Here's a very common
plastic package; the TO-92. Beware, not all parts in TO-92 share
this pinout. Here are some that do:
- 2N3903, 2N3904, 2N3905, 2N3906
- 2N4400, 2N4401, 2N4402, 2N4403
There are plenty of texts around on basic electronics, so
this is a very brief look at the three basic ways in which a
bipolar junction transistor (BJT) can be used. In each case, one
terminal is common to both the input and output signal. All the
circuits shown here are without bias circuits and power supplies
Common Emitter Configuration
Here the emitter terminal is common to both the input and
output signal. The arrangement is the same for a PNP transistor.
Used in this way the transistor has the advantages of a medium
input impedance, medium output impedance, high voltage gain and
high current gain.
Common Base Configuration
Here the base is the common terminal. Used frequently for RF
applications, this stage has the following properties. Low input
impedance, high output impedance, unity (or less) current gain
and high voltage gain.
Common Collector Configuration
This last configuration is also more commonly known as the
emitter follower. This is because the input signal applied at
the base is "followed" quite closely at the emitter with a
voltage gain close to unity. The properties are a high input
impedance, a very low output impedance, a unity (or less)
voltage gain and a high current gain. This circuit is also used
extensively as a "buffer" converting impedances or for feeding
or driving long cables or low impedance loads.