Power Amplifiers

Introduction:

* A power amplifier is desi gned for good power gain.It must handled large votage current savings.

* These high voltages and currents makes the power gain.

* It is very important to have good effiency in a power amlifier.

* An efficient power amplifier delivers the most signal power for the dc power it take from the supply.

Power Efficency:

The job of a amplifier is to charge dc power in to output signal power.Its efficiency is given by

    Power Efficiency=Pout(ac)  *100%

                                           Pin(dc)

Power Dissipation:

The power dissipation (heat) is the difference between input and output.

              

                         Power Dissipation=    Pin - Pout

Example:

What is the effiency of an amplifier that draws 2A  from a 40V supply when delivering 52w of signal power?How many watts are dissipated in this amplifier?

 Classes Of Amplifiers:

* Depending on the bias which controls the operating point of the amplifier power amplfiers can be classified under several amplifier classes.The main classes are class A,B,AB and C amplifiers.As simply we can said that these are various method to build amplifier circuits.

Class A power Amplifier:

* The class A power amplifier operates near the center of the load line.

* It is highly efficient.

* But it offer low distortion.

 * It has the most simple design.

Class A power Amplifier With Transformer Coupling:

* One reason why the class A amplifier is so wasteful is that Dc power is dissipated in the load

* A big improvement is possible by removing the load from the dc circuit.

* A transformer coupled to the load will produce twice as much signal power.

* A transformer impedence ratio is equal to the square of the turns ratio.

* The ac load line extends up to 32 V.

* A transformer is type of inductor.when the field collapses,a voltage is generated.This adds to the supply voltage.

* The dc load line is vertical.This is because have not changed.The change is the absence of 80 dc resistance.All the suppply now drops across the resistor.

* output power doubles.

* Maximum effiency.

* Transformers are expensive components ,so the cost ofa transformer coupled class A amplifier will be more expensive.

Class B power Amplifier:

* Class B amplifier is biased at cutoff.

* Efficiency is much better.

* Only half the input signal is amplified.

* This produces extreme distortion.

* Two Transistors can be operated in class B.

* One amplifier the positive going portion of the input and the other the negative going portion.

* Reduce distortion.

* This is called the 'push-pull' amplifier.

* is called the driver transformer 

* is called the output tranformer

* provides and with signal drives

* combines the two signals and  supplies output to the load

* With no signal input and are cut off

* When the input signal produces the secondary polrity in  is turned on.

* Current will flow through half of the primary of .

* Signal will appear across the secondary.

* So the positive going portion of the input signal has been amplified and appears across the load .

* By operating  two transistors in push-pull, much of the distortion has been eliminated. The circuit amplifiers almost the entire input signal.