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LAB 2: MODELING IN SPICE 1 Lab 2: Validity, Accuracy, Appropriateness, and Usefulness of Modeling in SPICE L. Schwappach, T. Thede, D. Wehnes EE600: Modern Solid State Devices Colorado Technical University 15 September 2011
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LAB 2: MODELING IN SPICE 2 AbstractThis lab report begins by testing the validity of an NMOS (MBreakN) transistor model builtusing SPICE. Next, given measured data is matched to a model using appropriate MOSFETtheory and equations as necessary to identify accurate parameter values that will correctly fit themeasured results. Once the parameter values are identified and verified for accuracy usingSPICE the appropriateness of modeling is gauged by using models of an NMOS and PMOStransistor to build a digital CMOS inverter using SPICE and analyzing the CMOS models digitalcharacteristics. Finally, the usefulness of modeling is obtained by comparing the CMOS modeldigital characteristics to the model of a BJT TTL inverter also built using SPICE.
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LAB 2: MODELING IN SPICE 3 Table of ContentsObjectivesTheory and Design Approaches / Trade-offsCircuit SchematicsAnalysis Part a - Validity: Model of a MOSFET Transistor in SPICE Equations and Given Parameter Values Hand Calculation Results SPICE Model Results Validity Comparison of Model and Hand Calculation Results Part b – Accuracy: Finding Parameters for Accurate MOSFET Modeling. Equations and Measured Values Hand Calculation Results Using Results in SPICE to Create an Accurate Model Accuracy Comparison of Model and Measured Values Part c: Appropriateness: Modeling a CMOS inverter in SPICE CMOS: DC Analysis Function Threshold Voltage Noise Margins Power Curve CMOS: Time Domain Analysis Propagation Delays Rise and Fall Times Max Switching Frequency CMOS: Frequency Analysis CMOS: Fanout Analysis Table of Results Conclusions
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LAB 2: MODELING IN SPICE 4 Part d: Usefulness: Comparison of BJT TTL and CMOS Using SPICE Modeling TTL: DC Analysis Function Threshold Voltage Noise Margins Power Curve TTL: Time Domain Analysis Propagation Delays Rise and Fall Times Max Switching Frequency TTL: Frequency Analysis Comparison of CMOS and TTL ResultsConclusions
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LAB 2: MODELING IN SPICE 5 Lab 2: Validity, Accuracy, Appropriateness, and Usefulness of Modeling in SPICEObjectives The lab is divided into four parts with different objectives for each part as shown below: A: The purpose of Lab 2A is to calculate by hand the drain currents Id for various valuesof VGS and VDS for a MOSFET in the cut-off, linear and saturation regions of operations. Thesecalculations will be compared to a SPICE model to verify that the SPICE model is valid. B: The purpose of Lab 2B is to determine the accuracy of the SPICE model. Measuredvalues of a MOSFET circuit will be used to hand calculate appropriate input values (VTO, k’and LAMBDA) for the SPICE model to determine if it provides similar results. C: The purpose of Lab 2C is to determine if the SPICE model is appropriate for designefforts by evaluating the digital characteristics of a CMOS inverter. The characteristics to beevaluated include the voltage transfer, power usage, pulse, frequency response and fanout. D: The purpose of Lab 2D is to use SPICE modeling to compare two circuits todetermine the usefulness of the SPICE model. The CMOS inverter circuit in Lab 2C will becompared to a BJT TTL inverter circuit by evaluating the characteristics of each circuit.Theory and Design Approaches / Trade-offs There are no specific design requirements for this project since it is not a design project,but an evaluation of the validity, accuracy, appropriateness, and usefulness of the SPICE model
