Class A Power Amplifiers with Transformer Coupled Load are also sometimes referred to as single ended power amplifier. The term "single ended" (denoting only one transistor) is used to distinguish it from push-pull amplifier using two transistors.
In case of a direct coupled class A power amplifier, the quiescent current flows through the collector resistive load and causes large wastage of dc power in it. This dc power dissipated in the load resistor does not contribute to the useful ac output power. Furthermore, it is generally inadvisable to pass the dc through the output device such as in a voice call of a loudspeaker. For these reasons an arrangement using a suitable transformer for coupling the load to the amplifier is usually employed, as shown in Fig. 1. This arrangement enhances the conversion efficiency by a factor of 2 by eliminating the dc power dissipation the load. Since the primary the of the transformer has negligible dc resistance, there is negligible dc power lost at this point. The ac power is, however, coupled magnetically across the transformer into the load RL. This coupling method also prevents a large dc current from flowing through the load, which otherwise could be harmful if the load were a loudspeaker, since it would cause saturation of the magnetic circuit and impair the reproduction of the audio signal.
Fig. 1. Transformer Coupled Class A Power Amplifier
This arrangement also permits impedance matching.
In a power amplifier circuit shown in Fig. 1, R1 and R2 provide potential divider biasing and emitter resistor RE is meant for bias stabilization. The emitter bypass capacitor CE is meant for preventing negative feedback in the emitter circuit. The input capacitor RL couples as signal voltage to the base of transistor but blocks any dc from the previous stage. A step-down transformer of suitable turn ratio (a = N1/N2) is provided to couple the high impedance collector circuit to low impedance load.
Impedance Matching
The power transferred from the power amplifier to the load will be maximum only if the amplifier output impedance equals the load impedance RL. This is in accordance with the maximum power transfer theorem. If we were not able to achieve the above condition, lesser power will be transferred to the load RL, though the amplifier is capable of delivering more power, and rest of power developed would be lost in the active device. 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. This is accomplished by using a step-down transformer of suitable turn ratio.
Circuit Operation
In this circuit dc (winding) resistance determines the load line. Typically, this resistance is quite small (assumed to be zero) providing dc load to be a vertical line rising from VCC, as shown in Fig. 2. When an ac signal is applied to the base of the transistor the collector current will vary around the operating point Q.
In order to have maximum ac power output, the peak value of collector current due to input ac signal alone should be equal to the zero signal collector current. To achieve this, the operating point Q is located at the centre of the load line. This is achieved by adjusting the biasing circuit R1, R2 and RE).
Fig. 2. Output Characteristics
When an ac signal is applied, collector current fluctuates from maximum to minimum (zero), and operating point Q moves up and down the load line. At the peak of the positive half cycle of the input signal, the total collector current Ic max = 2ICQ and collector-emitter voltage Vce min = 0 while at the peak of the negative half cycle of the input signal, the collector current Ic min = 0 and collector-emitter voltage Vce max = 2VCC. Thus collector-emitter voltage varies in opposite phase to the collector current. The variation of collector voltage appears across primary of the transformer. Now ac voltage is induced in the transformer secondary which in turn develops ac power and supplies to the load.
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