Using the circuit of Figure 6.50 assume the following values: VDD = 10 V, R1 = 40 kΩ, R2 = 100 kΩ, RS = 200 Ω, and MOSFET parameters: VTH = 0.5 V, μnCox = 100 µA/V² , W/L = 50, and λ = 0. Calculate the maximum allowable value of RD for the MOSFET to remain on the edge of saturation. Assume VGS

Question

Using the circuit of Figure 6.50 assume the following values: [latex]V_{DD}[/latex] = 10 V, [latex]R_1[/latex] = 40 kΩ, [latex]R_2[/latex] = 100 kΩ, [latex]R_S[/latex] = 200 Ω, and MOSFET parameters: [latex]V_{TH} [/latex] = 0.5 V, [latex]μ_nC_{ox}[/latex] = 100 µA/V² , W/L = 50, and λ = 0. Calculate the maximum allowable value of [latex]R_D[/latex] for the MOSFET to remain on the edge of saturation. Assume [latex]V_{GS} = 3 V[/latex].

Accepted Answer

From inspection of Figure 6.50, we can state that

[latex]V_{GG}=V_{GS}+I_DR_{DS} [/latex] (6.128)

In Equation (6.128) [latex]V_{GG}[/latex] is the gate voltage to ground. Since the gate current is negligible

[latex]V_{GG}=\frac{R_2}{R_1+R_2}V_{DD} =7.14 V[/latex] (6.129)

And since

[latex]I_D=\frac{1}{2}\mu _nC_{ox}\frac{W}{L}(V_{GS}-V_{TH})^2[/latex] (6.130)

using the values provided by the example, Equation (6.130) yields

[latex]I_D=15.63 mA[/latex]

The condition for the MOSFET to be on the edge of saturation is

[latex]V_{DS}=V_{GS}-V_{TH} [/latex]

and since [latex]V_{GS} = 3 V and V_{TH} = 0.5 V[/latex], it yields

[latex]V_{DS}=2.5 V[/latex]

By inspection of Figure 6.50, we see that [latex]V_{DD} = R_DI_D + V_{DS} + R_SI_D[/latex]. Using the given values in the above equation, 10 = [latex]R_D 0.01563 + 2.5 + 200 0.01563[/latex]. From the above equation we find the value of [latex]R_D[/latex] to be

[latex]R_D[/latex] ≅ 280 Ω.

Question 6.15: Using the circuit of Figure 6.50 assume the following values...

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