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LAB 2: MODELING IN SPICE 6Circuit SchematicsThe schematics for each of the circuits developed in SPICE for each part of the Lab 2 are shownbelow: GND_0 Lab2a VDrain PSpice Circuit 0Vdc VGate NMOS GND_0 I MbreaknLab2a 0Vdc L = 1u W = 14u GND_0 R 1 0 GND_0 Figure 1: MOSFET SPICE NMOS Transistor Model for DC Analysis of Lab 2A
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LAB 2: MODELING IN SPICE 7 GND_0 Lab2b VDrain PSpice Circuit 0Vdc VGate NMOS GND_0 I MbreaknLab2a 0Vdc L = 1u W = 24u GND_0 R 1 0 GND_0Figure 2: Modified MOSFET SPICE NMOS Transistor Model for DC Analysis of Lab 2B
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LAB 2: MODELING IN SPICE 8 GND_0 Vdd Lab2c CMOS Inverter 5Vdc PMOS MbreakpPMOS W = 24u L = 1u Vin Out GND_0 0Vdc NMOS MbreaknNMOS GND_0 W = 14u L = 1u C 90p 0 GND_0 GND_0 Figure 3: CMOS SPICE Model used for DC Analysis of Lab 2C
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LAB 2: MODELING IN SPICE 9 GND_0 Vdd Lab2c CMOS Inverter 5Vdc PMOS MbreakpPMOS W = 24u L = 1u Vin Out GND_0 V1 = 0 V2 = 5 NMOS PER = 10u MbreaknNMOS PW = 5u W = 14u TR = .01u L = 1u C TF = .01u 90p TD = 2u GND_0 GND_0 GND_0 0 Figure 4: CMOS SPICE Model used for Time Domain Analysis of Lab 2C
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LAB 2: MODELING IN SPICE 10 GND_0 Vdd 5Vdc Lab2c CMOS Inverter PMOS MbreakpPMOS W = 24u L = 1u Vin Out GND_0 1mVac 2.1771Vdc NMOS MbreaknNMOS W = 14u L = 1u GND_0 C1 90p 0 GND_0 GND_0 Figure 5: CMOS SPICE Model used for Frequency Analysis of Lab 2C
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LAB 2: MODELING IN SPICE 11 GND_0 Vdd 5Vdc Lab2c CMOS Inverter PMOS PMOS2 MbreakpPMOS MbreakpPMOS W = 24u W = 24u L = 1u L = 1u Vin OutGND_0 V1 = 0 V2 = 5 PER = 146.19u NMOS NMOS2 PW = 73.095u MbreaknNMOS MbreaknNMOS TR = 1n W = 14u W = 14u TF = 1n L = 1u L = 1u TD = 2u GND_0 C1 90p 0 GND_0 GND_0 GND_0 Figure 6: CMOS SPICE Model used for Fanout Analysis of Lab 2C at
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LAB 2: MODELING IN SPICE 12 GND_0 Vdd 5Vdc Lab2c CMOS Inverter PMOS PMOS2 MbreakpPMOS MbreakpPMOS W = 24u W = 24u L = 1u L = 1u Vin Out GND_0 V1 = 0 V2 = 5 PER = 1.4619m NMOS NMOS2 PW = 730.95u MbreaknNMOS MbreaknNMOS TR = 1n W = 14u W = 14u TF = 1n L = 1u L = 1u TD = 2u GND_0 C1 90p 0 GND_0 GND_0 GND_0 Figure 7: CMOS SPICE Model used for Fanout Analysis of Lab 2C at
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LAB 2: MODELING IN SPICE 13 GND_0 Vdd 5Vdc Lab2c CMOS Inverter PMOS PMOS2 MbreakpPMOS MbreakpPMOS W = 24u W = 24u L = 1u L = 1u Vin Out GND_0 V1 = 0 V2 = 5 PER = 14.619u NMOS NMOS2 PW = 7.3095u MbreaknNMOS MbreaknNMOS TR = 1n W = 14u W = 14u TF = 1n L = 1u L = 1u TD = 2u GND_0 C1 90p 0 GND_0 GND_0 GND_0 Figure 8: CMOS SPICE Model used for Fanout Analysis of Lab 2C at
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LAB 2: MODELING IN SPICE 14 Lab 2d BJT TTL Vcc R R1 R3 5Vdc 4k 1.6k 130 Q4 Q2N3904 Vin Q2 Q1 Q2N3904 Q2N3904 D2 0Vdc D1N4001 Out CL Q3 90p Q2N3904 GND_0 R2 1k 0 GND_0 Figure 9: BJT TTL SPICE Model used for DC Analysis of Lab 2D
