- 2082 Baisakh · 8 marks
Depletion MOSFET Q-points (IDSS 10 mA, VP -4 V)
Short answer
Assuming VDD = 20 V, RD = 500 Ω. For VGS = –1 V: ID = 5.625 mA, VDS = 17.19 V. For VGS = +2 V: ID = 22.5 mA, VDS = 8.75 V. Both Q-points have VDS > VGS – VP, so both are in pinch-off. Only VGS = +2 V (ID > IDSS) is enhancement mode.
The depletion type N channel MOSFET has IDSS= 10 mA and Vp= -4V. [Figure: n-channel depletion MOSFET with drain resistor RD from +VDD to drain, gate resistor RG from gate to -VGG, source grounded, signal Vin through coupling capacitor C to the gate, output Vout at the drain] a) Find the quiescent values of ID and VDS when VGS = -1V b) Repeat when VGS = +2V c) Which, if either, of these Q-points is in pinch off region? d) At which, if either, of these Q-points is the device operating in enhancement mode
The n-channel depletion MOSFET is biased by a negative gate supply -VGG (through RG, with no gate current, so VGS is set directly by the supply). The drain current follows the same equation as a JFET, but VGS may also be positive.
ID = IDSS (1 - VGS/VP)² , VDS = VDD - ID RD
IDSS = 10 mA, VP = -4 V. VDD and RD are not given in the question, so VDS is shown as a formula and then evaluated for an assumed VDD = 20 V and RD = 500 Ω (assumption).
a) VGS = -1 V
ID = 10 (1 - (-1)/(-4))² = 10 (1 - 0.25)² = 10 × 0.5625 = 5.625 mA
VDS = VDD - ID RD = 20 - 5.625 mA × 0.5 kΩ = 17.19 V
b) VGS = +2 V
ID = 10 (1 - 2/(-4))² = 10 (1 + 0.5)² = 10 × 2.25 = 22.5 mA
VDS = 20 - 22.5 mA × 0.5 kΩ = 8.75 V
c) Pinch-off (saturation) region
The condition for pinch-off operation is VDS ≥ VGS - VP (= VGS + 4 V).
- (a) VGS - VP = -1 + 4 = 3 V; VDS = 17.19 V > 3 V, so Q-point (a) is in the pinch-off region.
- (b) VGS - VP = 2 + 4 = 6 V; VDS = 8.75 V > 6 V, so Q-point (b) is also in the pinch-off region (for the assumed VDD and RD). In general the check is VDD - ID RD > VGS - VP.
d) Enhancement mode
A depletion MOSFET works in the depletion mode for VGS < 0 (channel width reduced, ID < IDSS) and in the enhancement mode for VGS > 0 (channel gets more carriers, ID > IDSS). Hence Q-point (b), VGS = +2 V with ID = 22.5 mA > IDSS, is in the enhancement mode; Q-point (a) (VGS = -1 V, ID = 5.6 mA < IDSS) is in the depletion mode.
Answer: (a) ID = 5.625 mA, VDS = VDD - 5.625 RD; (b) ID = 22.5 mA, VDS = VDD - 22.5 RD; (c) both are in pinch-off if VDS exceeds 3 V and 6 V respectively; (d) only (b).
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