Device behavioral expressions

All simple components can have a behavioral description, with syntax designed as an extension of the Spice time dependent sources. They are not necessarily physically realizable, but provide useful modeling capability. Some only work on particular types of analysis, or over a small range of values. Some can be used together, some cannot.

As of July 2009, these functions only work with Spice or ACS syntax. They do not work with Verilog or Spectre syntax.

All simple components are considered to have simple transformations. A function returns one parameter as a function of one other, as an extension of their linear behavior. For behavioral modeling or nonlinear values, replace the constant times input by an arbitrary function.

Gnucap also has some other simple components, which are compatible with some versions of Spice that also have them:

Sources are defined as functions of time:

These functions do not fit with the style of Verilog or Spectre, so they don't work with that syntax. As a workaround, you can build a subcircuit with Spice syntax, then call it as a component in any syntax.

spice
.subckt vsin (p n)
Vsrc (p n) sin {offset} {amplitude} {frequency} {delay} {damping}
.ends
verilog

vsin #(.frequency(1K)) vinput (in, 0);

Gnucap behavioral modeling conditionals are an extension of the “AC” and “DC” Spice source parameters.

The extensions …

• There are more choices, including an “else”.
• They apply to all elements (primitive components).
• Each section can contain functions and options.

The following are available:

A value or function with no conditional keyword is equivalent to “ALL”. For SPICE compatibility, use only “DC”, “AC”, or nothing.

They are interpreted like a “switch” statement. In case of a conflict, the last one applies. A set of precedence rules applies when some keys are missing. It is SPICE compatible to the extent the features overlap.

The following table describes the precedence rules:

V12 (1 0) AC 1 DC 3

This voltage source has a value of 1 for AC analysis, 3 for DC. OP, Transient, and Fourier inherit the DC value.

R44 (2 3) OP 1 ELSE 1g

This resistor has a value of 1 ohm for the “OP” analysis, 1 gig-ohm for anything else. This might be useful as the feedback resistor on an op-amp. Set it to 1 ohm to set the operating point, then 1 gig to measure its open loop characteristics, hiding the fact that the op-amp would probably saturate if it was really left open loop.

Gnucap behavioral modeling functions are an extension of the Spice source time dependent values.

• They apply to all elements (primitive components).
• All accept either Spice compatible order dependent parameters, or easier keyword=value notation.
• The syntax is identical for all supported components.
• They work a little different, depending on the type of device:

• complex Complex (re, im) value.
• exp Spice Exp source. (time dependent value).
• fit Fit a curve with splines.
• generator Value from Generator command.
• poly Polynomial (Spice style).
• posy Posynomial (Like poly, non-integer powers).
• pulse Spice Pulse source. (time dependent value).
• pwl Piece-wise linear.
• sffm Spice Frequency Modulation (time dependent value).
• sin Spice Sin source. (time dependent value).
• tanh Hyperbolic tangent transfer function.

In addition, you may name a “function” defined by a ”.model” statement. The following ”.model” types may be used here:

• table Fit a curve with splines.
• cap Spice semiconductor “capacitor” model.
• res Spice semiconductor “resistor” model.

These parameters are available with all functions. Some may not make sense in some cases, but they are available anyway.

Temperature adjustments and scaling use the following formula:

value *= _scale * (1 + _tc1*tempdiff + _tc2*tempdiff*tempdiff)

where tempdiff is t - _tnom.