Using BJT Models

The BJT model is used to develop BiCMOS, TTL, and ECL circuits. For BiCMOS devices, use the high-current Beta degradation parameters, IKF and IKR, to modify high injection effects. The model parameter SUBS facilitates the modeling of both vertical and lateral geometrics.

Selecting Models

To select a BJT device, use a BJT element and model statement. The element statement references the model statement by the reference model name. The following example uses the reference name MOD1. In this case an NPN model type is used to describe an NPN transistor.

Example

Q3 3 2 5 MOD1 <parameters>

.MODEL MOD1 NPN <parameters>

Parameters can be specified in both element and model statements. The element parameter always overrides the model parameter when a parameter is specified as both. The model statement specifies the type of BJT, for example, NPN or PNP.

Using Control Options

Control options affecting the BJT model are: DCAP, GRAMP, GMIN, and GMINDC. DCAP selects the equation that determines the BJT capacitances. GRAMP, GMIN, and GMINDC place a conductance in parallel with both the base-emitter and base-collector pn junctions. DCCAP invokes capacitance calculations in DC analysis. The BJT control options follow:

You can override global depletion capacitance equation selection that uses the .OPTION DCAP=<val> statement in a BJT model by including DCAP=<val> in the BJTs .MODEL statement.

Convergence

Adding a base, collector, and emitter resistance to the BJT model improves its convergence. The resistors limit the current in the device so that the forward-biased pn junctions are not overdriven.