The inclusion of transistors in circuit simulation requires a model for the transistor. That is, the simulator needs to know how the transistor behaves. Specifically, the simulator requires knowledge of the terminal characteristics of the transistor. For example, for a given dc voltage VBE applied between terminals B (base) and E (emitter), what will be the current into B (IB)? The mathematical relations are obtained from theory or experimental observation. Often, the theory is very complicated, but much more simple relations serve very well as good approximations.
TABLE B.1 | ||
---|---|---|
SPICE Name | Math Symbol | Description |
IS | IS | Description |
ISE | ISE | Saturation current. |
BF | βF | Base – emitter leakage saturation current. |
VAF | VAF | Ideal maximum forward current gain. |
BR | βR | Forward Early voltage. |
NF | nF | Reverse-current gain. |
NE | nE | Forward-current emission coefficient. |
The parameters for the BJT, which are to be determined in the projects as described in the following, are given in Table B.1. The parameters are obtained by fitting the mathematical expressions as used by SPICE to measured current – voltage relations.
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