Jul 5, 2016

Class-A Amplifier

Class-A amplifier, which has centered Q point. Therefore the transistors operate only over the linear region of its load-line.

So, it is an amplifier in which the circuit output current flows throughout the complete cycle of the input signal i.e., conduction angle is 360o. In other words, the transistors remain forward biased right through the input cycle.

Characteristics of Class-A Amplifier

  • While the transistor functions over the linear portion of the load-line, the input waveform and output waveform are exactly same. For this reason class-A amplifier are characterized by a high reliability of the output.
  • Since its operation is constrained only over a small central region of the load-line, this class-A amplifier is meant only for amplifying the input signals of small amplitude. Large signals will move Q-point into non-linear regions near saturation or cut-off. Therefore it creates distortion.
  • Due to the restriction of the input signals amplitude, ac power output for each active device is small.
  • The overall efficiency of the class-A amplifier circuit is
                           = ac power delivered to the load  / total power delivered by dc supply
                           = average ac power circuit  / average dc power input

The maximum possible collector efficiency of a class-A amplifier with resistive load is 50%.
  • The collector efficiency of a transistor is defined as
                            =average ac power output  / average dc power input to transistor

Class-A Amplifier: Power Distribution

Single stage class-A Amplifier

The figure-a shows that the connection of common-emitter transistor which forms the active element of a single stage class-A amplifier. The figure-b shows its output characteristics with a centered Q point.

When ac input signal is applied, “Q point” moves up and down from its central position. The output current also will increase or decreases from its quiescent value ICQ. Similarly collector-emitter voltage VCE will increase or decrease from its quiescent line, average value of collector current is ICQ because positive and negative input signal swings will produce equal changes in ICQ.

The output or collector characteristics of a common-emitter connected transistor are shown in the figure below with centered Q-point for class-A operation. When the input signal is given, then the Q-point shifts to positions Q1 and Q2 alternatively.

Collector characteristics of Class-A Amplifier

The output current varies around its quiescent value from its highest value of ICmax to lowest value of ICmin around its quiescent value of VCEQ.

Transformer coupled Class-A amplifier

Transformer Coupled Class-A Amplifier
  • The efficiency of a direct coupled calss-A amplifier is poor. This problem can be solved by using a proper transformer for coupling the load to the amplifier as shown in the fig below.
  • Since the load is not directly coupled to the collector terminal, the dc collector current does not flow through it. In an ideal transformer, primary winding resistance is zero. Therefore, dc power loss is zero in the load.
  • In short, what the transformer does is to substitute ac load in place of ohmic or dc load.


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