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146 2. Tutorial
D
G
S
VGS
gm*VGS
GDS
gmb*VBS
VBS
B
S
Figure 9.2: Low-frequency small-signal model for MOS transistors
To describe the MOSFET AC operation at higher frequencies more accurately a number of parasitic
capacitances are added to the low-frequency model, namely the gate-source and gate-drain capacitances
CGS and CGD, and the bulk-source and bulk-drain capacitances CBS and CBD. The resulting highfrequency
AC model is displayed in Figure 9.3. For simplicity, we have neglected the ohmic
resistances in the drain and source regions which are usually accounted for by two additional
resistors in between the physical drain and source connections and the corresponding terminals of
the internal MOSFET device.
CGD
D
CBD
G
VGS
CGS
GDS
CBS
B
VBS
S
gm*VGS
gmb*VBS
S
Figure 9.3: High-frequency small-signal model for MOS transistors
Implementation of Small-Signal MOS Transistor Models
For the following calculations we do not need the high-frequency MOSFET equivalent circuit.
Therefore, we only implement the low-frequency model at this point. The former will be dealt with
later when you have learned more about advanced techniques for associating numerical design-point
values with symbolic subcircuit parameters.
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