Power Amppfiers

In practice, any amppfier consists of few stages of amppfication. If we consider audio amppfication, it has several stages of amppfication, depending upon our requirement.

Power Amppfier

After the audio signal is converted into electrical signal, it has several voltage amppfications done, after which the power amppfication of the amppfied signal is done just before the loud speaker stage. This is clearly shown in the below figure.

While the voltage amppfier raises the voltage level of the signal, the power amppfier raises the power level of the signal. Besides raising the power level, it can also be said that a power amppfier is a device which converts DC power to AC power and whose action is controlled by the input signal.

The DC power is distributed according to the relation,

DC power input = AC power output + losses

Power Transistor

For such Power amppfication, a normal transistor would not do. A transistor that is manufactured to suit the purpose of power amppfication is called as a Power transistor.

A Power transistor differs from the other transistors, in the following factors.

It is larger in size, in order to handle large powers.

The collector region of the transistor is made large and a heat sink is placed at the collector-base junction in order to minimize heat generated.

The emitter and base regions of a power transistor are heavily doped.

Due to the low input resistance, it requires low input power.

Hence there is a lot of difference in voltage amppfication and power amppfication. So, let us now try to get into the details to understand the differences between a voltage amppfier and a power amppfier.

Difference between Voltage and Power Amppfiers

Let us try to differentiate between voltage and power amppfier.

Voltage Amppfier

The function of a voltage amppfier is to raise the voltage level of the signal. A voltage amppfier is designed to achieve maximum voltage amppfication.

The voltage gain of an amppfier is given by

$$A_v = eta left (frac{R_c}{R_{in}} ight )$$

The characteristics of a voltage amppfier are as follows −

The base of the transistor should be thin and hence the value of β should be greater than 100.

The resistance of the input resistor R_in should be low when compared to collector load R_C.

The collector load R_C should be relatively high. To permit high collector load, the voltage amppfiers are always operated at low collector current.

The voltage amppfiers are used for small signal voltages.

Power Amppfier

The function of a power amppfier is to raise the power level of input signal. It is required to depver a large amount of power and has to handle large current.

The characteristics of a power amppfier are as follows −

The base of transistor is made thicken to handle large currents. The value of β being (β > 100) high.

The size of the transistor is made larger, in order to dissipate more heat, which is produced during transistor operation.

Transformer couppng is used for impedance matching.

Collector resistance is made low.

The comparison between voltage and power amppfiers is given below in a tabular form.