GAIN VERSUS FREQUENCY

In any particular bipolar transistor, the gain decreases as the signal frequency increases. There are two expressions for gain-versus-frequency behavior.
The gain bandwidth product, abbreviated f_T, is the frequency at which the current gain becomes equal to unity (1) with the emitter connected to ground. This means, in effect, that the transistor has no current gain; the output current amplitude is the same as the input current amplitude, even under ideal operating conditions. The alpha cutoff is the frequency at which the current gain becomes 0.707 times (that is, 70.7 percent of) its value at exactly 1 kHz (1,000 Hz). Most transistors can work as current amplifiers at frequencies above the alpha cutoff, but no transistor can work as a current amplifier at frequencies higher than its gain bandwidth product.