Environmental Engineering Reference
In-Depth Information
where is a technology parameter commonly referred to as the base-transit-
time constant. The total base-emitter capacitance,
is then given by
In contrast, since the base-collector junction is reverse-biased, its relative
charge is stored in the depletion region. Assuming a graded junction, the
corresponding capacitance, is expressed by an equation similar to (1.16)
multiplied by the effective area of the base-collector interface,
that is
1.4.5
Active Region Small Signal Model
The most commonly used small signal model is the model. This
model is similar to the small signal model for a MOSFET in saturation
region and is shown in Fig. 1.18. Note that for historical reasons, small
signal subscripts that refer to the base-emitter components, i.e.
be,
are
substituted by the Greek letter and small signal subscripts that refers to the
base-collector components, i.e.
bc,
are substituted by the Greek letter
As in the MOSFET case, the most important parameter is the dependent
current generator,
whose transconductance,
is defined as
The small signal resistance,
is defined as
The resistor
models the Early effect and is defined as
