![]() ![]() If the collector current flows only during the positive half-cycle of the input signal, it is called as class B power amplifier. The only disadvantage of class A power amplifier is the low output power and low collector efficiency. Since the output wave shape is exactly similar to the input wave shape, hence, such amplifiers have least distortion. The operating point Q is so selected that collector current flows at all times throughout the full cycle of the applied signal. The operation of a class A power amplifier as a.c. This is accomplished by using a step-down transformer of suitable turn-ratio. Hence for transfer of maximum power from amplifier to the output device matching of amplifier output impedance with the impedance of output device is necessary. If we were not able to achieve the above condition, lesser power will be transferred to the load R L, though the amplifier is capable of delivering more power, and rest of power developed would be lost in the active device. This is in accordance with the maximum power transfer theorem. The power transferred from the power amplifier to the load such as loudspeaker, will be maximum only if the amplifier output impedance equals the load impedance R L. A step-down transformer of suitable turn ratio is provided to couple the high impedance collector circuit to low impedance load. The input capacitor C in couples ac signal voltage to the base of the transistor but blocks any dc from the previous stage. The emitter bypass capacitor C E is meant for R E to prevent ac voltage. In a power amplifier circuit shown R 1 and R 2 provide potential divider biasing and emitter resistor R E is meant for bias stabilization. This arrangement also permits impedance matching. Hence, it is generally inadvisable to pass t he current through the output device such as in a voice coil of a loudspeaker.įor these reasons an arrangement using a suitable transformer for coupling the load to the amplifier is usually employed, as shown above. As a result this dc power dissipated in the load resistor does not contribute to the useful ac output power. ![]() In case of a direct-coupled class A power am plifier as shown above, the current flows through the collector resistive load causes large wastage of dc power in it. ![]()
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