A Power amplifier is an electronic device which provides sufficient power to an output load to drive a speaker or other power device, typically a few watts to tens of watts. The main purpose of this amplifier is to raise the power level of the input signal, to get the large power at the output. To match the input signal shape with larger amplitude, power amplifier takes the energy from the power supply & controls the output. In our previous article, we explain Amplifiers, Transistors, diodes, rectifiers in detail. In this article, we are going to explain Power amplifier working, performance and classifications i.e. Class A, Class B, Class C, Class AB amplifier in detail.
Power Amplifier Basics:
It is the mostly used amplifier in electronics circuit also known as large signal amplifiers. The main features of the large signal amplifier are the circuit power efficiency, impedance matching to the output device and the maximum amount of power that the circuit is capable of handling.
(i) Power Amplification:
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As per the law of conservation of energy, the power amplifier cannot amplify power. The power amplifier is just a DC to AC power converter whose action is controlled by the input signal. During operation, it takes DC power from the supply connected to the output circuit and converts it into useful AC signal. The type of AC power developed at the output of the power amplifier is controlled by the input signal. Thus, whatsoever power is developed is fed to the load i.e. loudspeaker.
(ii) Power Amplifier Properties:
The performance of the power amplifier is understood on the basis of quantities like collector efficiency, power dissipation capability and distortion. We discuss all such properties one by one. Have a look at it.
The change of the output waveform from the input waveform of an amplifier is known as Distortion. When the output wave shape is not an exact replica of the input wave shape, the amplifier is said to have some distortion. As compare to voltage amplifier, power amplifiers handle larger signals, therefore, the problem of distortion occurs immediately. So in these type of amplifiers, we have to take care of this factor a lot.
(ii) Collector Efficiency:
It is defined as the ratio of AC output power to the DC input power or zero signal power of a power amplifier. Collector efficiency tells us the percentage of DC power converted into AC power by the amplifier. For example, if the DC power supplied by the source is 10W and AC output power is 4W, then the collector efficiency is 40% i.e. the greater the collector efficiency, better is the amplifier.
Calculation of Collector efficiency:
Mathematical expression of collector efficiency is given as:
η = AC power output / DC power input
η = Pac / Pdc
As we know,
η = Vce Ic / Vcc Ic
Therefore, collector efficiency is also given as;
η = [Vce (peak to peak) * Ic (peak to peak)] / 8 Vcc Ic
(iii) Power Dissipation capability:
During operation, the ability of power transistor to dissipate heat developed in it is known as its power dissipation capability. As we know, power transistors used in a power amplifier carries large current during operation which heats up the collector junction. The rise in temperature influences the operating conditions of the transistor. Therefore, the transistor used must be capable of dissipating this heat to the surroundings.
Classification of Power Amplifier:
When power amplifier handles large signals, sometimes many of them are driven so hard by the input signals that the collector current reaches either in the cut-off or in the saturation regions during its peak values. Mainly it divides into two categories:
(i) Audio Power Amplifier:
These are known as the small signal amplifier generally raise the power levels of signals that have the audio frequency range of 20 Hz to 20 KHz. In an audio playback system, it is the electronic final stage.
(ii) Radio Power Amplifier:
These are known as the large signal amplifiers generally raise the power levels of signals that have the radio frequency range. It basically converts a low-power radio-frequency signal into a higher power signal and amplify a specific frequency or narrow band of frequencies & rejecting all other frequencies.
On the basis of mode of operation i.e. the portion of the input cycle during which the collector current flows through the circuit, it is classified into four classes:
- Class A power amplifier
- Class B power amplifier
- Class C power amplifier
- Class AB power amplifier
As soon as possible in our next article, we explain all classes of the power amplifier in detail.
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