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Home Transistor High Frequency Applications The Transistor at High Frequencies Current Gain  
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Current GainAuthor: Leonard Krugman where α is the current gain of the operating frequency f; α_{1} is the low frequency current gain; and f_{c} 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} cutoff frequency of f_{c} = 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 r_{c} decreases rapidly for a ratio of greater than 0.15, falling to about 10% of its low frequency value at = 1, and then remains at that value. The collector junction capacitance C_{c} also decreases as the operating frequency increases above an greater than 0.15, but does not decrease as rapidly as r_{c}. In a typical characteristic, C_{c} drops to approximately 75% of its low frequency value a =1 and to about 50% at=10, after which the curve levels out.Due to the coupling betweenthe input and output circuits, , the input impedance contains a reactive componentbeyond the emitter shunt capacitance. At the α cutoff frequency f_{c}, 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).


Home Transistor High Frequency Applications The Transistor at High Frequencies Current Gain 