What is Power amplifier? Class.....

Power amplifier

1.  It is a large signal amplifier. Due to large signal variation,it has large AC output  current and voltage.Hence it can supply large signal power to load.
2. Power amplifier can cause harmonic distortion in the output due to non-linerity of transistor.Hence power amplifier should be used at final stage potput stage to keep the distortion minimum.
3.  A power transistor is used in power amplifier.
4.   Power transistor is operated at greater I_c and V_ce  (I_c is a few ampere & V_ce is in 10’s of volt).
5.  Due to greater power dissipation,power transistor produce greater heat.Hence “Heat sink” should be used to increase the rate of heat transfer to surroundings (heat sink is made from a good conductor of Heat such as Aluminimum).
6. Performance of power amplifier is measured in terms of conversion efficiency and figure of merit.

Power amplifier

       .     Conversion Efficiency:

·        A power amplifier supplies a large AC power to load because it internally converts a part of DC power drawn from biasing supply into AC power.

·   Conversion Efficiency is a measure of ability of a power amplifier to convert DC power into AC power.

·        Efficiency  = Ac signal power supplied to Load / DC power drawn  from   biasing supply

_·        % efficiency = P_{ac /} P _{dc * 100%}                          

2    .    Figure of Merit:

·        It is a ratio of maximum power dissipation in transistor and maximum AC power which can be supplied to load.

·   As power dissipation in transistor is unwanted, figure of merit (F.O.M.) should be smaller.

·        For ideal power amplifier; F=0 & Efficiency= 100%

·        F = P_{d max.} / P_{ac max.}

 Classification of power amplifier:

 According to position of operating point on load line , amplifiers are divided into 4       type

        1    Class-A

        2      Class-B

        3      Class-AB

        4    Class-C  

    1.  Class-A amplifier :

o   An amplifier in which operating point is approximately in the middle of load line.

  o If Q point is at centre of load line then collector current can have max                             symmetrical.Hence Class-A amplifier provide distortion less output.

  o   When Q-point is at centre of load line,Quiescent power dissipation will be the                highest.Hence in class-A amplifier power dissipation of transistor is greater                   (Disadvantage).

            

Class-A amplifier

     o   Efficiency= 25% , Vmin = 0

  o   Due to low efficiency , Class-A direct coupled amplifier is not preferred as                    a power amplifier.

  o   Efficiency's is low due to- 

•       Greater power dissipation in Transistor.
•         DC power dissipation in load.

  o   Figure Of Merit , F= 2

 2.Class-B amplifier :

  o   An amplifier in which transistor is operated either in cut-off region or in saturation        region.Operating point is either at bottom end (or) at top end of load line.

          

Class-B amplifier

  o  In Class-b amplifier Quiescent power dissipation is zero i.e. transistor dissipate              zero power in absence of AC input.

    Ex: If Q-point in cut-off.

Q-point in cut-off

   When Q-point in cut-off,collector current can very only in positive(+) direction.

    Ex: If Q-point in saturation.

Q-point in saturation

  When Q-point is in saturation ,collector current can vary only in negative(-) direction.

 Application :

  o  Class-b push-pull amplifier is used as untuned power amplifier (or) Audio                      Frequency power amplifier.

3.Class-AB power amplifier :

  o   An amplifier in which operating point is between Class-A and Class-B position.

       

Class-AB power amplifier

  o   Practically, Q-point in Class-AB amplifier is slightly above cut-off (or) slightly              below saturation.

  o When Q-point is above cut-off, clipping occours in Ic waveform during negative            cycle and clipping occours in Vce waveform during positive cycle.

  o    Hence output of Class-AB amolifier will be more than half sinusoidal.

  o   Maximum efficiency lies between 50% to 78.5%

  o   Power dissipation is less than class-A but more than class-B.

  o   Distortion in output is greater than class-A but less than class-B.

4.Class-C power amplifier :

  o   An amplifier in which transistor is operated either in deep cut-off (or) in strong              saturation.

  o   Deep cut-off: if V_be < V_be (cut-off)

Vbe (cut-off) is the Vbe value at which transistor becomes fully OFF.                  I_c  = I_co

o   Strong saturation:  I_b >>  I _bmin

o  Practically , in a class-C amplifier       transistor is operated in deep cut-off by      applying a  negative voltage between base and emitter.

  o   When Ac input is applied,transistor remains off in the negative cycle and conducts for a short interval during positive cycle.

  o   Hence output signal of Class-C amplifier will be less than half sinusoidal.

  o   In class-C amplifier has the highest efficiency (90-100%)

 

Application:

  o   Class-C amplifier is used as Tuned power amplifier.

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