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MFETSimTMod - The simpli f ed T model is shown in Fig.2...

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Simpli fi ed T Model with Body E ff ect Figure 1 shows the MOSFET T model with a Thévenin source in series with the gate and the body connected to signal ground. We wish to solve for the equivalent circuit in which the sources i sg and i sb are replaced by a single source which connects from the drain node to ground having the value i 0 d = i 0 s . We call this the simpli fi ed T model. The fi rst step is to look up into the branch labeled i 0 s and form a Thévenin equivalent circuit. With i 0 s = 0 , we can use voltage division to write v s ( oc ) = v tg r sb r s + r sb = v tg r s r s + r s = v tg 1 + χ (1) With v tg = 0 , the resistance r 0 s seen looking up into the branch labeled i 0 s is r 0 s = r s k r sb = r s 1 + χ = 1 (1 + χ ) g m (2) Figure 1: T model with Thévenin source connected to the gate and the body connected to signal ground.
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Unformatted text preview: The simpli f ed T model is shown in Fig.2. Compared to the corresponding circuit for the BJT, the MOSFET circuit replaces v tb with v tg / (1 + χ ) and r e with r s = r s / (1 + χ ) . Because the gate current is zero, set α = 1 and β = ∞ in converting any BJT formulas to corresponding MOSFET formulas. The simpli f ed T model is derived with the assumption that the body lead connects to signal ground. In the case that the body lead connects to the source lead, it follows from Fig. 1 that i sb = 0 . Connecting the body to the source is equivalent to setting χ = 0 in the MOSFET equations. Figure 2: Simpli f ed T model. 1...
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