High-frequency transistor modeling for circuit design

It has been found that certain high-frequency transistor types cannot be modeled accurately over wide frequency ranges with the hybrid pi or high-frequency T models even though reasonable extrinsic elements are added. Modifying the hybrid pi by replacing r/SUB /spl pi//C/SUB /spl pi// with an RC ladder extends the useful frequency range for that model to f/SUB T//2. A computer optimization program is used to determine the appropriate element values for the extended hybrid-pi model. For the 2N918, a two-section ladder gives a 3 : 1 improvement in the usable frequency range of the model.