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LAB 2: MODELING IN SPICE 15 Lab 2d BJT TTL Vcc R R1 R3 5Vdc 4k 1.6k 130 Q4 Q2N3904 Vin Q2 Q1 Q2N3904 V1 = 0 Q2N3904 D2 V2 = 5 D1N4001 PER = 10u PW = 5u TR = .01u Out TF = .01u TD = 2u CL Q3 90p Q2N3904 GND_0 R2 1k 0 GND_0 Figure 10: BJT TTL SPICE Model used Time Domain Analysis of Lab 2D
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LAB 2: MODELING IN SPICE 16 Lab 2d BJT TTL Vcc R R1 R3 5Vdc 4k 1.6k 130 Q4 Q2N3904 Vin Q2 Q1 Q2N3904 Q2N3904 D2 1mVac D1N4001 1.3925Vdc Out CL Q3 90p Q2N3904 GND_0 R2 1k 0 GND_0 Figure 11: BJT TTL SPICE Model used Frequency Analysis of Lab 2D
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LAB 2: MODELING IN SPICE 17AnalysisLab 2A - Validity: Model of a MOSFET in SPICE Equations and Given Parameter Values A MOSFET transistor has 3 modes of operation: Cut-off mode, Triode or linear mode,and saturation mode. The mode of operation is determined by the values of (the voltagefrom the Gate to Source), (the Voltage from the Gate to Source), (the ThresholdVoltage). The following simplified equations can be utilized to calculate the effects of the draincurrent . When the transistor is in Cut-off mode: Equation 1: MOSFET in Cut-off Mode When the transistor is in Triode or Linear mode: Equation 2: MOSFET in Triode Mode When the transistor is in Saturation mode: Where Equation 3: MOSFET in Saturated Mode
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LAB 2: MODELING IN SPICE 18 We were also provided with the following parameters: Parameter values for & VGS VDS a) 0.5 V 3.2 V b) 1.5 V 0.3 V c) 1.5 V 1.0 V d) 1.5 V 2.3 V e) 3.0 V 0.5 V f) 3.0 V 2.3 V g) 3.0 V 5.0 V h) 5.0 V 0.3 V i) 5.0 V 5.0 V Table 1: Table of Values Providing and
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LAB 2: MODELING IN SPICE 19 Using the given values of , and ; the mode of operation was checked for eachrow of Table 1 above. The results are shown by Table 2 below. Modes of Operation MODE a) Cut-off b) Linear c) Saturated d) Saturated e) Linear f) Saturated g) Saturated h) Linear i) Saturated Table 2: Modes of Operation
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LAB 2: MODELING IN SPICE 20 Hand Calculation Results Hand Calculations were then compiled using the appropriate formulas correspondingwith the various appropriate modes of operation and confirmed by all group members. Theseresults are displayed by Figure 12 below. Figure 12: Hand Calculations for Lab 2A
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LAB 2: MODELING IN SPICE 21 SPICE Model Results Next a SPICE project file was created by modeling a NMOS MOSFET using a MbreakNpart in SPICE as shown by Figure 1 in the schematics section. The attributes of the MbreakNwere then modified to contain and display the correct width and length values of the model . The model for the part was then modified for the correct given model parametervalues as shown by Figure 13. Figure 13: Model Parameters Used to Modify MbreakN (NMOS) Transistor in Lab 2A A SPICE DC analysis simulation was ran for analyzing whether or not the modeldeveloped in SPICE would correctly provide the same drain current results calculated usingthe fundamental theory and formulas for a MOSFET device. The results are shown in Figure14.
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LAB 2: MODELING IN SPICE 22 Figure 14: Spice DC Analysis Results for Lab 2A Validity Comparison of Model and Hand Calculation Results The hand calculation results were next compared against the SPICE models results todetermine the validity of the SPICE model NMOS MOSFET. The comparison results are shownin Table 3.
