The BJT
Notation
The notations used here for voltages and currents correspond to the following conventions: Dc bias
values are indicated by an upper case letter with upper case subscripts, e.g. VDS, IC. Instantaneous
values of small-signal variables are indicated by a lower-case letter with lower-case subscripts, e.g.
vs, ic. Total values are indicated by a lower-case letter with upper-case subscripts, e.g. vBE, iD.
Circuit symbols for independent sources are circular and symbols for controlled sources have a
diamond shape. Voltage sources have a ± sign within the symbol and current sources have an
arrow.
Device Equations
Figure 1 shows the circuit symbols for the npn and pnp BJTs. In the active mode, the collector-base
junction is reverse biased and the base-emitter junction is forward biased. For the npn device, the
active-mode collector and base currents are given by