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itself, or can be produced by copying the voltage at terminal Y acting across
an external impedance connected to terminal X.
As can be argued from Fig. 2.1b, the small-signal model of the generic
transistor is derived by merging the BJT and MOSFET models. In order to
further simplify the model, we could relate the bulk transconductance,
with transconductance
via the bulk transconductance parameter,
(do
not mistake
this
parameter
for
the
MOS
channel-length
modulation
parameter,
Parameter
is always much lower than 1. Its value is around 0.2-0.3 for
MOSFETs and is equal to zero for BJTs. Thus, since resistance,
is infinite
for MOSFETs, the following relationship
always
holds
An important parameter, not shown in the model, is the current gain,
equal to
Its value is usually in the range of 50 to 200 for BJTs, and, of course, is
infinitely large for MOSFETs.
Resistance is for any kind of transistor large enough to justify its
neglect in discrete realisations employing discrete (load) resistors, while it
must be often considered in IC applications. Hence, when appropriate and
for the sake of completeness, we will include
in ouranalytical derivations.
2.2 AC SCHEMATIC DIAGRAM AND LINEAR ANALYSIS
In this topic, we will be mostly interested in the small-signal properties of
(feedback) configurations rather than their bias details. Thus, to simplify our
description and analysis, and to focus only on the performance of interest,
we will regularly make use of the
AC schematic diagram,
i.e., a circuit
diagram in which biasing details are not shown.
Although small-signal analysis could be performed directly on the
original schematic diagram (experienced designers do this), in this chapter
we will not follow this procedure. For the sake of clarity (and for educational
