BJT amplifier circuits

An amplifier is a circuit that, typically, takes an input voltage or current (or vice versa) and provides an amplified voltage or current (or vice versa) at its output. They all exploit some semiconductor physical properties that confer the device the capability of multiplying current by a factor, just by slightly changing the input quantity; the fact that most of the amplifiers work as voltage amplifiers is just a matter of convenience, even if, under some design requirements, the use of a current amplifier should be preferred. BJT amplifiers are not so common anymore since they have been replaced with another type of more efficient, complex, accurate, clean (low-noise), low-cost, small amplifiers. However, it could be interesting to know how a single transistor amplifier works, which are its configurations, which are its main parameters, with pros and cons.

BJT amplifiers come in three configurations: common-base, common-emitter and common-collector; in the following, we will analyze all of them, through the Giacoletto model, which is shown below.

This is the most used configuration, especially for low-frequency input signals that require a high gain and low distortion at the output.