Question 11.8: MOSFET Switch Determine the operating points of the MOSFET s......

Known Quantities: Drain resistor; V_{DD}; signal source output voltage as a function of time.

Find: Q point for each value of the signal source output voltage.

Schematics, Diagrams, Circuits, and Given Data: R_{D}  =  125  \Omega ; V_{DD}  =  10  V; v_{signal}  =  0  Vfor t < 0; v_{signal}(t)  =  2.5  V for t = 0.

Assumptions: Use the drain characteristic curves for the MOSFET (Figure 11.19) .

Analysis: We first draw the load line, using the drain circuit equation

V_{DD}  =  R_D i_D  +  v_{DS} \quad 10  =  125i_D  +  v_{DS}

recognizing a v_{DS} axis intercept at 10 V and an i_D axis intercept at 10/125 = 80 mA.

1. t < 0 s. When the signal source output is zero, the gate voltage is zero and the MOSFET is in the cutoff region. The Q point is

v_{GSQ} = 0  V \quad i_{DQ} = 0  mA \quad v_{DSQ} = 10  V

2. t ≥ 0 s. When the signal source output is 2.5 V, the gate voltage is 2.5 V and the MOSFET is in the saturation region. The Q point is

v_{GSQ}  =  0  V \quad i_{DQ}  =  60  mA \quad v_{DSQ}  =  2.5  V

This result satisfies the drain equation, since R_Di_D  =  0.06 \times 125  =  7.5  V .

Comments: The simple MOSFET configuration shown can quite effectively serve as a switch, conducting 60 mA when the gate voltage is switched to 2.5 V.