CMOS Analog Integrated Circuits

7.2.2    MOSFET-C circuits

In this approach, the metal oxide semiconductor field-effect transistors (MOSFET) operating in triode region are used as variable resistors that are automatically adjusted to provide accurate RC products by an on-chip control circuit. According to the n-channel transistor shown in Figure 7.56, if V_GS, V_DS, V_T, and V_B are the gate-source, drain-source, threshold, and substrate voltages, respectively, the drain current, I, in the triode region will be given by

$I=K\left[2\left(V_{GS}-V_T\right)V_{DS}-V_{DS}^2\right],$

(7.163)

where $K={\mu }_n{C}_{ox}\left(W/2L\right),V_{GS}>V_T,V_{DS}<V_{DS\left(sat\right)}=V_{GS}-V_T$, and V_B ≤ V_s. Assuming that V_GS = V_C, the current, I, can be written as [42]

$I=\frac{V_{DS}}{R}+f\left(V_{DS}\right),$

(7.164)

where $f\left(V_{DS}\right)=-K{V}_{DS}^2$ and the tunable resistor, R, is of the form

$R=\frac{1}{}$