BJT Modeling Statement
DR. SPICE simulates many types of BJT’s, for both small signal and power applications, working at frequencies from DC through 1GHz. DR. SPICE is not recommended for microwave frequency simulations.
DR. SPICE transistor model parameters cover most low frequency and rf transistors, but you may not need to use them. For many applications, the default statement generates the desired model.
The default statement for a BJT is:
.DATASHEET <DEVICE NAME> <DEVICE TYPE>
The <device type> = npn or pnp.
The statement format for a BJT with parameters is:
.DATASHEET <DEVICE NAME> <DEVICE TYPE><PARAMETERS>
Use parameters to define the following BJT characteristics:
Saturation and DC Current Gain (Beta) Parameters
These parameters come from the on characteristics section of a datasheet; they determine the IS, N, and BF parameters for the BJT .MODEL statement. In the following table, the first three parameters contain saturation information, and the rest specify DC current gain (beta) characteristics.
NOTE: If you specify any of the beta characteristics, you should specify all four beta, ic pairs. If you do not have a beta vs. ic table as part of the datasheet, do not specify values in this section. Instead, set the bf parameter in the .MODEL statement to be whatever the maximum beta for the device is.
Saturation and DC Current Gain (Beta) Parameters
| Parameter | Description |
| b1 | The low current value of beta where beta is ½ its maximum, or the value of beta at the minimum current given if the current for ½ maximum beta is not available |
| b2 | A value larger than b2, but smaller than the maximum beta, where the beta is still increasing |
| b3 | The maximum value of beta |
| b4 | The high-current beta where beta is ½ its maximum, or the value of beta at the maximum current given if the current for ½ maximum beta is not available |
| bmult | If the actual values of beta are given in the beta vs. IC curve, omit bmult. If the beta values in beta vs. IC curves are given normalized to one, rather than as their actual values, set bmult to the maximum value of beta |
| bvce | The vce at which beta is measured |
| ic1 | The collector current at which b1 is measured |
| ic2 | The collector current at which b2 is measured |
| ic3 | The collector current at which b3 is measured |
| ic4 | The collector current at which b4 is measured |
| icsat | The collector current at which vbesat and vcesat are measured |
| vbesat | VBE in saturation |
| vcesat | VCE in saturation |
Small Signal and High Frequency Parameters
These parameters come from the small signal characteristics or output characteristics section of a datasheet. In the following table, the first two parameters define FT characteristics, the last three parameters define small signal output admittance, and the rest define input and output capacitances.
Small Signal and High Frequency Parameters
| Parameter | Description |
| ci1 | Input capacitance from the left side of the input capacitance chart |
| ci2 | Input capacitance from the right side of the input capacitance chart |
| co1 | Output capacitance from the left side of the output capacitance chart |
| co2 | Output capacitance from the right side of the output capacitance chart |
| ft | The ft of the transistor |
| ftic | The current at which ft is measured |
| hoe | The output admittance from the datasheet, or from a point on the output admittance curve |
| hoeoic | The IC at which hoe is measured |
| hoevce | The vce at which hoe is measured |
| vci1 | Reverse bias voltage at which ci1 is measured |
| vci2 | Reverse bias voltage at which ci2 is measured |
| vco1 | Reverse bias voltage at which co1 is measured |
| vco2 | Reverse bias voltage at which co2 is measured |
BJT Modeling Example
The following .DATASHEET statement generates a model for a q2n3903:
.datasheet q2n3903 npn + vbesat=0.75 vcesat=0.2 icsat=1e-2 + hoe=20e-6 hoeic=1e-3 hoevce=10 + ft=250e6 ftic=1e-3 + bmult=1 bvce=1 + b1=30 ic1=1e-4 b2=50 ic2=1e-3 b3=100 ic3=1e-2 b4=45 ic4=5e-2 + ci1=4.2e-12 vci1=0.1 ci2=2.8e-12 vci2=2 + co1=3.5e-12 vco1=0.1 co2=2.5e-12 vco2=2
The MODEL Maker output file for this example (q2n3903.mod) is as follows.