Environmental Engineering Reference
In-Depth Information
model,
and
ideal switch model
are the most widely used circuit-level
switch models.
1.1
Full-Transistor Models
A full-transistor model of MOS switches takes into account both the
intrinsic and parasitic parameters of MOS devices [21]. It is capable
of capturing the rapidly varying characteristics of the time-domain be-
havior of the switches. Simulation based on the full-transistor model,
however, is costly, mainly due to the large size of the equivalent circuit
of MOS switches and the small value of the parasitic parameters of MOS
switches. The same holds true for other types of switches as well. The
use of the full transistor model is therefore warranted only if a detailed
analysis of the transient behavior of the circuits is required. For practical
applications, because most mixed-mode switching circuits are designed
in such a way that the transient behavior of the circuits in a clock phase
dies out before reaching the end of the clock phase, the full-transistor
models are rarely needed.
1.2
Voltage-Modulated Resistor Models
When the rapidly changing transient characteristics of the circuits are
not of a critical concern, switches can be modeled as a voltage-modulated
resistor that has a small resistance in the ON state, usually a few
depending upon the technology in which MOS switches are realized and
the width of the switches, and a large resistance in the OFF state, often
in the range of several hundred The large difference between the
ON and OFF resistances of the voltage-modulated resistor gives rise to
largely distinct time constants for the ON and OFF states, respectively.
As a result, stiff systems with numerically ill-conditioned circuit matrices
occur. The time-domain analysis of these circuits requires fine time steps
in order to capture the rapid transient behavior of the circuits in the ON-
state. On the other hand, simulation needs to be executed over a long
period of time in order to account for the large time constant associated
with the OFF state, resulting in excessive computation and exceedingly
long simulation time.
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