Phy 415 Electronics
This tutorial demonstrates the use of EXCEL to
The excel sheet data shown in Figure 1 represents transistor
currents. To calculate the transistor beta (Ic/Ib) at different
- Select cell E6 as shown.
Click on the equal sign above the spread sheet area. As shown, type
"D6/C6" in the box following the equal sign. D6 and C6 are
the cells containing Ic and Ib. Hit "ENTER" and their quotient
will appear in the selected cell, E6.
Place the cursor over the square dot in the lower right corner of
the selected cell, E6. The square dot becomes a cross. Drag
the cross down the column to cell E14. This copies the formula
to these cells. The correct Ib and Ic are referenced to produce
the appropriate beta values.
More complex formulas can be evaluated.
Problem:   Plot the log of Ib and Ic vs Vbe
- Select the data to be plotted as shown in Figure 2.
Click on the chart icon,
shown on the EXCEL sheet in Figure 2.
In the titles option window of the chart wizard, fill in the
desired graph and axis titles as shown in Figure 3.
Either select the axis option of the chart wizard or
finish the plot and then
select the vertical axis. With the vertical axis selected,
click the right mouse button. In the window that opens, select "format axis".
The window shown in Figure 4 results.
Figure 4   When selecting a logarithmic axis,
be sure zero or negative values are not included.
Select a minimum greater than zero and check "logarithmic scale".
Fitting Data to a Formula
A formula representation of the collector current data can be obtained.
As shown below, eliminate data that does not look like it is
on a straight line in the gummel plot. This results in a more
accurate formula over a smaller Vbe range.
Select the curve representing Ic in the graph.
Click the right mouse button.
In the window that comes up, select "Add Trendline"
Select   Type > exponential
Select   Option > display equation on chart
EXCEL formula shown in graph:     Ic = 2E-15 e40.709 Vbe
SPICE uses a formula
Ic = IS eVbe/(NF Vt)
where IS and NF are SPICE model parameters and Vt = KT/q = 0.0259V
at T=300 degrees Kelvin.
Matching the EXCEL and SPICE formulas for Ic allows two important SPICE model
parameters to be determined.
IS = 2E-15
NF = 0.95