  A recently introduced Early modeling of transistors is applied to the study
of the common collector amplifier (or emitter follower), an important type of
electronic circuit typically employed as buffer, being characterized by near
unit voltage gain, high input resistance, and low output resistance. The Early
equivalent model is applied to derive a simple representation that is simple
and yet capable of incorporating the transistor non-linearities implied by the
Early effect. Mathematical expressions are obtained describing completely the
circuit operation in terms of currents and voltages, allowing accurate
estimation of the average voltage gain, total harmonic distortion (THD), and
average input and output resistances. Prototypes of small signal silicon
transistors of types NPN and PNP obtained in a previous work are used to
discuss the respectively implied properties of the common collector transistor.
In addition to confirming the importance of the trade-off of current gain for
the desired properties, it is also shown that sub-optimal performance can be
obtained in case the base and emitter resistances are not properly chosen. Even
so, the limited current gain implied by real-world NPN and PNP small signal
silicon devices implies some performance constraints. In particular, it has
been observed that the THD tends to be larger for PNP devices than NPN
counterparts with the same average current gain. The obtained results pave the
way not only to complementary analytical studies, but also provide guidance for
design and implementation of improved common collector configurations.
