User`s guide

AC Sweep Analysis 10
-7
How PSpice A/D Treats
Nonlinear Devices
An AC Sweep analysis is a linear or small-signal analysis. This
means that nonlinear devices must be linearized to run the
analysis.
What’s required to linearize a device
If you were to manually linearize a device such as a transistor
amplifier, you would need to do the following:
1
Compute the DC bias point for the circuit.
2
Compute the complex impedance and/or transconductance
values for each device at this bias point.
3
Perform the linear circuit analysis at the frequencies of
interest by using simplifying approximations.
What PSpice A/D does
PSpice A/D automates this process for you. PSpice A/D
computes the partial derivatives for nonlinear devices at the bias
point and uses these to perform small-signal analysis.
Example: Nonlinear behavioral modelin
g
block
Suppose you have an analog behavioral modeling block that
multiplies V(1) by V(2). Multiplication is a nonlinear operation.
To run an AC sweep analysis on this block, the block needs to
be replaced with its linear equivalent. To determine the linear
equivalent block, PSpice A/D needs a known bias point.
Example: Replace a bipolar
transistor in common-emitter
mode with a constant
transconductance (collector
current proportional to base-
emitter volta
g
e) and a number of
constant impedances.