A class A power amplifier is defined as a power amplifier in which output current flows for the entire cycle (360 degree) of the input signal. In other words, the transistor remains forward biased throughout the input cycle.
A schematic circuit of series fed class A large signal amplifier using resistive load RC is shown in Figure 2. The term series fed is derived from the fact that the load RC is connected in series with the transistor output. The only difference between this circuit and the small signal amplifier circuits considered previously is that the signals handled by the large signal circuit are in the range of few watts. This circuit is seldom used for power amplification because of its poor collector efficiency but will give clear understanding of class A operation to the readers.
The output characteristics with operating point Q are shown in Fig.3. ICQ and VCEQ represent no signal collector current and collector-emitter voltage respectively. When an input ac signal is applied to to the amplifier circuit given in Fig. 2, the output will vary from its dc bias operating voltage and current. A small input input signal, as indicated in Fig. 3, will cause the base current to vary above and below the dc bias point, which will then cause the collector current (output) IC to vary from the dc bias point set as well as the collector-emitter voltage Vce to vary around its dc bias value. With the strengthening of the input signal. the output will vary further around the established dc bias point until either the current or voltage attains a limiting condition. For the current this limiting condition is either zero current at the lower end or VCC/RC at the upper end. For the collector-emitter voltage this limit is either 0 V or the supply voltage VCC. Let the output current vary between limits Ic min and Ic max and similarly the collector-emitter voltage between limits Vce min and Vce max.
Fig. 1
A schematic circuit of series fed class A large signal amplifier using resistive load RC is shown in Figure 2. The term series fed is derived from the fact that the load RC is connected in series with the transistor output. The only difference between this circuit and the small signal amplifier circuits considered previously is that the signals handled by the large signal circuit are in the range of few watts. This circuit is seldom used for power amplification because of its poor collector efficiency but will give clear understanding of class A operation to the readers.
Fig. 2
The output characteristics with operating point Q are shown in Fig.3. ICQ and VCEQ represent no signal collector current and collector-emitter voltage respectively. When an input ac signal is applied to to the amplifier circuit given in Fig. 2, the output will vary from its dc bias operating voltage and current. A small input input signal, as indicated in Fig. 3, will cause the base current to vary above and below the dc bias point, which will then cause the collector current (output) IC to vary from the dc bias point set as well as the collector-emitter voltage Vce to vary around its dc bias value. With the strengthening of the input signal. the output will vary further around the established dc bias point until either the current or voltage attains a limiting condition. For the current this limiting condition is either zero current at the lower end or VCC/RC at the upper end. For the collector-emitter voltage this limit is either 0 V or the supply voltage VCC. Let the output current vary between limits Ic min and Ic max and similarly the collector-emitter voltage between limits Vce min and Vce max.
Fig. 3
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