ELECTRONICS I SUGGESTED EXERCISES 3 Problem 1 The bias circuit below is used in a design with VG=5V and RS=1kΩ. For an enhancement MOSFET with kn’(W/L) =2mA/V2, the source voltage was measured and found to be 2V. What must Vt be for this device? If a device for which Vt is 0.5V less is used, what does Vs become? What bias current results? Problem 2 In the circuit below, let RG=10MΩ, RD=10kΩ, and VDD=10V. For each of the following two transistors, find the voltages VD and VG. a) Vt=1V and kn’W/L=0.5mA/V2. b) Vt=2V and kn’W/L=1.25mA/V2. Problem 3 Using the feedback bias arrangement shown in the circuit of problem 2, with a 9V supply and an NMOS device for which Vt=1V and kn’W/L=0.4mA/V2, find RD to establish a drain current of 0.2mA. If resistor values are limited to those on the 5% resistor scale( See appendix G in Sedra), what value would you choose? What values of current and VD result? Problem 4 In the circuit below, the NMOS transistor has |Vt|=0.9V and VA=50V and operates with VD=2V. What is the voltage gain vo/vi ? What do VD and the gain become for I increased to 1mA? Problem 5 A CS amplifier using an NMOS transistor biased in the manner of the circuit below, for which gm=2mA/V is found to have an overall voltage gain Gv of -16V/V. What value should a resistance Rs inserted in the source lead have to reduce the voltage gain by a factor of 4? Problem 6 The overall voltage gain of the amplifier below was measure with a resistance Rs of 1kΩ in place and found to be -10 V/V. When RS is shorted, but the circuit operation remained linear the gain doubled. What must gm be? What value of RS is needed to obtain an overall voltage gain of -8V/V?
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