1. EXERCISE 24: BiCMOS Circuit Comparisons 1 Exercise 24: BiCMOS Circuit Comparisons L. Schwappach, T. Thede, D. Wehnes EE660: Modern Electronic Design Colorado Technical University November 2011
2. EXERCISE 24: BiCMOS Circuit Comparisons 2 Exercise 24: BiCMOS Circuit ComparisonsObjectives: The objective of this exercise is to evaluate new BiCMOS technologies and systems andidentify elements which could be used to facilitate or improve a design using SPICE modeling.To achieve these goals four circuits, to include a CMOS inverter, a BiCMOS emitter follower, aBiCMOS common emitter and a BiCMOS gated diode must be evaluated. In addition, eachcircuit must be evaluated to determine the viability of using such circuits in a design once thepower supply voltages and device sizes have been reduced.Theory and Design Approaches/Trade-offs: First of all, all four circuits needed to be created in SPICE as shown below with themodel parameters given by the instructor as shown in the table below: Device/Parameter VTO KP LAMBDA 2 PMOS -1.0 V 32uA/V 0.004 NMOS 1.5 V 72uA/V2 0.002 Table 1: PMOS and NMOS Model Specifications for Circuits. Initially all four circuits need to be provided an input voltage (Vdd) of +5 VDC and givendevice sizes: PMOS: W=84um, L=1um; NMOS: W=40um, L=1um. These four circuits mustthen be analyzed using SPICE to determine their critical characteristics. In part 2 of the exercise,the input voltage will be lowered to +3.3 VDC and the size of the devices were lowered to:PMOS: W=56um, L=.67um; NMOS: W=26.4um, L=.67um. The circuits must again beanalyzed in order to determine their critical characteristics by using SPICE and comparing thecircuits to the performance of the circuits at +5 VDC. In part 3 of this exercise, the input voltage
3. EXERCISE 24: BiCMOS Circuit Comparisons 3is lowered to +1.8 VDC and the size of the devices is lowered to: PMOS: W=30um, L=.36um;NMOS: W=14.4um, L=.36um. The circuits are again analyzed to determine their criticalcharacteristics using SPICE and compared to the performance of the circuits at +5 VDC and +3.3VDC. Each circuit must be compared in terms of speed, power, and noise immunity with thebenefits and disadvantages of each circuit discussed. The required device sizes and power requirement changes for the four circuits modeled inPSPICE are illustrated by Table 2 below. Evaluation NMOS W/L PMOS W/L Power Supply Number 1 40um/1um 84um/1um 5V 2 26.4um/.67um 56um/.67um 3.3V 3 14.4um/.36um 30um/.36um 1.8V Table 2: PMOS and NMOS Physical Specifications for Circuits Finally as an additional step to this exercise, in order to avoid the Vthreshold problems thatwere discovered while decreasing device sizes to allow for the 1.8VDD power supplies used inExercise 23 the PMOS and NMOS models for the 1.8V models in this exercise were adjustedaccording to device shrinking scaling rules. The 1.8V circuits PMOS Vthreshold values wereadjusted to -.4356mV and the 1.8V circuits NMOS Vthreshold values was adjusted to 653mV.
4. EXERCISE 24: BiCMOS Circuit Comparisons 4Schematics: Figure 1: Schematic Used for Circuits where VDD = 5V, VGate modified according to Analysis
5. EXERCISE 24: BiCMOS Circuit Comparisons 5 Figure 2: Schematic Used for Circuits to Isolate Power Supplies where VDD = 5V
6. EXERCISE 24: BiCMOS Circuit Comparisons 6 Figure 3: Schematic Used for Circuits when VDD = 3.3V, VGate modified according to Analysis
7. EXERCISE 24: BiCMOS Circuit Comparisons 7Figure 4: Schematic Used for Circuits to Isolate Power Supplies when VDD = 3.3V
8. EXERCISE 24: BiCMOS Circuit Comparisons 8Figure 5: Schematic Used for Circuits when VDD = 1.8V, VGate modified according to Analysis
9. EXERCISE 24: BiCMOS Circuit Comparisons 9Figure 6: Schematic Used for Circuits to Isolate Power Supplies when VDD = 1.8
