The current gain of a bipolar transistor drops at high frequencies because of

Solution (By Examveda Team)

Option A: Transistor capacitances is correct because at high frequencies, the capacitances present in a bipolar transistor (such as base-emitter and base-collector junction capacitances) start to have a significant effect. These capacitances limit the current gain, causing a drop in the transistor's performance at high frequencies. The capacitive effects become more dominant as frequency increases, leading to a reduction in the current gain.

Option B: High current effects in the base is incorrect because high current effects in the base generally affect the transistor's thermal stability and the possibility of transistor saturation. While they can affect the performance, they are not the primary reason for the drop in current gain at high frequencies.

Option C: Parasitic inductive elements is incorrect because while parasitic inductive elements may have some impact on the high-frequency behavior of a transistor, they do not directly cause the current gain to drop. The primary limiting factor at high frequencies is the capacitance, not inductance.

Option D: The early effect is incorrect because the Early effect refers to the variation of the transistor's collector current due to changes in the collector-base voltage. While the Early effect can influence the transistor's operation, it is not the main cause for the reduction in current gain at high frequencies.

Conclusion: The correct answer is Option A: Transistor capacitances because at high frequencies, the transistor's junction capacitances play a significant role in limiting the current gain, leading to a reduction in performance.