Gain versus frequency:
Another important specification for a transistor is the range of frequencies over which it can be used as an amplifier. The transistors have an amplification factor, or gain, which decreases as signal frequency increases. Some of the devices will work well only up to a few megahertz; others can be used to several gigahertz.
Gain can be expressed in various different ways. In the above topic, you learned a little about current gain, expressed as a ratio. You will also at times hear about voltage gain or power gain in amplifier circuits. These, can be expressed as ratios. For instance, if the voltage gain of the circuit is 15, then output signal voltage (RMS, peak, or peak to peak) is 15 times input signal voltage. If power gain of the circuit is 25, then output signal power is 25 times input signal power.
There are 2 expressions commonly used for gain versus frequency behavior of a bipolar transistor. The gain bandwidth product, represented as fT, is frequency at which gain becomes equal to 1 with the emitter connected to the ground. If you try to make an amplifier by using a transistor at a frequency higher than its fT, you will fail! Hence fT represents the absolute upper limit of sorts.
The alpha cutoff frequency of the transitor is frequency at which gain becomes 0.707 times its value at 1 kHz. A transistor may still have considerable gain at its alpha cutoff. By looking at alpha cutoff frequency, you can get the idea of how rapidly the transistor loses gain as frequency goes up. Some devices die-off faster than the other devices.
The figure given below shows the gain band width product and alpha cutoff frequency for the hypothetical transistor, on a graph of gain versus frequency. Note that scales of this graph are nonlinear; they are scrunched up at higher values. This type of graph is useful for showing some functions. It is called as log-log graph as both scales are logarithmic instead of linear.
Figure--Alpha cutoff and gain bandwidth product for a hypothetical transistor.