10. EXERCISE 24: BiCMOS Circuit Comparisons 10Analysis VThreshold and Noise Margins Using SPICE, the DC transfer characteristics were determined by running a DC Sweep ofVGate from 0V to (VDD)V in .001V increments. The results of all three simulations aredisplayed below. The traces of Vout verified that the CMOS, BiCMOS Emitter Follower andBiCMOS Gated Diode circuits functioned as inverters while the BiCMOS Common Emittercircuit was not functioning as an inverter due to a 180 degree phase shift. The Vin thresholdvoltage was determined by looking at the middle of the vertical linear regions whereVout=((Vout(High) – Vout(Low))/2 + Vout(Low)) with the results shown in the table below. Figure 7: VTH and Noise Margin Results where VDD = 5V
11. EXERCISE 24: BiCMOS Circuit Comparisons 11 Figure 8: VTH and Noise Margin Results where VDD = 3.3V Figure 9: VTH and Noise Margin Results where VDD = 1.8V
12. EXERCISE 24: BiCMOS Circuit Comparisons 12 The noise margins for these circuits were calculated as follows and are shown in the table: Noise Margin (High) = Vout(High) – Vin(High) Noise Margin (Low) = Vin(Low) – Vout(Low)VDD Parameter Ideal CMOS Emitter Common GatedCase Follower Emitter Diode5V VThreshold 2.5V 2.73V 3.61V 2.79V 2.72V5V NMH 2.5V 1.651V 1.129V 1.594V 1.246V5V NML 2.5V 2.096V 3.203V 2.319V 1.689V3.3V VThreshold 1.65V 1.89V 2.17V 1.9V 1.88V3.3V NMH 1.65V 1.213V 237mV 1.172V 998mV3.3V NML 1.65V 1.685V 1.874V 1.658V 1.499V1.8V VThreshold 900mV 1.002V 1.338V 1.010V 988mV1.8V NMH 900mV 623mV 315mV 588mV 451mV1.8V NML 900mV 820mV 996mV 783mV 638mV Table 3: Vthreshold and Noise Margin Results High noise margins are needed to avoid errors. The CMOS circuit displayed a goodVthreshold and the best NMH. The BiCMOS Emitter Follower displayed the best NML but hadthe worst Vthreshold and NMH results. The BiCMOS Common Emitter displayed poorVthreshold results and a range of sufficient to poor NMH and NML results. The BiCMOS GatedDiode had the closest Vthreshold to the Ideal however it suffers from a poor NMH and the worstNML. As the devices shrink in size the Noise margin performance continues with the exceptionof the CMOS inverter whose NML gets better than the BiCMOS Common Emitter circuit.However due to the common emitters 180 degree phase shift it cannot function as an inverter forlogic functions.
13. EXERCISE 24: BiCMOS Circuit Comparisons 13 Power Usage Results Each circuit was provided an independent VDD for the power usage comparison of thefour CMOS/BiCMOS devices. The simulation results follow: Figure 10: Power Usage where VDD = 5V
14. EXERCISE 24: BiCMOS Circuit Comparisons 14 Figure 11: Power Usage where VDD = 3.3V Figure 12: Power Usage where VDD = 1.8V
15. EXERCISE 24: BiCMOS Circuit Comparisons 15 VDD Parameter CMOS Emitter Common Gated Case Follower Emitter Diode 5V Power Used 25pW 157pW 109mW 257pW At Vgate = 0V 5V Power Used 25pW 448pW 90mW 6nW At Vgate = 5V 5V Max Power Used 11mW 179mW 855mW 207mW 3.3V Power Used 11pW 66pW 24mW 96pW At Vgate = 0V 3.3V Power Used 11pW 166pW 15mW 2.2nW At Vgate = 3.3V 3.3V Max Power Used 731uW 12mW 108mW 57nW 1.8V Power Used 3pW 18pW 4mW 22pW At Vgate = 0V 1.8V Power Used 3pW 37pW 3mW 513pW At Vgate = 1.8V 1.8V Max Power Used 316uW 4mW 47mW 5nW Table 4: Power Usage Results From the analysis above the BiCMOS Common Emitter uses large amounts of powereven while offline. The BiCMOS Gated Diode is second in power usage but gains a slightadvantage over the BiCMOS Emitter Follower and CMOS inverters at smaller device sizes. AC Characteristics Using SPICE, the AC characteristics were also determined for the circuits. The AC smallsignal characteristics were obtained by changing the input voltage source from a DC source to anAC source and by changing the DC value of the new source to each threshold voltage. A biaspoint small signal analysis was developed in SPICE with the output next below and compared ina table to follow.
