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Current Gain

Author: Leonard Krugman

In view of these limitations, the basic circuit illustrated in Fig. 7-1 is not a useful approximation of transistor performance at high frequencies. To modify this circuit for accurate representation of high frequency equivalence requires that all of the internal parameters be specified in a complex form (magnitude and phase angle) as functions of the frequency. In most cases, however, it is sufficiently accurate to modify Fig. 7-1 to include only the variation of α with frequency, since few design problems justify the details required for exact equivalence. The variation in current gain can be satisfactorily approximated by the relationship:


where α is the current gain of the operating frequency f; α1 is the low frequency current gain; and fc is the frequency at which the current gain is 0.707 of its low frequency value (3 db down).

As a numerical example of the above, compute the current gain tor a junction transistor having a low frequency current gain of ai = 0.95, an a cut-off frequency of fc = 10 me, and an operating frequency of 7.5 me. Then


Including only the junction capacitance and variation in a in the low frequency circuit makes all the computed values far from exact. In addition to the capacitive reactance of the emitter, there is also considerable variation with frequency in the collector resistance and collector junction capacitance. The collector resistance rc decreases rapidly for a ratio of transistor_basics_07-8.gif greater than 0.15, falling to about 10% of its low frequency value at transistor_basics_07-9.gif = 1, and then remains at that value. The collector junction capacitance Cc also decreases as the operating frequency increases above an transistor_basics_07-10.gif greater than 0.15, but does not decrease as rapidly as rc. In a typical characteristic, Cc drops to approximately 75% of its low frequency value a =1 and to about 50% at=10, after which the curve levels out.transistor_basics_07-11.gifDue to the coupling betweentransistor_basics_07-12.gifthe input and output circuits, transistor_basics_07-13.gif, the input impedance contains a reactive componentbeyond the emitter shunt capacitance. At the α cut-off frequency fc, the reactive component is approximately equal to the resistive input component. This causes the input impedance to be inductive for the grounded base connection, and capacitive for the grounded emitter connection (due to phase reversal).

Last Update: 2010-11-17