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LAB 2: MODELING IN SPICE 23 Validity Comparison Hand Calculated SPICE Model % error ID ID a) 0 3.21 pA NA b) 6.615 µA 6.7041 µA 1% c) 8.052 µA 8.2291 µA 2% d) 8.513 µA 8.6898 µA 2% e) 55.125 µA 56.358 µA 2% f) 127.701 µA 139.021 µA 8% g) 140.01 µA 154.292 µA 10% h) 72.765 µA 73.728 µA 1% i) 526.68 µA 617.23 µA 17% Table 3: Comparison of Model and Hand Calculations The model closely matched the hand calculated results in the cut-off and linear or trioderegions (%error was less than 3%). The only significant amount of error 10% and 17% wasobserved in the extremely saturated regions at large values of . These differences in valuescould be accounted for the fact that SPICE uses additional equations and factors in itscalculations. With such small percent error, it is our conclusion that this SPICE model is a validmodel. We have also gained confidence in using SPICE for modeling NMOS MOSFET devices.
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LAB 2: MODELING IN SPICE 24Lab 2B – Accuracy: Finding Parameters for Accurate MOSFET Modeling Equations and Measured Values The MOSFET Equations 1-3 were again used along with given measured values andcurrents through a MOSFET in order to develop a more accurate MOSFET model using SPICE.The given measured voltages and currents are shown by Table 4. Validity Comparison VGS VDS ID a) 0.5 V 0.5 V 0.51 pA b) 0.5 V 2.5 V 2.51 pA c) 0.5 V 5V 5 pA d) 1.5 V 0.5 V 895 nA e) 1.5 V 1V 1 µA f) 1.5 V 2.5 V 1 µA g) 3V 0.5 V 3.5778 µA h) 3V 2.5 V 9.153 µA i) 3V 5V 9.26 µA j) 5V 0.5 V 7.15 µA k) 5V 2.5 V 27 µA l) 5V 5V 33 µA Table 4: Measured Values for Lab 2B Hand Calculation Results In order to create an accurate model our group first needed to identify the mode ofoperation that each row of Table 4 above was in. Since we did not know the value of ,weneeded to provide a range of values for . Thus we assumed that was in the range of (0.5 V< < 1.5 V). Using this range it was discovered that rows g and j were most likely linearwhile row I was most likely saturated. With W/L’s ratio given as 24 we were able to use the
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LAB 2: MODELING IN SPICE 25linear equation (Equation 2) with the data from rows g and j using linear algebra (substitutionmethod) in order to solve for a common value of and that could be utilized in SPICE. Figure 15: Hand Calculation Results for finding Vt and K’ in Lab 2B
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LAB 2: MODELING IN SPICE 26 From the hand calculated results using data that followed the linear equations for a n-channel MOSFET we obtained a VT (VTO) of 746.865 mV and (KP) of 148.842 nA/V2.Now by using both of these values with the formula for a MOSFET in saturation (Equation 3)and the data from row i (most likely to be saturated) we could find (LAMBDA) as shown byFigure 16 below. Figure 16: Hand Calculation Results for finding in Lab 2B The value of (LAMBDA) was calculated to be .0077342/V.
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LAB 2: MODELING IN SPICE 27 Using Results in SPICE to Create an Accurate Model Using these values a new MOSFET model was created (Figure 2) this time providing aW (width) of 24 um and a L (length) of 1 um. The model for the MbreakN was changed usingthe calculated values for LAMBDA, KP, and VTO. A DC Sweep simulation was thencompleted to see how the SPICE model approximated the premeasured values. The results areshown in Figure 17 below. Figure 17: SPICE Simulation Results for NBreakN (NMOS) Model used in Lab 2B
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LAB 2: MODELING IN SPICE 28 Accuracy Comparison of Model and Measured Values The values provided as measured data were then evaluated against the results obtained bythe SPICE model simulation. This comparison is showed by Table 5 below. Measured drain SPICE model % VGS VDS current drain current error ID ID a) 0.5 V 0.5 V 0.51 pA 510 fA 0 b) 0.5 V 2.5 V 2.51 pA 2.51 pA 0 c) 0.5 V 5V 5 pA 5.01 pA 0 d) 1.5 V 0.5 V 895 nA 902.142 nA 1 e) 1.5 V 1V 1 µA 1.0209 µA 2 f) 1.5 V 2.5 V 1 µA 1.0327 µA 3 g) 3V 0.5 V 3.5778 µA 3.5983 µA 1 h) 3V 2.5 V 9.153 µA 9.2427 µA 1 i) 3V 5V 9.26 µA 9.4177 µA 2 j) 5V 0.5 V 7.15 µA 7.233 µA 1 k) 5V 2.5 V 27 µA 27.343 µA 1 l) 5V 5V 33 µA 33.557 µA 2 Table 5: Comparison of SPICE Model and Given Measured Values From the comparison results, it seems our SPICE model created by fitting the values ofVTO, KP, and LAMBDA to actual measured data was ninety eight percent accurate at modelingthe results provided by given measured values. Thus, this model acted as a highly accurate(<5% error) model of our real world NMOS MOSFET. It is now apparent that SPICE canachieve results with an even greater accuracy when the SPICE model uses parameters that best-fit the real world device. Overall, this is portion of the lab was a success in modeling and inshowing how SPICE models can handle accuracy and complexity.
