Alpha cutoff and gain bandwidth product for a hypothetical transistor.
Another important specification for a transistor is the range of frequencies over which it can be used as an amplifier. All transistors have an amplification factor, or gain, that decreases as the signal frequency increases. Some devices work well only up to a few megahertz; others can be used to several gigahertz.
Gain can be expressed in various ways. In the preceding discussion, you learned a little about current gain, expressed as a ratio. You will also hear about voltage gain or power gain in amplifier circuits. These, too, can be expressed as ratios. For example, if the voltage gain of a circuit is 15, then the output signal voltage (rms, peak, or peak-to-peak) is 15 times the input signal voltage. If the power gain of a circuit is 25, then the output signal power is 25 times the input signal power.
Two expressions are commonly used for the gain-versus-frequency behavior of a bipolar transistor. The gain bandwidth product, abbreviated fT, is the frequency at which the gain becomes equal to 1 with the emitter connected to ground. If you try to make an amplifier using a transistor at a frequency higher than its fT specification, you are bound to fail. The alpha cutoff frequency of a transistor is the frequency at which the gain becomes 0.707 times its value when the input signal frequency is 1 kHz. A transistor can have considerable gain at its alpha cutoff frequency. By looking at this specification for a particular transistor, you can get an idea of how rapidly it loses its ability to amplify as the frequency goes up. Some devices die off faster than others.
Above figure shows the gain bandwidth product and alpha cutoff frequency for a hypothetical transistor, on a graph of gain versus frequency. Note that the scales of this graph are not linear; that is, the divisions are not evenly spaced. This type of graph is called a log-log graph because both scales are logarithmic rather than linear.