Bias Circuit

B.3 BJT Base, Emitter, and Collector Currents in the
Active Mode
The discussion of the transistor mechanism of Unit B.1 is extended here to include basecurrent
mechanisms, which exist in the real transistor, and the effect of base-width modulation
as discussed in Unit B.2. Initially, the case of active-mode operation is discussed, and this is
followed by the general-bias case, which includes the possibility of both pn junctions becoming
forward biased.
In the device model for the bipolar junction transistor, the parameter βDC is
Equation B.6
where IB is the composite of all contributions to the base current. (The use of "dc" is consistent
with SPICE.) This relates the output current, IC, to the input current, IB, for the transistor
operated in the common-emitter terminal configuration.
In the following, the discussion is based on the npn transistor, as shown now with the
schematic symbol in Fig. B.7. The npn is chosen over the pnp, as it is substantially more basic
to BJTs than the pnp. This probably has to do mostly with the fact that a common-emitter
stage is consistent with a positive power supply (a holdover from the vacuum-tube days), that
the npn is superior in terms of frequency response, and that it has consistently been the
device of BJT digital switches including the TTL. The pnp was used in the discussion above on
the fundamentals of transistor action, as it is somewhat more intuitively satisfying to have the
current and particle flow (where the illustration is with the hole) in the same direction.
Figure B.7. BJT (npn) in the common-base configuration, showing
terminal voltages and branch currents.
The assignment of polarities of voltages VBE and VBC is consistent with forward bias for both the
emitter – base and collector – base junctions. VBC will be negative in active-mode operation.
The assignment of VCE is standard, as it will be positive in active-mode operation. Current
directions are assigned to correspond to the actual directions of the currents in the forwardactive
mode.
Base currents do not couple between junctions. Rather, as suggested in Fig. B.8, a given base
current is associated with a given pn junction. In the forward-active mode, a base-current
component is added to the emitter current as indicated in Fig. B.7. However, since the base –
collector junction is reverse biased, the base-current contribution to the collector current is
negligible and (B.3) still represents the total collector current. The collector-current relation
with the base-width-modulation effect (Unit B.2) now included becomes
Equation B.7