Power Amplifiers

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

Power Amplifier

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

Audio Signal

While the voltage amplifier raises the voltage level of the signal, the power amplifier raises the power level of the signal. Besides raising the power level, it can also be said that a power amplifier 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 amplification, a normal transistor would not do. A transistor that is manufactured to suit the purpose of power amplification 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 amplification and power amplification. So, let us now try to get into the details to understand the differences between a voltage amplifier and a power amplifier.

Difference between Voltage and Power Amplifiers

Let us try to differentiate between voltage and power amplifier.

Voltage Amplifier

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

The voltage gain of an amplifier is given by

$$A_v = \beta \left (\frac{R_c}{R_{in}} \right )$$

The characteristics of a voltage amplifier 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 Rin should be low when compared to collector load RC.

  • The collector load RC should be relatively high. To permit high collector load, the voltage amplifiers are always operated at low collector current.

  • The voltage amplifiers are used for small signal voltages.

Power Amplifier

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

The characteristics of a power amplifier 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 coupling is used for impedance matching.

  • Collector resistance is made low.

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

S.No Particular Voltage Amplifier Power Amplifier
1 β High (>100) Low (5 to 20)
2 RC High (4-10 KΩ) Low (5 to 20 Ω)
3 Coupling Usually R-C coupling Invariably transformer coupling
4 Input voltage Low (a few m V) High (2-4 V)
5 Collector current Low (≈ 1 mA) High (> 100 mA)
6 Power output Low High
7 Output impendence High (≈ 12 K Ω) Low (200 Ω)