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Thermal resistance is not an absolute constant of the transistor because it can be different, depending on the measurement conditions.
The approximate effect of Le and re can easily be shown considering the simplified T-equivalent circuit of Fig. A.
Fig. A. T-equivalent circuit with output shorted.
At high frequencies, it is convenient to represent β by the equation:
From Fig. A2,
Writing the voltage-loop equation for the input circuit, using relations (1) and (2), results in the equation:
which results in an input impedance of:
At normal frequencies of interest for normal rf power transistors,
Eq. 4 is then simplified to the final result:
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