# 10 - t = v o(1/R S2 g m2 R o = v o/i t = 1(1/R S2 g m2 = R...

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EE 2001 Fall 2008 Problem Set 10 ______________________________________________________________________ Large Signal FET For the circuit shown: a) Find i D and v DS . b) Estimate transistor parameters K, V To , and r d Solution v GS = (20 v)*(100k/(100k + 300k)) = 5 v i D = 36 ma (from chart) v DS = 20 v - (194 ohm)(36 ma) = 13 v V To = 2; (this is an estimate - looks like nothing starts until after 2 volts) K = 0.004; (this fits the equation, i D = K(v GS – V To ) 2 ) r d = infinity (the curves look flat in active region) 300 kΩ 100 kΩ 194 Ω + 20 V + 20 V 10 20 30 40 i D (ma) v DS v GS = 5.0 v GS = 4.5 v GS = 4.0 v GS = 3.5 v GS = 3.0 v GS = 2.5 5 10 15 0 0

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Small Signal FET a) Draw the small signal model of the amplifier shown. Include all component values. b) Calculate A v , R i and R o . Solution R i = 50 k ohm R o = ? Apply v o and calculate current driven into circuit, i t . i t = v o /R S2 - g m2 v gs2 , and v gs2 = -v o , so i
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Unformatted text preview: t = v o (1/R S2 + g m2 ), R o = v o /i t = 1/(1/R S2 + g m2 ) = R S2 ||(1/g m2 ) = 47.62 ohms KCL at d1: g m1 v gs1 +v ds1 /(R D1 ||R G2 ) = 0, so v ds1 = -45 v gs1 = -45 v i [Note: A v1 = -45] KCL at s2:-g m2 v gs2 + v o /R S2 = 0, and v gs2 = v ds1 – v o , so -g m2 (-45 v i – v o ) + vo/R S2 = 0 Therefore, g m2 v o + v o /R S2 = -45 gm2 v i , A v = v o /v i = -45 g m2 /(g m2 + 1/R S2 ) [Note: A v2 = g m2 /(g m2 + 1/R S2 ) = R S2 /(1/g m2 + R S2 ) = 0.9524] A v = A v1 A v2 =-42.86 150 kΩ 75 kΩ 10 kΩ + 20 V + 20 V 90 kΩ + 20 V + 20 V v i v O 10 kΩ 1 kΩ Q1 Q2 Common Source Source Follower g m1 = 0.005 g m2 = 0.02 50 k 10 k 0.005 v gs1 9 0 k 0.02v gs2 + v gs1 _ + v gs2-1 k g1 g2 d1 s1 d2 s2 + vi _ + vo _...
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10 - t = v o(1/R S2 g m2 R o = v o/i t = 1(1/R S2 g m2 = R...

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