Various Stages in a Practical Power Amplifier and Block Diagram:
Power amplifier is meant to amplify a weak signal until sufficient power is available to operate an output device such as a loudspeaker, a solenoid or a relay. Power amplifier, to provide the desired power amplification, has generally 3 stages (voltage amplification stage, driver stage and output stage), as shown by a block diagram in Fig. 17.3. The Various Stages in a Practical Power Amplifier and Block Diagram will discuss here
1. Voltage Amplification Stage: As we know already that, the signal developed by the input transducer is very small (of the order of few millivolts) and needs sufficient amplification so as to operate the output device. Therefore, for raising the level of the weak input signal, it is amplified in two or more stages, R-C coupling is usually employed.
2. Driver Stage: The stage that immediately precedes the output stage is called the driver stage. The output from the last voltage amplification stage is fed to the driver stage and output from the driver stage is fed to the output stage. The driver stage renders power amplification as usual. Here, concentrated effort is made to provide maximum power gain and so transformer coupled class A power amplifier is employed in this stages in a practical power amplifier. The driver transformer is usually a step-down transformer and facilitates impedance matching.
3. Output Stage: The output stage essentially consists of a power amplifier and is meant for transferring maximum power to the output device. In order to transfer maximum power at high efficiency, push-pull arrangement is employed in the output stage (if a single transistor is used as a class A amplifier in the output stage to provide faithful amplification, the operation efficiency will be quite low). In this arrangement two transistors are used in class B operation and are fed from the centre-tapped secondary of the transformer whose primary forms the collector load for the driver stage.