What is Power Amplifier ?
A power amplifier usually electronic amplifier designed to increase the magnitude of power of a given input signal. It's constitutes the last stage of a multistage amplifier system and serves to supply sizable power to the loads of output devices like speakers, headphones, RF transmitter and so on.
The efficiency of a power amplifier is the ratio of the ac output power to the power delivered by the supply source. The efficiency increased from classes A to class C via class AB and class B Amplifiers.
The operations of classes AB, B and C are nonlinear and so generate harmonics of the signal frequency at the output current. Two methods are employed to reduce or eliminate the harmonic components. One method employs the push-pull connection over a wide band of frequencies in the low or AF range.
The second method uses a parallel resonant circuit as the amplifier load over a narrow band of frequencies in the RF range. Class C operation of power amplifiers is not used in the AF range owing to the excessive harmonic distortion and the requirement of a parallel resonant circuit.
Why Power amplifier is used ?
This is because they are all the power MOSFET power BJTs are using in this particular power amplifier general transistor has some limitation to amplify in the range of temperature on the parameter of temperature on the parameter of voltage on the parameter of current and so on.
Purpose of a Power Amplifiers :-
The purpose of a power amplifier is to convert dc power from the supply source to ac signal power at the load. A special bipolar transistor, referred to as a power transistor, is employed as the active amplifying element in a power amplifier.
The special features of a Power transistor are as follows :
- The emitter and the base regions of a power transistor and heavily doped.
- the ohmic resistance between the emitter and the base is considerably reduced in a power transistor by increasing the areas of contact.
- the area of the collector layer is made large and the collector is connected to a metallic heat sink which quickly removes the heat produced at the collector base junction.
Circuit Diagram of Power Amplifier :
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since a power amplifier delivers in large output power, a power transistor operate at high collector current and voltage that a transistor serving as a voltage amplifier.
The three important operating limits of power transistor are the voltage, current and the power dissipation ratings. This rating are specified by manufacturers.
When the collector junction voltage exceeds the rated value, the junction breaks down. if the junction voltage is close to the breakdown voltage and the collector current exceeds the rated limit, a second breakdown occurs.
Consequently , the transistor may be damaged. also the power is dissipated as heat at the rivers biased collector junction which have a high resistance. The collector junction thus becomes hot.
When the power dissipation is larger than the rated value the excessive junction temperature may destroy the transistor.
A power amplifier handles a large signal and the excursion of the operating point may go beyond the linear region of the amplifiera active device characteristics.
A graphical method using the load line is therefore an employed in the analysis of a powers amplifier.
This is unlike the case of a voltage amplifier handling is small signal; hence the excursion of operating point is confined to a small linear region of the amplifier device characteristics and an equivalent circuit analysis can be used.
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