Question 4.SP.12: For the n-channel enhancement-mode MOSFET of Fig. 4-18, gate......
For the n-channel enhancement-mode MOSFET of Fig. 4-18, gate current is negligible, I_{\text{Don}} = 10 \text{mA}, and V_T = 4 \text{V}. If R_S = 0, R_1 = 50 kΩ, V_{DD} = 15 \text{V}, V_{GSQ} = 3 \text{V}, and V_{DSQ} = 9 \text{V}, determine the values of (a) R_1 and (b) R_D.
(a) Since i_G = 0, V_{GSQ} = V_{GG} of (4.3). Solving for R_2 gives
R_G = \frac{R_1R_2}{R_1 + R_2} \quad \text{and} \quad V_{GG} = \frac{R_1}{R_1 + R_2} V_{DD} (4.3)
(b) By (4.6),
i_D = I_{\text{Don}} \left( 1 – \frac{v_{GS}}{V_{T}} \right)^2 (4.6)
I_{DQ} = I_{\text{Don}} \left(1 – \frac{V_{GSQ}}{V_T} \right)^2 = 10 × 10^{-3} \left(1 – \frac{3}{4} \right)^2 = 0.625 \text{mA}
Then KVL around the drain-source loop requires that
R_D = \frac{ V_{DD} – V_{DSQ}}{I_{DQ}} = \frac{ 15 – 9}{0.625 × 10^{-3}} = 9.6 kΩ