The general form of the instance of a BJT is the following:

Q<name> <nodeC> <nodeB> <nodeE> [nodeS] <model name> [area] [OFF] [IC=<VBE0,VCE0>] [TEMP=<device temperature>]



Is the name of the component and the initial letter Q identifies the BJTs.

nodeC, nodeB, nodeE

Are the collector, base, and emitter nodes, respectively.


Is the (optional) substrate node. If unspecified, ground is used.

model name

Is the model name.


The area factor determines the number of equivalent parallel devices of a specified model. The affected parameters are marked with an asterisk under the heading 'area' in the model descriptions. If the area factor is omitted, a value of 1.0 is assumed.


OFF indicates an (optional) initial condition on the device for the dc analysis.


The (optional) initial condition specification using IC=VBE, VCE is intended for use with the UIC option on the .TRAN control line, when a transient analysis is desired starting from other than the quiescent operating point. See the .IC control line description for a better way to set transient initial conditions.


The (optional) TEMP value is the temperature at which this device is to operate. This value, if specified, takes precedence over the analysis temperature.


Two different forms of initial conditions may be specified for some devices.

The first form is included to improve the dc convergence for circuits that contain more than one stable state. If a device is specified OFF, the dc operating point is determined with the terminal voltages for that device set to zero. After convergence is obtained, the program continues to iterate to obtain the exact value for the terminal voltages. If a circuit has more than one dc stable state, the OFF option can be used to force the solution to correspond to a desired state. If a device is specified OFF when in reality the device is conducting, the program still obtains the correct solution (assuming the solutions converge) but more iterations are required since the program must independently converge to two separate solutions. The .NODESET control line serves a similar purpose as the OFF option. The .NODESET option is easier to apply and is the preferred means to aid convergence.

The second form of initial conditions are specified for use with the transient analysis. These are true 'initial conditions' as opposed to the convergence aids above. See the description of the .IC control line and the .TRAN control line for a detailed explanation of initial conditions.


Q1 1 2 3 QMOD
Q2 1 2 3 IC=0.5,3.0

See also

BJT Models
BJT Instance Parameters