# How to determine Frequency response in BJT

What do I want to look for on a BJT datasheet to find out the maximum operating frequency before distortion occurs? Or perhaps the question should be: How do I know how fast a transistor can cause the main current to change in response to a change in the base signal current?

Also, is there a noteable difference in PNP and NPN BJTs in this regard? I read that P-Channel mosfets generally have higher slew rates than N-Channel, making them favorable if you need extremely fast response. Is there any rule of thumb like that for BJTs?

• New acronym time: SINFR. – Kaz Jun 10 '13 at 22:55

In the datasheets of the BJT transistors, common emitter cutoff frequency, $f_T$ is usually given. At this frequency the magnitude of the common emitter current gain equals to one, $\beta_{f_T} = 1$. As a rule of thumb for BJT's, we can say that a BJT transistor as an amplifier is usable up to the frequency $f_u = f_T/10$.
The distortion depends not only on the characteristic of the transistor (such as $f_T$), but also on the circuit the transistor is biased to. For example, the capacitors being used to block DC currents reduce the upper (and also lower) usable frequency of the transistor.