Manuals >Nonlinear Device Models Volume 1 >Circuit Modeling
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Defining a Circuit

The process of defining a circuit in IC-CAP is similar to defining a single device. The main difference is the interconnection of the components and the use of subcircuit lines to define the circuit block. For detailed information on defining circuits, refer to the appropriate Reference chapter (Chapter 3, "SPICE Simulators," Chapter 4, "SPECTRE Simulator," Chapter 5, "Saber Simulator,").

Supported Circuit Components

Circuits in IC-CAP support the standard components that can be simulated with SPICE:

Passive elements    R, L, C, Transmission lines

Semiconductors    Bipolar, MOS, GaAs, JFET, Diode

Sources    V, I, VCVS, VCIS, ICVS, ICIS

The syntax for defining a circuit in IC-CAP is similar to a SPICE simulation input deck. Each line contains a component, its node numbers, value, and (if applicable) an associated model name reference. Proper specification and use of these components is critical to the success of circuit simulation and parameter extraction.

In general, independent voltage sources are specified as inputs within a given setup. This allows you to specify their values and use them in additional numerical or graphic analysis. Some of the differences between SPICE and IC-CAP circuit definitions are listed.

    • The .OPTIONS statement (if used) must be the first line in the circuit description. All options must be on one line (no continuation).
    • The next line of the circuit is .SUBCKT.
    • A TITLE specification is automatically generated by IC-CAP and should not be included in the circuit definition.
    • The last line of the circuit is .ENDS
    • An .END statement is automatically generated by IC-CAP and should not be included in the circuit definition.

The following figure shows an example circuit description. This circuit defines the input section of an ECL OR/NOR logic gate. (Figure 218 shows the schematic.) This circuit is referenced several times in this chapter. You can create it using the circuit editor or read it from the file $ICCAP_ROOT/data/ECLornor.mdl.

Figure 217 Circuit Description for an ECL OR/NOR Logic Gate 
 .SUBCKT ECLORNOR  1=IN1 2=IN2 3=OR 4=NOR
+ 5=VCC 6=VEE 7=VREF
* ECL OR/NOR LOGIC GATE
Q1   4  1  8  NPN1
Q2   4  2  8  NPN1
Q0   3  7  8  NPN2
.MODEL NPN1 NPN
+ IS = 2E-14  NF = 0.998  BF = 120
+ RB = 225  CJC = 300p  TF = 20p
.MODEL NPN2 NPN
+ IS = 4E-14  NF = 0.998  BF = 120
+ RB = 110  CJC = 530p  TF = 18p
RL1  5  4  300
RL0  5  3  300
RIEE 8  6  1.2K
.ENDS

When you enter the circuit description in the Circuit folder of the model window, moving the mouse out of the Circuit folder automatically causes the circuit to be parsed, that is, the specified circuit elements are read and entries are created for them in Model Parameters. When they are added initially, they assume the value specified in the circuit description. To change a value subsequently, you must change it in Model Parameters. To change all entries in Model Parameters to the values in the circuit description, choose Reset.

Note the difference in the Parameters table parameter names for a transistor in a circuit. In a single transistor circuit, the model parameter names of the transistor are the entries in the Parameters table. In a multi-component circuit the transistor's model parameters must be associated with a specific model, so the parameters take on a prefix of that model's name. Thus, the forward Beta model parameter BF for a model named NPN1 is listed in the Parameter Editor as NPN1.BF. In the example above, transistors Q1 and Q2 both use the NPN1 model, while transistor Q0 uses the NPN2 model.

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