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LAB 2: MODELING IN SPICE 29Lab 2C: Appropriateness: Modeling a CMOS inverter in SPICEDC Analysis Function A CMOS inverter was developed in SPICE (Figure 3) to examine the digital characteristics of the circuit using the given values shown in Figure 18 and Figure 19 below. Figure 18: Model Parameters for MbreakN (PMOS) MOSFET used for Lab 2C Figure 19: Model Parameters for MbreakN (NMOS) MOSFET used for Lab 2C A plot of the transfer characteristics was then created in SPICE for Vout vs. Vin as shownin Figure 20 on the next page. From the results it is observed that when a logic low (0) input (0V) is provided to the circuit a logic high (1) output (5 V) results. Likewise when a logic high (1)input (5 V) is provided a logic low (0) output (0 V) results. Thus the circuit is functioning as aninverter.
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LAB 2: MODELING IN SPICE 30 Inverter Truth Table In Out 0 1 1 0 Table 6: Inverter Truth Table Threshold Voltage There are several ways to identify the circuit’s logic threshold or switching point. Onemethod used is to draw a line with a slope of one across the output results. For an inverter this isthe point where Vin equals Vout and occurs for this CMOS inverter circuit at 2.1771 V makingthis value our switching point also known as voltage threshold VT=2.1771 V as shown by Figure20 on the next page. Figure 20: DC Analysis Plots for Vout vs. Vin
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LAB 2: MODELING IN SPICE 31 Noise Margins The noise margins for the circuit can also be found using the previously identified (VinA(low), Vout(high)) and (VinA (high), Vout(low)) points in Figure 20 by finding the valueswhere the slope of Vout equals -1 (identified in the top plot of Figure 20). The Logic NoiseMargin is the difference between what the circuit outputs as a valid logic voltage and what thecircuit expects to see as a valid logic voltage. The two equations used to find noise margins are: Noise Margin High = NMH = Vout(high) – Vin(high) Equation 4: Noise Margin High Noise Margin Low = NML = Vin(low) – Vout(low) Equation 5: Noise Margin Low The higher the noise margins, the better the circuit will be able to handle a diverse rangeof logic values. You can find Vout(high), Vout(low), and thus Vin(low), and Vin(high) by usingthe method previously mentioned (where slope of Vout equaled -1) or by estimating and usingminimum numbers for high output and maximum numbers for the low output. Since Vout(high)= 4.7138 V and Vin(high) = 2.5164 V, NMH is 2.197 V. Since Vin(low) = 1.740 V andVout(low) = 347mV, NML is 1.393 V. Ideal noise margins would be approximately 2.5 V forthis inverter circuit. Thus the CMOS inverter circuit has a good NMH and a poor NML.
