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File name BJT Excel Transistor SPICE Model Generator How To.pdf

excel http://www.analogservices.com/excel.htm Introduction Although SPICE models of integrated circuits are often supplied by vendors, models for discrete components can be difficult to get. Since SPICE uses roughly 40 different parameters to describe a BJT, generating a model seems a daunting task. However, if the BJT will be used only in its forward active region, then its data sheet often contains enough information to create an useful model. The rest of the parameters aren't needed and can be left at SPICE default values. The Excel [1] Spreadsheet provided here streamlines the process of converting BJT data sheet information. Click here to download it as a .zip file. The static SPICE parameters found are IS -- Saturation Current NF -- Forward Emission Coefficient BF -- Max. Beta NE -- Leakage Emission Coefficient ISE -- Leakage Saturation Current IKF -- Upper Current Corner The dynamic SPICE parameters found are MJE -- Base-Emitter Junction Grading Coefficient CJE -- Zero-Bias Base-Emitter Depletion Capacitance VJE--Base-Emitter Built-In Potential MJC -- Base-Collector Junction Grading Coefficient CJC -- Zero-Bias Base-Collector Depletion Capacitance VJC -- Base-Collector Built-In Potential TF -- Ideal Forward Transit Time In addition to finding the parameters, the spreadsheet also creates a text file of a SPICE .MODEL statement that incorporates the parameters. The remainder of this note explains how to use the spreadsheet. An example using a 2N5190 is given. An appendix is also included to present the theory behind the spreadsheet. Graphical techniques to find at least the static SPICE parameters have been suggested or demonstrated in [2] and [3]. However, by using the Excel Solver feature and a least squares fit to several values of (HFE, Ic), you get better results. To use the technique you need a BJT data sheet with at least the following information: 1. A graph of HFE versus IC (collector current). 2. A graph of VBE versus IC in the forward active region. 3. A graph of Ccb and Ceb versus reverse junction voltages. Fortunately, these are often provided in data sheets. If you know the value of the Early voltage (SPICE parameter VAF) this is also helpful. It can be found from a plot of IC versus VCE or from a known value of HOE (small signal output resistance) at a given IC. 1 of 10 07/06/2010 8:01 PM excel http://www.analogservices.com/excel.htm Your downloaded file should be called "BJT.XLS" and should contain 3 sheets. The first is used to fit the HFE versus collector current, and is used to find IS, NF, BF, NE, ISE, and IKF. The second fits Ceb versus emitter-base reverse voltage, and is used to find MJE, CJE, and VJE. The third fits Cob versus collector-base reverse voltage, and is used to find MJC, CJC, and VJC. Note that Ceb(0) (zero bias) = CJE and Ccb(0) = CJC. Fitting HFE To fit HFE data use Sheet 1. An example of the use of sheet 1 is given in figure 1. The values you need to enter are at A8 through D8, B13 through C13,