Gain Versus Frequency:

In any specific bipolar transistor, the gain reduces as the signal frequency rises. There are two expressions for gain-versus-frequency behavior.

The gain bandwidth product, abbreviated as fT, is the frequency at which the current gain becomes equivalent to unity (i.e., 1) with the emitter connected to ground. This means that the transistor has no current gain; the output current amplitude is similar as the input current amplitude, even under ideal operating situations. The alpha cutoff is the frequency at which the current gain becomes 0.707 times (that is, 70.7 % of) its value at precisely 1 kHz (1,000 Hz). Most of the transistors can work as current amplifiers at frequencies above the alpha cutoff, though no transistor can work as a current amplifier at frequencies higher than its gain bandwidth product.

PROBLEM:

The bipolar transistor has a current gain, under ideal situations, of 23.5 at an operating frequency of 1,000 Hz. The alpha cutoff is identified as 900 kHz. Determine the maximum possible current gain which the device can have at 900 kHz?

SOLUTION:

Multiply 23.5 by 0.707 to acquire 16.6. This is the maximum possible current gain which the transistor can generate at 900 kHz.