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LAB 2: MODELING IN SPICE 32 Power Curve The power used was next analyzed next using the plot in Figure 21 showing power vs.Vin. As shown in the figure, the power used at Vin = 0 V and Vin = 5 V are both at 25 pW withthe maximum power used when the circuit is switching (inverting) Vin = 2.2020 of 240 µW.This is an advantage for CMOS, since nearly all of the power used is during the relatively smalltime taken for switching. Figure 21: DC Analysis Plot for Power vs. Vin for CMOS Inverter for Lab 2C
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LAB 2: MODELING IN SPICE 33CMOS Time Domain Analysis Propagation Delays The circuit was modified as shown by Figure 4 with a 5 us digital pulse (10 µs period, 2µs delay and 0.01 µs rise and fall times). The low to high propagation delay time for this circuit(tPLH) is calculated by taking the time at the point the output has risen to fifty percent of theinputs maximum range plus the inputs minimum value and subtracting the time at which theinput voltage had dropped to fifty percent of its maximum range plus the inputs minimum value.The high to low propagation delay time for this circuit (tPHL) is calculated by taking the time atthe point the output has dropped to fifty percent of the inputs maximum range plus the inputsminimum value and subtracting the time at which the input voltage had risen to fifty percent ofits maximum range plus the inputs minimum value. The total propagation delay is the sum of thetwo propagation delays (tP = tPLH + tPHL). The following formulas were used for calculating thepropagation delay times. The results are shown by Figure 22 on the next page. Equation 6: Propagation Delay Low to High Equation 7: Propagation Delay High to Low Equation 8: Total Propagation Delay
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LAB 2: MODELING IN SPICE 34‘ Figure 22: Pulse Analysis Plot for tPLH and tPHL of CMOS Inverter used in Lab 2C From the results show by Figure 22 tPLH =721 ns and tPHL = 396 ns. The totalpropagation delay tP = 1.117 µs.
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LAB 2: MODELING IN SPICE 35 Rise and Fall Times The rise time for this circuit is calculated by taking the time at the point the output hasrisen from its minimum to ninety percent of its maximum output range plus the outputsminimum and subtracting the time at which the output has risen from its minimum to ten percentof the maximum output range plus its minimum. The fall time for this circuit is calculated bytaking the time at the point the output has fallen from its maximum to ten percent of itsmaximum output range plus the outputs minimum and subtracting the time at which the outputhas fallen from its maximum to ninety percent of the maximum output range plus its minimum.These are defined by the following formulas: Equation 9: Rise Time Equation 10: Fall Time
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LAB 2: MODELING IN SPICE 36 Figure 23: Pulse Analysis Plot for tR and tF of CMOS Inverter used in Lab 2C From the results show by Figure 23 tR =1.688 µs and tF = 969 ns. Max Switching Frequency The maximum switching frequency for a circuit is normally defined by the time it takesthe circuit to rise and fall from to its maximum and minimum output values. This is normallycomputed using the circuits rise and fall times as shown by the formula: Equation 9: Max Switching Frequency Using this formula fmax = 376 kHz.
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LAB 2: MODELING IN SPICE 37CMOS Frequency Analysis The power supply in the SPICE model was changed for the next part of the analysis asshown by Figure 5 and a frequency analysis of the circuit was completed by biasing the circuitat the threshold voltage. The corner frequency (-3db) of this circuit occurred at f3dB = 6.84 kHzas shown by Figure 24. The corner frequency represents the -3dB point at which the power isreduced to ½ of the maximum and the voltage gain is reduced to .707 of maximum. Figure 24: Frequency Analysis Plot for CMOS Inverter used in Lab 2C
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LAB 2: MODELING IN SPICE 38CMOS Fanout Analysis The pulse input in the SPICE model was changed for the next part of the analysis asshown in Figure 6 with a second inverter added driven by the output of the first inverter usingthe same power supply with a Period of (1/f3dB) = 146.2 µs and PW=73.1 µs. A time domainanalysis of this circuit is shown by Figure 25 below. The pulse input was then modified for aPeriod of (1/(.1*f3dB)) = 1.462 ms and PW=731 µs as shown by Figure 7. A time domainanalysis of this circuit is shown by Figure 26 on the next page. The pulse input was thenmodified for a Period of (1/(10*f3dB)) = 14.62 µs and PW=7.31 µs as shown by Figure 8. Atime domain analysis of this circuit is shown by Figure 27 on the next page. Figure 25: Results for CMOS Inverter w/2nd Inverter Added and Period at 1/f3dB.
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LAB 2: MODELING IN SPICE 39 Figure 26: Results for CMOS Inverter w/2nd Inverter Added and Period at 1/(.1*f3dB). Figure 27: Results for CMOS Inverter w/2nd Inverter Added and Period at 1/(10*f3dB).
