Large signal amplifier or power amplifier:- Class A,Class B ,Class C

INTRODUCTION :-

• Large signal amplifier means a small signal applied to the input of amplifier where amplifier get amplifying the signal means a weak signal are changed in strong signal by the help of signal amplifier. 
• The power amplifier are the large signal amplifier which Rises the power level of the input signal.
• The power amplifier is also defined as the device which convert DC power to AC power ,whose action is controlled by the input signal.
• DC POWER =AC POWER +LOSS
• The power amplifier is divided into the following category:- 

1. Radio power amplifier 
2. Audio power amplifier 

1. Radio power amplifier:-

Radio power amplifier is also known as tuned power amplifier,which Rises the power level of the signal which has radio frequency range.

On the basis of operation the radio power amplifier are divided into following categories :-

1. CLASS  A POWER  AMPLIFIER  
2. CLASS  B POWER  AMPLIFIER  
3. CLASS  C POWER  AMPLIFIER  

In short term :-

CLASS A POWER AMPLIFIER:-

When the collector current flows during whole cycle that type of power amplifier is known as class A power amplifier.

CLASS B POWER AMPLIFIER:- 

When the collector current flows only during in positive cycle of the input cycle is said to be class B power amplifier. 

CLASS C POWER AMPLIFIER:- 

When the collector current flows for less than half cycle of the input signal or less than 180 degree of the input signal, then the amplifier is known as Class C power amplifier.

EXPLANATION OF CLASS A POWER AMPLIFIER :- 

• Class a power amplifier has defined as the power amplifier in which output current flow for the entire whole cycle of the input signal. 
• They worked in active region. 
• Their distortion is less.
•  Their efficiency is less :- 25% percent can increase to 50 %.
•  Conduction angle 0 degree to 360 degree.
• Efficiency(η) is 50 degree.

Working :- 

When the transistor remains forward bias throughout the input cycle. Class A power amplifier is one in which output current flow during the entire cycle of input signal. Thus the operating point or Q point is selected in such a way that the transistor operate through only over the linear region of its load line.

So this amplifier can amplify input signal of a small magnitude.

The transistor operate over the linear portion of the load line the output waveform exactly similar to the input waveform. Hence this amplifier is used where the freedom from distortion is prime AIM .figure (a) shows CE connected transistor forming an active element of class a amplifier and figure B Shows the output characteristic with operating point Q. Here Ic(Q) and Vce(Q) is represent that no signal is applied. when the signal is given operating point Q shift either to Q1 and Q2 the output current increase to Ic max and decreases to Ic min . Similarly the collector emitter voltage increased to Vce Max and decrease to Vce min.

DC power drawn from Collector battery Vcc is given by 

Pin = voltage × current 

       = Vcc × Ic (Q)

This power is used in following two part:- 

 (a) Power dissipated in collector load and heat is given by

 P(RL)= (current)^2 × resistance 

           = Ic^2 × Rc 

 (b) power gain to the transistor is given by

 P (tr) = P(in) - P(RL) 

          =Vcc .Ic(Q) - Ic^2 (Q).RC