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Courtesy : Prof Andrew Mason CMOS Inverter: DC Analysis By Dr.S.Rajaram, Thiagarajar College of Engineering

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Courtesy : Prof Andrew Mason CMOS Inverter: DC Analysis Analyze DC Characteristics of CMOS Gates by studying an Inverter DC Analysis –DC value of a signal in static conditions DC Analysis of CMOS Inverter –Vin, input voltage –Vout, output voltage –single power supply, VDD –Ground reference –find Vout = f(Vin) Voltage Transfer Characteristic (VTC) –plot of Vout as a function of Vin –vary Vin from 0 to VDD –find Vout at each value of Vin

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Courtesy : Prof Andrew Mason Inverter Voltage Transfer Characteristics Output High Voltage, V OH –maximum output voltage occurs when input is low (Vin = 0V) pMOS is ON, nMOS is OFF pMOS pulls Vout to VDD –V OH = VDD Output Low Voltage, V OL –minimum output voltage occurs when input is high (Vin = VDD) pMOS is OFF, nMOS is ON nMOS pulls Vout to Ground –V OL = 0 V Logic Swing –Max swing of output signal V L = V OH - V OL V L = VDD

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Courtesy : Prof Andrew Mason Inverter Voltage Transfer Characteristics Gate Voltage, f(Vin) –V GSn =Vin, V SGp =VDD-Vin Transition Region (between V OH and V OL ) –Vin low Vin < Vtn –Mn in Cutoff, OFF –Mp in Triode, Vout pulled to VDD Vin > Vtn < ~Vout –Mn in Saturation, strong current –Mp in Triode, V SG & current reducing –Vout decreases via current through Mn –Vin = Vout (mid point) ≈ ½ VDD –Mn and Mp both in Saturation –maximum current at Vin = Vout –Vin high Vin > ~Vout, Vin < VDD - |Vtp| –Mn in Triode, Mp in Saturation Vin > VDD - |Vtp| –Mn in Triode, Mp in Cutoff Error in Fig : Replace V OH to V OL + V GSn - + V SGp - Vin < V IL input logic LOW Vin > V IH input logic HIGH Drain Voltage, f(Vout) –V DSn =Vout, V SDp =VDD-Vout

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Courtesy : Prof Andrew Mason Transistor operating regions RegionnMOSpMOS ACutoffLinear BSaturationLinear CSaturation DLinearSaturation ELinearCutoff

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Courtesy : Prof Andrew Mason Noise Margin Input Low Voltage, V IL –Vin such that Vin < V IL = logic 0 –point ‘a’ on the plot where slope, Input High Voltage, V IH –Vin such that Vin > V IH = logic 1 –point ‘b’ on the plot where slope =-1 Voltage Noise Margins Error in Fig : Replace V OH to V OL –measure of how stable inputs are with respect to signal interference –VNM H = V OH - V IH = VDD - V IH –VNM L = V IL - V OL = V IL –desire large VNM H and VNM L for best noise immunity

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Courtesy : Prof Andrew Mason Switching Threshold Switching threshold = point on VTC where Vout = Vin –also called midpoint voltage, V M –here, Vin = Vout = V M Calculating V M –at V M, both nMOS and pMOS in Saturation –in an inverter, I Dn = I Dp, always! –solve equation for V M –express in terms of V M Error in Fig : Replace V OH to V OL –solve for V M

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Courtesy : Prof Andrew Mason Effect of Transistor Size on VTC Recall If nMOS and pMOS are same size –(W/L)n = (W/L)p –Coxn = Coxp (always) If Effect on switching threshold –if n p and Vtn = |Vtp|, V M = VDD/2, exactly in the middle Effect on noise margin –if n p, V IH and V IL both close to V M and noise margin is good since L normally min. size for all tx, can get betas equal by making Wp larger than Wn

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