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LAB 2: MODELING IN SPICE 40 It is apparent from Figures 25-27 that the CMOS inverter creates good output pulseswhen the frequency is at (Figure 25) or below (Figure 26) the f3dB point. However as theoutput rectangular pulse quickly fails to retain its shape once the frequency is driven beyond thef3dB frequency.Table of Results Ideal This Evaluation Procedure Parameter Inverter Inverter Transfer VThreshold 2.5 V 2.1771 V Characteristic NMH 2.5 V 2.197 V Noise Margins NML 2.5 V 1.393 V P @ VinA = 0 0W 25 pW V Power Used P @ VinA = 5 0W 25 pW V PMax 0W 240 uW tPHL 0s 396 ns Propagation Delays tPLH 0s 721 ns tP 0s 1.117 us Rise Time tR 0s 1.688 us Fall Time tF 0s 969 ns 3dB Corner Frequency F3dB inf. 6.84 kHz Max Frequency (Using P-delay fMax inf. 376 kHz method) Dual Inverter Pulse Digital Pulse Perfect Good Output at f3dB Quality Dual Inverter Pulse Digital Pulse Perfect Excellent Output at .1*f3dB Quality Dual Inverter Pulse Digital Pulse Perfect Poor Output at 10*f3dB Quality Table 7: CMOS Results
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LAB 2: MODELING IN SPICE 41Part C Conclusions: In summary, SPICE provided an appropriate model for analyzing the real world CMOSinverter. Overall, this portion of the lab was a success in modeling and in showing how SPICEmodeling can provide a variety of appropriate analyses results for evaluating integrated circuits.It took only a few hours in a group to complete all of part 2’s analyses in SPICE making it amuch more efficient use of time than would have occurred by measuring each result physically.Aspiring engineers need to understand and use the SPICE in order to conduct quality ICevaluations. The CMOS inverter studied in this section offers great advantages in power overthe TTL inverter studied in Lab 1 and has a good NMH. However the NML, propagation delays,rise and fall times were worse than the TTL inverter. It was also observed that the CMOS circuitresponded poorly when an additional CMOS circuit was added and the clock frequency waspushed higher than the f3dB frequency. As a result this CMOS circuit will not function at ashigh of speeds as the TTL circuit and has poor fanout.
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LAB 2: MODELING IN SPICE 42Lab 2D: Usefulness: Comparison of BJT TTL and CMOS Using SPICE ModelingDC Analysis Function A BJT TTL inverter was developed in SPICE (Figure 9) to obtain the DC characteristicsof the circuit so they can be used to compare the circuit with the CMOS inverter in Lab 2C. A plot of the transfer characteristics was then created in SPICE for Vout vs. Vin as shownin Figure 28 on the next page. From the results it is observed that when a logic low (0) input (0V) is provided to the circuit a logic high (1) output (5 V) results. Likewise when a logic high (1)input (5 V) is provided a logic low (0) output (0 V) results. Thus the circuit is functioning as aninverter. Inverter Truth Table In Out 0 1 1 0 Table 8: Inverter Truth Table Threshold Voltage There are several ways to identify the circuit’s logic threshold or switching point. Onemethod used is to draw a line with a slope of one across the output results. For an inverter this isthe point where Vin equals Vout and occurs for this CMOS inverter circuit at 1.3925 V makingthis value our switching point also known as voltage threshold VT=1.3925 V as shown by Figure28 on the next page.
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LAB 2: MODELING IN SPICE 43 Figure 28: DC Analysis Plots for Vout vs. Vin for BJT TTL Inverter for Lab 2D
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LAB 2: MODELING IN SPICE 44 Noise Margins The noise margins for the circuit can also be found using the previously identified (Vin(low), Vout(high)) and (Vin (high), Vout(low)) points in Figure 28 by finding the values wherethe slope of Vout equals -1 (identified in the top plot of Figure 28). The Logic Noise Margin isthe difference between what the circuit outputs as a valid logic voltage and what the circuitexpects to see as a valid logic voltage. Once again Equation 4 and Equation 5 were used tocompute noise margins. Since Vout(high) = 4.7423 V and Vin(high) = 1.4370 V, NMH is 3.305 V. SinceVin(low) = 606 mV and Vout(low) = 23 mV, NML = 583 mV. Ideal noise margins would bebalanced at approximately 2.5 V for this inverter circuit. Thus the TTL inverter circuit has agreat NMH and a very poor NML.
