From Wideband Amplifier Design

2.8 AN EXAMPLE

For an example, I will use a real-world transistor NEC part number NE851M03. It is an RF NPN small-signal device with an f t approximately equal to 8 GHz at I C = 20 mA, and V CE = 3 V. For purposes of this book, it has the advantage of being fairly well characterized with SPICE parameters, packaging parameters and "s-parameters" provided by the manufacturer. The actual parameters are given in Appendix A. With these data, one can now begin to compare the accuracy of the high-frequency models against the actual SPICE model.

Assume that we are using the NE851M03 in the emitter-follower circuit shown in Figure 2-22 with a 4 pF load. The operating point is V CE = 3 V, I C = 20 mA, making g m = 0.7737 S. There is a 10 nH inductor in the base. The NE851M03 has f t = 7.95 GHz at this operating point.

This gives T t = = 20.03 ps and C ? = g mT t = 15.5 pF.

From the SPICE parameters table, we see that the transistor has an emitter resistance R eb = 1.7 ?. (The actual SPICE parameter is labeled R E.) This gives


Because C eb is in series with C ?, the combination is C bt:


R bb, as listed in the SPICE table, is 1 ? . R

Copyright SciTech Publishing Inc. 2007 under license agreement with Books24x7

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