DETERMINING THE CHARACTERISTICS OF THE AMPLIFIER CIRCUITS

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Student
Instructor
Electronics
Date
LO1: DETERMINING THE CHARACTERISTICS OF THE AMPLIFIER CIRCUITS.

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1.1. Operational characteristics of class A, B and AB power amplifiers
Class A amplifier
This is the amplifier whose topology uses one output transistor that is biased around the Q-point.
Biasing around the Q-point, midpoint of its load line, ensures that the transistor is never operated
in either saturation region or cut-off region. This enables 360 degrees of transistor’s current
conduction which never turns OFF class-A amplifier topology.
Output stage of class A amplifier is biased ON throughout so as to realize high gain and linearity.
The output idling current of the class A amplifier at the production stage is usually equal or
greater than extreme current of the load so as to generate the largest signal at the output. Class A
amplifier is same as current source since it operates in the linear region of its distinctive
characteristic curve conducting complete 360 degrees of the resultant waveform. Continuous
current flow in this type of the amplifier results to continuous amplifier power loss thus
generating a lot of heat that is not suitable for high power amplifier application (Electronics
Tutorials 2019).
Applications
Loudspeakers.
Class B amplifier.
In this type of amplifier, the conduction cycle of every active device is 180 degrees. For instance,
positive biased transistor conducts while negative biased transistor is off when input waveforms
is on the positive cycle and vice versa. The transistors therefore alternatively conduct a half the
cycle. Combination of two halves at the output stage results to a full linear output waveform to
supply load with continuous current.

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Application.
It is used in radio frequency.
Class AB amplifier.
This amplifier is the amalgamation of class A and B amplifier. The crossover distortion is
mitigated against by allowing both transistors to conduct concurrently around the crossover point
of the waveform.
It is made possible by incorporating small biasing voltage, provided by diodes, make conducting
devices be on ON conduction state for more than a half cycle but less than full conduction cycle.

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Depending on the bias point, the conduction of this type of the amplifier lies between 180 to 360
degrees (Electronics Tutorials 2019).
Application
Used in audio amplifiers.
1.2 Differences for class A, B and AB amplifier in performance characteristics.
Class A amplifier
 It has an excellent linearity
 It has a high gain
 When designed correctly, it has low level of signal distortion
 Rarely applied in high power amplification because of the thermal power supply
applications and low efficiency, approximately 30%.
 They are mainly suitable for audio amplification due to their best sounding quality.
Class B amplifier.
 It has higher efficiency of about 50%
 Transistors dead band voltages ¿) result to distortion at zero-crossing point of the
waveforms known as crossover distortion. Until these voltages are exceeded, the
transistors are not biased thus distorting waveforms that fall within this bracket thus not
suitable for audio amplification (Electronics Tutorials 2019).
Class AB amplifier
 It more efficient with excellent linearity than class A and B amplifiers
 The conversion efficiency operates between 50 to 60%.
1.3 A practical test on the amplifier’s performance and commenting on the results.
The output of the amplifier is as shown in the figure below. The amplified output voltage signal
is 180 degrees out of phase with the input signal.

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Simulation of the operational amplifier in multisim
The circuit was constructed in Multisim as shown in the diagram below.

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The output of the circuit with reference to the input voltage signal is inverted and amplified. The
signals are operating with the same frequency.