is a time interval between the positive and negative
alternations of the input when neither transistor is conducting, as shown in
Figure.
 The resulting distortion in the output waveform is called crossover distortion.
 Crossover distortion in the output signal refers to the fact that during the
signal crossover from positive to negative or vice versa there is some
nonlinearity in the output signal . The circuit does not provide exact switching
of one transistor off and the other on at the zero voltage condition.Both
transistor may be partially off so.
 Biasing the transistors in class AB improves the operation by biasing both
transistors to be on for more than half cycle.
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Class C amplifiers
 Class C amplifiers are biased so that conduction occurs for much less than 180o
 Class C amplifiers are more efficient than either class A , B, or AB
 The output amplitude is a nonlinear function of the input, so class C amplifiers

are not used for linear amplification.
 They are generally used in radio frequency (RF) applications, including resonance
circuits
Basic Class C Operation
 A class C amplifier is normally operated with a

resonant circuit load, so the resistive load is used
only for the purpose of illustrating the concept.
 The ac source voltage has a peak value that exceeds
the barrier potential of the base-emitter junction for
a short time near the positive peak of each cycle,
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Millers theorem
High frequency response of CS amplifier
Miller Approximation
References
• Chapter 11: Frequency Response
• millers theorem :page Number 546
• Solve Example 11.10,
• Study 11.2 : High frequency model of transistor from Razavi

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Power amplifiers , Millers Theorem and High frequency model of transistors

• Power amplifiers Chapter , Boylestad :

• Millers Theorem : examples Page No:553 (Razavi Text book)

 A disadvantage of class B or class AB is that it is more difficult to implement the

 The Circuit only conducts for the

EEDP - Basem ElHalawany

 The input transformer thus

 The output transformer combines the signals by permitting current in both

EEDP - Basem ElHalawany

EEDP - Basem ElHalawany

 The output amplitude is a nonlinear function of the input, so class C amplifiers

Basic Class C Operation

 A class C amplifier is normally operated with a

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