What are the applications of bipolar transistors (bipolar junction transistors: BJTs) ?

In recent years, MOSFETs have been used more often than bipolar-junction transistors (BJTs). This is because MOSFETs generally require no drive current and are easy to handle. However, BJTs are used in amplifiers, oscillators, switching at low voltages, etc. 
This is because the BJT has higher gain (hFE)*, better linearity, better 1/f noise, and can be turned on by applying a voltage of about 0.7 V between the base and emitter. In addition, BJTs are sometimes used in environments with large disturbances due to their high resistance to static electricity (ESD).

An example circuit using BJTs is shown below.

Common-collector amplifier (emitter follower)
Common-collector amplifier (emitter follower)
Common-emitter amplifier
Common-emitter amplifier
Common-base amplifier
Common-base amplifier
Switching circuit
Switching circuit

*hFE varies with the collector-emitter voltage, which depends on external circuitry.
For a description of hFE, also see the FAQ entry, “What is the relationship between the base current and collector current of a bipolar junction transistor (BJT) ?”.

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