Important features of a power amplifier:

Impedance matching:

As the power amplifiers are handling a large amt of power it is important to transfer maximum power to the load. To do so, impedance matching between the o/p impedance of the power amplifier and load has to be ensure.

As the loads like loud speakers have low impedance, the o/p imp of a power amp. Also must be low. Therefore, the common collector or emitter follower circuit is normally used as the power amplifier because it has a low o/p imp.

A transformer may also be used for imp matching on the o/p side.

Use of power transistors.

As the power amplier is designed to handle large powers, the transistors used must be capable of withstanding to large voltages and currents.

A large power gets dissipated in these transistors in the form of heat. Therefore, we cannot use the ordinary logic level transistors.

Instead specially manufactured power transistors are used.

Power amp are bulky:

The power transistors are bigger in size than the low ones and they are mounted on “heat sinks” which are of large surface area.

The “heat sinks” help to reduce the temp of power transistors by dissipating the heat to the surroundings.

Due to the use of heat sinks and large size power transistors, the power amplifiers’ become bulky.

Harmonic distortion:

Due to non-linear characteristics of transistors, distortion will be present on the o/p side of the amp.

That means, frequency components which are present on the i/p side will be present in the o/p.

This will distort the shape of the o/p of waveform. To measure the percentage of distortion, analysis of

o/p waveform is carried out.

H-parameters cannot be used for analysis:

The h-parameters are called as small signal h-parameters. They are valid if and only if the i/p signal is small enough to operate the amp close to Q pt. on the linear portion of transfer characteristics of the transistor.

Power amp cannot satisfy this.

Efficiency:

The efficiency is defined as ratio of o/p power to total input power.

$\% (n) \text{Efficiency} (n) = \frac{p_{out}}{p_{in}}\times 100$

But $P_{in}=P_{out}+P_{losses}$

$\%(n)=\frac{P_{out}}{P_{out}+P_{losses}} \times 100$

Where $P_{losses}$ is the power lost in power transistor

i)If transistor is biased to operate in active region, then power loss taking in it will be high and efficiency will be low.

ii) But if the transistor is biased in the cut-off region or below cut off and in saturation region then the power loss taking place in its low and efficiency will be increased.