User manual ORCAD PSPICE

[. . . ] OrCAD PSpice® User's Guide Copyright © 1998 OrCAD, Inc. Trademarks OrCAD, OrCAD Layout, OrCAD Express, OrCAD Capture, OrCAD PSpice, and OrCAD PSpice A/D are registered trademarks of OrCAD, Inc. OrCAD Capture CIS, and OrCAD Express CIS are trademarks of OrCAD, Inc. Microsoft, Visual Basic, Windows, Windows NT, and other names of Microsoft products referenced herein are trademarks or registered trademarks of Microsoft Corporation. [. . . ] To complete the example, one could add a circuit which produces the family of I-V curves (shown in Figure 48). Figure 48 Triode subcircuit producing a family of I-V curves. 173 Chapter 6 Analog behavioral modeling PSpice-equivalent parts PSpice-equivalent parts respond to a differential input and have double-ended output. These parts reflect the structure of PSpiceE and G devices, thus having two pins for each controlling input and the output in the part. Table 1 summarizes the PSpice-equivalent parts available in the part library. Table 1 PSpice-equivalent parts Part EVALUE GVALUE ESUM GSUM EMULT GMULT Table look-up ETABLE GTABLE Frequency table look-up Laplace transform EFREQ GFREQ ELAPLACE GLAPLACE general purpose general purpose general purpose EXPR TABLE EXPR TABLE EXPR XFORM special purpose (none) Description general purpose Properties EXPR Category Mathematical expression There are no equivalent "F" or "H" part types in the part library becayse PSpice "F" and "H" devices do not support the ABM extensions. PSpice-equivalent ABM parts can be classified as either E or G device types. The E part type provides a voltage output, and the G device type provides a current output. The device's transfer function can contain any mixture of voltages and currents as inputs. Hence, there is no longer a division between voltage-controlled and current-controlled parts. Rather the part type is dictated only by the output requirements. If a voltage output is required, use an E part type. If a current output is necessary, use a G part type. 174 PSpice-equivalent parts Each E or G part type in the ABM. OLB part file is defined by a template that provides the specifics of the transfer function. Other properties in the model definition can be edited to customize the transfer function. By default, the template cannot be modified directly choosing Properties from the Edit menu in Capture. Rather, the values for other properties (such as the expressions used in the template) are usually edited, then these values are substituted into the template. However, the part editor can be used to modify the template or designate the template as modifiable from within Capture. This way, custom parts can be created for special-purpose application. Implementation of PSpice-equivalent parts Although you generally use Capture to place and specify PSpice-equivalent ABM parts, it is useful to know the PSpice command syntax for "E" and "G" devices. This is especially true when creating custom ABM parts since part templates must adhere to PSpice syntax. The general forms for PSpice "E" and "G" extensions are: E <name> <connecting nodes> <ABM keyword> <ABM function> G <name> <connecting nodes> <ABM keyword> <ABM function> where <name> <connecting nodes> is the device name appended to the E or G device type character specifies the <(+ node name, - node name)> pair between which the device is connected 175 Chapter 6 Analog behavioral modeling <A BM keyword> specifies the form of the transfer function to be used, as one of: VALUE TABLE LAPLACE FREQ table CHEBYSHEV arithmetic expression lookup table Laplace transform frequency response Chebyshev filter characteristics <A BM function> specifies the transfer function as a formula or lookup table as required by the specified <ABM keyword> Refer to the online OrCA D PSpice A /D Reference Manual for detailed information. Modeling mathematical or instantaneous relationships The instantaneous models (using VALUE and TABLE extensions to PSpice "E" and "G" devices in the part templates) enforce a direct response to the input at each moment in time. For example, the output might be equal to the square root of the input at every point in time. Such a device has no memory, or, a flat frequency response. These techniques can be used to model both linear and nonlinear responses. Note For AC analysis, a nonlinear device is first linearized around the bias point, and then the linear equivalent is used. EVALUE and GVALUE parts The EVALUE and GVALUE parts allow an instantaneous transfer function to be written as a mathematical expression in standard notation. These parts take the input signal, perform the function specified by the EXPR property on the signal, and output the result on the output pins. 176 PSpice-equivalent parts In controlled sources, EXPR may contain constants and parameters as well as voltages, currents, or time. [. . . ] If the capacitance model parameters are specified (e. g. , CJS) then the junction has (voltage-dependent) capacitance but no DC current. This can lead to a sneaky problem: if the junction is inadvertently forward-biased it can create a very large capacitance. The capacitance goes as a power of the junction voltage. Normal junctions cannot sustain much forward voltage because a large current flows. [. . . ]