What is the term for the frequency at which the grounded-base current gain of a bipolar junction transistor has decreased to 0.7 of the gain obtainable at 1 kHz?
Why In a bipolar transistor, alpha is the current gain in the grounded-base (common-base) configuration, the ratio of collector current to emitter current. Like any gain, it rolls off at high frequencies, and the point where it has fallen to about 0.7 (the 0.707 or -3 dB point) of its low-frequency value, referenced to 1 kHz, is called the alpha cutoff frequency. It is a useful figure of merit because it indicates roughly the highest frequency at which the device is still useful as an amplifier.
Watch out Beta cutoff frequency is the analogous -3 dB point for beta, the grounded-emitter current gain, and it occurs at a much lower frequency than alpha cutoff; "alpha rejection frequency" is not a real transistor parameter.
Alpha goes with the base grounded, beta with the emitter grounded. Grounded base in the question means alpha cutoff.
Pair them alphabetically: Alpha with Base (A, B), Beta with Emitter (B, E). Grounded base means alpha cutoff.