What is an audio power amplifier ? Explain the difference between voltage amplifier and power amplifier.
Transistor Audio Power Amplifier
A transistor amplifier which raises the power level of the signals that have audio frequency range is known as transistor audio power amplifier.
A practical amplifier always consists of a number of stages that amplify a weak signal until sufficient power is available to operate a output device such as loud speaker.
The first few stages in a multistage amplifier have the function of only voltage amplification however, the last stage should provide maximum power. Hence, this final stage is known as power stage.
The block diagram of an audio amplifier is shown in the figure below.
The last stage i.e. the power amplifier differs from all the previous stages in that here a concentrated ehhort is made to obtain maximum power.
A transistor which is suitable for power amplification is known as a power transistor.
It differs from other transistors mostly in size. It is normally larger for handling the large amount of power it delivers.
Audio power amplifiers are used to deliver a large amount of power to a low resistance load. Typical values of load resistance range from 300 Ω for transmission antenna to 8 Ω for loudspeaker.
The typical power output rating of a power amplifier is 1 W or more.
Difference Between Voltage Amplifier and Power Amplifier
A voltage amplifier is designed to achieve maximum voltage amplification. However, it is not important to raise the power level.
On the other hand, a power amplifier is designed to obtain maximum output power.
The voltage gain of an amplifier is given by :
In order to achieve high voltage amplification, the following points should be noted :
- The transistor with high β, more than 100 is used in the circuit. In other words transistors with thin base are used in this case.
- The input resistance of the transistor should be low as compared to the collector load resistance.
- A relatively high load resistance is used in the collector. To permit this condition, voltage amplifiers are always operated with at low collector currents nearly 1 mA.
A power amplifier is needed to deliver a large amount of power and hence it has to handle a large amount of current. In order to achieve high power amplification, the following points should be noted :
- The size of the power transistor is made considerably larger in order to dissipate the heat generated in the transistor during operation.
- The base is made thicker to handle large current. In other words transistors with smaller β (normally between 5 to 20) are used in this case.
- Transformer coupling is used for impedance matching.