Presentation on theme: "DC Characteristics of a CMOS Inverter"— Presentation transcript:
1 DC Characteristics of a CMOS Inverter A complementary CMOS inverter consists of a p-type and an n-type device connected in series.The DC transfer characteristics of the inverter are a function of the output voltage (Vout) with respect to the input voltage (Vin).The MOS device first order Shockley equations describing the transistors in cut-off, linear and saturation modes can be used to generate the transfer characteristics of a CMOS inverter.Plotting these equations for both the n- and p-type devices produces voltage-current characteristics shown below.
4 DC Response DC Response: Vout vs. Vin for a gate Ex: Inverter When Vin = 0 -> Vout = VDDWhen Vin = VDD -> Vout = 0In between, Vout depends ontransistor size and currentBy KCL, we thatIdsn = |Idsp|We could solve equationsBut graphical solution gives more insight
5 Transistor Operation Current depends on region of transistor behavior For what Vin and Vout are nMOS and pMOS inCutoff?Linear?Saturation?
24 DC Transfer CurveTranscribe points onto Vin vs. Vout plot
25 Operating Regions Revisit transistor operating regions Region nMOS pMOSABCDE
26 Operating Regions Revisit transistor operating regions Region nMOS pMOSACutoffLinearBSaturationCDE
27 Beta Ratio If bp / bn 1, switching point will move from VDD/2 Called skewed gateOther gates: collapse into equivalent inverter
28 DC Characteristics of a CMOS Inveter The DC transfer characteristic curve is determined by plotting the common points of Vgs intersection after taking the absolute value of the p-device IV curves, reflecting them about the x-axis and superimposing them on the n-device IV curves.We basically solve for Vin(n-type) = Vin(p-type) and Ids(n-type)=Ids(p-type)The desired switching point must be designed to be 50 % of magnitude of the supply voltage i.e. VDD/2.Analysis of the superimposed n-type and p-type IV curves results in five regions in which the inverter operates.Region A occurs when 0 leqVin leq Vt(n-type).The n-device is in cut-off (Idsn =0).p-device is in linear region,Idsn = 0 therefore -Idsp = 0Vdsp = Vout – VDD, but Vdsp =0 leading to an output of Vout = VDD.Region B occurs when the condition Vtn leq Vin le VDD/2 is met.Here p-device is in its non-saturated region Vds neq 0.n-device is in saturationSaturation current Idsn is obtained by setting Vgs = Vin resulting in the equation:
30 CMOS Inverter Transfer Characteristics In region B Idsp is governed by voltages Vgs and Vds described by:Region C has that both n- and p-devices are in saturation.Saturation currents for the two devices are:Region D is defined by the inequalityp-device is in saturation while n-device is in its non-saturation region.Equating the drain currents allows us to solve for Vout. (See supplemental notes for algebraic manipulations).
31 CMOS Inverter Static Charateristics In Region E the input condition satisfies:The p-type device is in cut-off: Idsp=0The n-type device is in linear modeVgsp = Vin –VDD and this is a more positive value compared to Vtp.Vout = 0nMOS & pMOS Operating pointsVout =Vin-VtpAVDDBVout =Vin-VtnBoth in satCnMOS in satOutput VoltagepMOS in satDEVDD/2VDD+VtpVDDVtpVtn