The beta cutoff frequency of a bipolar transistor refers to the frequency at which?

The beta cutoff frequency of a bipolar transistor is defined as the frequency at which the emitter current gain, often represented as beta (β), has decreased to 0.707 of its maximum value. This is a crucial point in the operation of the transistor, as it delineates the frequency range within which the transistor can effectively amplify signals.

At this cutoff frequency, the gain begins to drop significantly, and the transistor may not provide the necessary current amplification for high-frequency signals, impacting the overall performance of amplification circuits. The value of 0.707 is significant because it corresponds to the -3 dB point on a gain versus frequency graph, which indicates that the power gain has fallen to half of its maximum value.

This concept is essential for understanding the high-frequency response of bipolar junction transistors, and it's a critical factor when designing circuits that operate at varying frequency ranges. While the other options speak to different aspects of transistor operation, they do not define the cutoff frequency in the same context of current gain reduction at high frequencies.