Efficiency Stabilization

Author: Leonard Krugman

Figure 5-8 represents a typical Class A transistor amplifier using d-c operating biases as described in the preceding paragraphs. The stabilizing resistor in the emitter circuit is made equal to the load impedance in this case. This condition provides maximum protection against variations in I_co, since the power available from the battery is effectively limited to the maximum collector dissipation. While the arrangement shown in Fig. 5-8 prevents transistor damage due to excessive collector variations, half of the d-c power is dissipated across the stabilizing resistor. The maximum efficiency of a class A transistor is 50%; using a stabilizing resistor whose value is equal to the load resistor reduces the efficiency to a maximum of 25%.

In general, this amount of stability control is needed only in mass production applications if transistors having a wide tolerance range are to be used. Actually, the reproducibility of transistor characteristics has improved rapidly during the past few years. There is no reason why this trend should not continue, and eventually permit the attainment of amplifier efficiency values very close to the theoretical maximum. In most circuits, between 5 and 10% of the collector d-c power (E_cI_c)is satisfactory for normal stabilization. In the circuit shown in Fig. 5-8, for example, a resistance of 100 ohms between the emitter and ground would be sufficient.

Fig. 5-8. Class A amplifier.