lec6-dctran_jae.ppt - Lecture 6 DC Transient Response 1 Outline Pass Transistors DC Response Logic Levels and Noise Margins Transient Response RC Delay

lec6-dctran_jae.ppt - Lecture 6 DC Transient Response 1...

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Lecture 6: DC & Transient Response 1
2 Outline Pass Transistors DC Response Logic Levels and Noise Margins Transient Response RC Delay Models Delay Estimation
3 Pass Transistors We have assumed source is grounded What if source > 0? (Which side is S?) e.g. pass transistor passing V DD V g = V DD If V s > V DD -V t , V gs < V t Hence transistor would turn itself off nMOS pass transistors pull no higher than V DD -V tn Called a degraded “1” (See Prob. 2.14 – noninv) Approach degraded value slowly (low I ds ) pMOS pass transistors pull no lower than V tp Transmission gates are needed to pass both 0 and 1 V DD V DD
4 Pass Transistor Ckts Output = S (why)?
5 DC Response DC Response: V out vs. V in for a gate (SPICE model) Ex: Inverter (Vout swings rail to rail) – V in = 0 -> V out = V DD – V in = V DD -> V out = 0 In between, V out depends on transistor size and current By KCL, must settle such that I dsn = |I dsp | (current in = current out) We could solve equations But graphical solution gives more insight I dsn I dsp V out V DD V in
6 Transistor Operation Current depends on region of transistor behavior For what V in and V out are nMOS and pMOS in Cutoff? Linear? Saturation?
7 nMOS Operation Cutoff Linear Saturated V gsn < V tn V in < V tn V gsn > V tn V in > V tn V dsn < V gsn – V tn V out < V in - V tn V gsn > V tn V in > V tn V dsn > V gsn – V tn V out > V in - V tn I dsn I dsp V out V DD V in V gsn = V in V dsn = V out
8 pMOS Operation Cutoff Linear Saturated V gsp > V tp V in > V DD + V tp V gsp < V tp V in < V DD + V tp V dsp > V gsp – V tp V out > V in - V tp V gsp < V tp V in < V DD + V tp V dsp < V gsp – V tp V out < V in - V tp I dsn I dsp V out V DD V in V gsp = V in - V DD V dsp = V out - V DD V tp < 0
9 I-V Characteristics Vgs(n)=Vdd*n/5 Make pMOS is wider than nMOS such that n = p
10 Current vs. V out , V in V in5 V in4 V in3 V in2 V in1 V in0 V in1 V in2 V in3 V in4 I dsn , |I dsp | V out V DD
11 Load Line Analysis V in5 V in4 V in3 V in2 V in1 V in0 V in1 V in2 V in3 V in4 I dsn , |I dsp | V out V DD For a given V in : Plot I dsn , I dsp vs. V out – V out must be where |currents| are equal in I dsn I dsp V out V DD V in
12 Load Line Analysis V in = 0 V in5 V in4 V in3 V in2 V

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