User Guide

AC sweep analysis
239
How PSpice 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 transform a device into a linear
circuit
In order to transform a device (such as a transistor
amplifier) into a linear circuit, you must 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 does
PSpice automates this process for you. PSpice computes
the partial derivatives for nonlinear devices at the bias
point and uses these to perform small-signal analysis.
Example: nonlinear behavioral modeling 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 needs a
known bias point.
Examp
l
e: Rep
l
ace a
b
ipo
l
ar transistor in
common-emitter mode with a constant
transconductance (collector current
proportional to base-emitter voltage) and a
number of constant impedances.
Pspug.book Page 239 Wednesday, November 11, 1998 1:14 PM