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LAB 2: MODELING IN SPICE 45 Power Curve The power used was next analyzed next using the plot in Figure 29 showing power vs.Vin. As shown in the figure, the power used at Vin = 0 is 5.386 mW and the power used at Vin= 5 is 16.772 mW with the maximum power used when the circuit is switching (inverting) at Vin= 1.43 V is 165 mW. This is a disadvantage of TTL, even when the circuit is not switching ituses several milliwatts of power. Figure 29: DC Analysis Plot for Power vs. Vin for BBJT TTL Inverter for Lab 2D
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LAB 2: MODELING IN SPICE 46TTL Time Domain Analysis Propagation Delays The circuit was modified as shown by Figure 10 with a 5 us digital pulse (10 µs period, 2µs delay and 0.01 µs rise and fall times) matching the values used for the CMOS inverter. Onceagain Equations 6-8 were used for calculating the propagation delay times. The simulationresults are shown by Figure 30. ‘ Figure 30: Pulse Analysis Plot for tPLH and tPHL of TTL Inverter used in Lab 2D From the results show by Figure 30 tPLH =267 ns and tPHL = 3 ns. The total propagationdelay tP = 270 ns.
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LAB 2: MODELING IN SPICE 47 Rise and Fall Times The rise times for the TTL circuit were calculated using Equation 9 and Equation 10,the formulas for rise and fall time. The simulation results are displayed by Figure 31 below. Figure 31: Pulse Analysis Plot for tR and tF of CMOS Inverter used in Lab 2C From the results show by Figure 31 tR =35.4 ns and tF = 4 ns.
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LAB 2: MODELING IN SPICE 48 Max Switching Frequency Using Equation 9 to compute the maximum frequency the TTL circuit has fmax = 25.4MHz.TTL Frequency Analysis The power supply in the SPICE model was changed for the next part of the analysis asshown by Figure 11 and a frequency analysis of the circuit was completed by biasing the circuitat the threshold voltage. The corner frequency (-3db) of this circuit occurred at f3dB = 18.9 MHzas shown by Figure 32 below. Figure 32: Frequency Analysis Plot for TTL Inverter used in Lab 2D
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LAB 2: MODELING IN SPICE 49Comparison of CMOS and TTL Results Ideal CMOS TTL Evaluation Procedure Parameter Inverter Inverter Inverter Transfer VThreshold 2.5 V 2.177 V 1.393 V Characteristic NMH 2.5 V 2.197 V 3.305 V Noise Margins NML 2.5 V 1.393 V 583 mV P @ VinA = 0 0W 25 pW 5.386 mW V Power Used P @ VinA = 5 0W 25 pW 16.773 mW V PMax 0W 240 uW 165 mW tPHL 0s 396 ns 3 ns Propagation Delays tPLH 0s 721 ns 267 ns tP 0s 1.117 us 3 ns Rise Time tR 0s 1.688 us 35.4 ns Fall Time tF 0s 969 ns 4 ns 3dB Corner Frequency F3dB inf. 6.84 kHz 18.9 MHz Max Frequency (Using P-delay fMax inf. 376 kHz 25.4 MHz method) Table 9: Comparison of CMOS and TTL Circuits Table 9 shows a comparison of the characteristics for the BJT TLL inverter with theCMOS inverter from Lab 2C. As shown in the table, the BJT TTL inverter is must faster with asignificantly lower rise time, fall time and propagation delays and is able to handle faster clockspeeds. The CMOS inverter makes up for its lack of speed by using significantly less power andonly using that power during switching. Therefore, if speed is the most important circuitcharacteristic, the BJT TTL inverter would win. However, if minimum power usage is the mostimportant characteristic, the CMOS inverter would win. Finally the TTL has a much smallerNML than the CMOS inverter making the device less resistant to noise interference.
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LAB 2: MODELING IN SPICE 50Conclusion for Lab 2D: In summary, SPICE provided a valid, accurate, appropriate and useful model foranalyzing the BJT TTL and CMOS inverters. This portion of the lab was a success in modelingand in showing how SPICE modeling can provide a variety of useful analyses for comparingintegrated circuits. Overall this lab demonstrated the power and features of modeling usingSPICE.
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