7.2.4 Further Small-Signal Elements

The output conductance ${\it g}_{\mathrm{ds}}$ is given in Fig. 7.13. The extracted RF- ${\it g}_{\mathrm{ds}}$ is always positive. The qualitative and the quantitative behavior of ${\it g}_{\mathrm{ds}}$ are modeled correctly. A typical value of ${\it g}_{\mathrm{ds}}$= 50 mS/mm is observed for a class A bias, i.e. ${\it I}_{\mathrm{D}}$ = 50% ${\it I}_{\mathrm{Dmax}}$, for this pseudomorphic device. The dependence of the phase term ${\it\tau }$ to ${\mit g}_{\mathrm{m}}$ is found in Fig. 7.14 in comparison with measurements. Both the qualitative and the quantitative are reproduced by the simulations correctly. $\tau$ rises as a function of ${\it V}_{\mathrm{GS}}$ for ${\it V}_{\mathrm{DS}}$= 2.0 V. A maximum can be observed for the maximum ${\mit g}_{\mathrm{m}}$. For higher ${\it V}_{\mathrm{GS}}$ $\tau$ slightly drops. When the diode opens for ${\it V}_{\mathrm{GS}}$$\approx$ 0.7 V $\tau$ sharply rises in the simulation in agreement with the measurements shown.

Figure 7.13: Simulated and measured RF-$g_{ds}$ as a function of bias $V_{GS}$ for $V_{DS}$= 2 V.

Figure 7.14: $\tau$ as a function of $V_{GS}$ bias for $V_{DS}$= 2 V.

Fig. 7.15 shows the bias dependence of ${\it C}_{\mathrm{ds}}$ as a function of ${\it V}_{\mathrm{GS}}$ for ${\it V}_{\mathrm{DS}}$= 2 V. A nearly constant behavior is also observed in the simulation which underestimates the available data.

Figure 7.15: The drain-source capacitance $C_{ds}$ as a function of $V_{GS}$ bias for $V_{DS}$= 2 V.