Radio Frequency Integrated Circuit Design - Webs

3.4 Small-Signal Model
A Brief Review of Technology
Once the bias voltages and currents are determined for the transistor, it is
necessary to determine how it will respond to alternating current (ac) signals
exciting it. Thus, an ac small-signal model of the transistor is now presented.
Figure 3.6 shows a fairly complete small-signal model for the bipolar transistor.
The values of the small-signal elements shown, r� , C� , C� , g m , and ro , will
depend on the dc bias of the transistor. The intrinsic transistor (shown directly
under the emitter region in Figure 3.1) is shown at the center. The series
resistances to the base, emitter, and collector are shown respectively by r b , r E ,
and rc . Also, between each pair of terminals there is some finite capacitance
shown as C bc , C ce , and C be . This circuit can be simplified by noting that of
the extrinsic resistors, r b is the largest, and as a result r E and rc are often omitted,
along with the capacitances C bc , C ce , and C be , as shown in Figure 3.7. Resistor
r E is low due to high doping of the emitter, while rc is reduced by a heavily
Figure 3.6 Small-signal model for bipolar transistor.
Figure 3.7 Simplified small-signal model for bipolar transistor.
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