5.50) An n-channel JFET with = .444 mA/V 2 and V p = -1.5C is to be used as a vo

Question

5.50) An n-channel JFET with = .444 mA/V2and Vp= -1.5C is to be used as a voltage controlledresistor that spans the range:                                            1 k RN-JFET 100k a) Find the two values of vGS that give therespective limiting values of resistance b) Estimate the maximum permissible amplitude ofvDS for this resistor to operate as a lineardevice 5.50) An n-channel JFET with = .444 mA/V2and Vp= -1.5C is to be used as a voltage controlledresistor that spans the range:                                            1 k RN-JFET 100k a) Find the two values of vGS that give therespective limiting values of resistance b) Estimate the maximum permissible amplitude ofvDS for this resistor to operate as a lineardevice

Explanation / Answer

For an N-JFET to operate as a voltage controlledresistor , it must be used in the linear region This requires that VGS > VP and VDS Resistance RN = VD/ID = 1/[2 { (VGS-VP)- 0.5VDS }] => VGS = VP + 0.5VDS + 1/(2RN) As can be seen that increasing VDS increases VGS and hencedecreases RN => At VDS = 0 V , RN is max = 100k ohm => VGS min = -1.5V +1/(2*0.444*100) V = -1.4887 V At RNmin = 1 kohm , VGS max= -1.5V + 1/(2*0.444*1) = -0.3738 V (b) Since transistor must always be in linear region VDS max = (VGS - VP) in worst case => VDS max = (VGS min - VP) in worst case => VDSmax = -1.4887+1.5 V = 0.0113 V =11.3 mV I hope this helps you

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