User Guide

PSpice-equivalent parts
181
Frequency-domain device models
Frequency-domain models (ELAPLACE, GLAPLACE,
EFREQ, and GFREQ) are characterized by output that
depends on the current input as well as the input history.
The relationship is therefore non-instantaneous. For
example, the output may be equal to the integral of the
input over time. In other words, the response depends
upon frequency.
During AC analysis, the frequency response determines
the complex gain at each frequency. During DC analysis
and bias point calculation, the gain is the zero-frequency
response. During transient analysis, the output of the
device is the convolution of the input and the impulse
response of the device.
Laplace transforms (LAPLACE)
The ELAPLACE and GLAPLACE parts allow a transfer
function to be described by a Laplace transform function.
The ELAPLACE and GLAPLACE parts are defined, in
part, by the following properties (default values are
shown):
ELAPLACE
EXPR V(%IN+, %IN-)
XFORM 1/s
GLAPLACE
EXPR V(%IN+, %IN-)
XFORM 1/s
The LAPLACE parts use a Laplace transform description.
The input to the transform is the value of EXPR, where
EXPR follows the same rules as for VALUE expressions
(see EVALUE and GVALUE parts
on page 6-176). XFORM is
an expression in the Laplace variable, s. It follows the rules
for standard expressions as described for VALUE
expressions with the addition of the s variable.
Moving
b
ac
k
an
d
f
ort
h
b
etween t
h
e time
and frequency-domains can cause
surprising results. Often the results are
quite different than what one would
intuitively expect. For this reason, we
strongly recommend familiarity with a
reference on Fourier and Laplace
transforms. A good one is:
1 R. Bracewell,
The Fourier Transform
and Its Applications
, McGraw-Hill,
Revised Second Edition (1986)
We also recommend familiarity with the
use of transforms in analyzing linear
systems. Some references on this subject:
2 W. H. Chen,
The Analysis of Linear
Systems
, McGraw-Hill (1962)
3 J. A. Aseltine,
Transform Method in
Linear System Analysis
, McGraw-Hill
(1958)
4 G. R. Cooper and C. D. McGillen,
Methods of Signal and System
Analysis
, Holt, Rinehart, and Winston
(1967)
Vo
l
tages, currents, an
d
TIME cannot appear
in a Laplace transform.
Pspug.book Page 181 Wednesday, November 11, 1998 1:14 PM