16. EXERCISE 24: BiCMOS Circuit Comparisons 16 Figure 13: Small Signal Characteristics of CMOS, BiCMOS Emitter Follower, BiCMOS Common Emitter, and BiCMOS Gated Diode when VDD = 5V
17. EXERCISE 24: BiCMOS Circuit Comparisons 17 Figure 14: Small Signal Characteristics of CMOS, BiCMOS Emitter Follower, BiCMOS Common Emitter, and BiCMOS Gated Diode when VDD = 3.3V
18. EXERCISE 24: BiCMOS Circuit Comparisons 18 Figure 15: Small Signal Characteristics of CMOS, BiCMOS Emitter Follower, BiCMOS Common Emitter, and BiCMOS Gated Diode when VDD = 1.8V
19. EXERCISE 24: BiCMOS Circuit Comparisons 19VDD Parameter Ideal CMOS Emitter Common GatedCase Follower Emitter Diode5V Gain Inf. -538 -11 31 -935V Input Inf. Ohms 1e20 ohms 1e20 ohms 1e20 ohms 1e20 ohms Impedance5V Output Zero 77k ohms 73 ohms 100 ohms 377 ohms Impedance Ohms.3.3V Gain Inf. -10 -30 127 -1.2k3.3V Input Inf. Ohms 1e20 ohms 1e20 ohms 1e20 ohms 1e20 ohms Impedance3.3V Output Zero 4.78k ohms 446 ohms 495 ohms 9.4e8 ohms Impedance Ohms.1.8V Gain Inf. -1.88k -3 142 -1.17k1.8V Input Inf. Ohms 1e20 ohms 1e20 ohms 1e20 ohms 1e20 ohms Impedance1.8V Output Zero 951k ohms 386 ohms 593 ohms 5.047G Impedance Ohms. ohms Table 5: Small Signal Characteristic Results From the results shown the CMOS inverter offers the best choice as an amplifier circuitwhen connected to a 5V and 1.8V power supply, however when connected to the 3.3V powersupply it functioned the worst. The BiCMOS emitter follow had the best input and outputimpedance of the four circuits and thus would function great as a buffer and with severaladditional output devices. The small signal characteristics remain steady as the devices decreasehowever the output impedance becomes much greater after each technology reduction. Frequency Analysis A frequency analysis of the circuits was obtained by running an AC Sweep/Noiseanalysis of the circuits. Bode plots were created highlighting the corner frequency (-3dB) pointof the circuits. The results follow:
20. EXERCISE 24: BiCMOS Circuit Comparisons 20 Figure 16: Frequency Analysis Results of CMOS Circuit when VDD = 5VFigure 17: Frequency Analysis Results of BiCMOS Emitter Follower Circuit when VDD = 5V
21. EXERCISE 24: BiCMOS Circuit Comparisons 21Figure 18: Frequency Analysis Results of BiCMOS Common Emitter Circuit when VDD = 5V Figure 19: Frequency Analysis Results of BiCMOS Gated Diode Circuit when VDD = 5V Figure 20: Frequency Analysis Results of CMOS Circuit when VDD = 3.3V
22. EXERCISE 24: BiCMOS Circuit Comparisons 22Figure 21: Frequency Analysis Results of BiCMOS Emitter Follower Circuit when VDD = 3.3VFigure 22: Frequency Analysis Results of BiCMOS Common Emitter Circuit when VDD = 3.3V
23. EXERCISE 24: BiCMOS Circuit Comparisons 23Figure 23: Frequency Analysis Results of BiCMOS Gated Diode Circuit when VDD = 3.3V Figure 24: Frequency Analysis Results of CMOS Circuit when VDD = 1.8V
24. EXERCISE 24: BiCMOS Circuit Comparisons 24Figure 25: Frequency Analysis Results of BiCMOS Emitter Follower Circuit when VDD = 1.8VFigure 26: Frequency Analysis Results of BiCMOS Common Emitter Circuit when VDD = 1.8V
25. EXERCISE 24: BiCMOS Circuit Comparisons 25Figure 27: Frequency Analysis Results of BiCMOS Gated Diode Circuit when VDD = 1.8VVDD Parameter Ideal CMOS Emitter Common GatedCase Follower Emitter Diode5V F3db Inf. Hz 52 Hz 56k Hz 39k Hz 10 k Hz3.3V F3db Inf. Hz 830 Hz 8.9k Hz 7.8k Hz 1.4 Hz1.8V F3db Inf. Hz 4.4 Hz 10k Hz 6.5k Hz 1.4 Hz Table 6: Corner Frequency Results From the results of the frequency analysis it seems that the BiCMOS Emitter Followerhas the best frequency response and as device sizes shrink the circuits f3dB frequency decreases.With the benefits of low power usage the BiCMOS Emitter Follower looks to be the idealpower/speed inverter technology combination, however the results of the propagation delayanalysis should offer more insight.
26. EXERCISE 24: BiCMOS Circuit Comparisons 26 Time Domain Analysis (Rise and Fall Times, Propagation Delays) The rise time for the circuits is calculated by taking the time at the point each output rosefrom low to ninety percent of the high output and subtracting the time at which each output rosefrom low to ten percent of the high output. This is defined by the following formula: TR = Trise(.9Vout) – Trise(.1Vout) or t4 – t2 below. The fall time for the circuits is calculated by taking the time at the point each output fellfrom high to ten percent of the high output and subtracting the time at which each output fellfrom high to ninety percent of the high output. This is defined by the following formula: TF = Tfall(.1Vout) – Tfall(.9Vout) or t8-t6 below. The low to high propagation delay time (tPLH) for a circuit is calculated by taking the timeat the point the output has risen from low to fifty percent of the high input voltage andsubtracting the time at which the input voltage dropped to its fifty percent voltage point. tPLH isdefined by t3-t1 on the following results. The high to low propagation delay time (tPHL) for a circuit is calculated by taking the timeat the point the output has fallen from high to fifty percent of the high input voltage andsubtracting the time at which the input voltage rose to its fifty percent voltage point. tPHL isdefined by t7-t5 on the following results. The total propagation delay is the sum of tPLH and tPHL. There are several methods forstating fMAX. This lab will simplify fMAX to 1/tP.
27. EXERCISE 24: BiCMOS Circuit Comparisons 27 Figure 28: Time Domain Analysis Results of CMOS at VDD = 5V Figure 29: Time Domain Analysis Results of BiCMOS Emitter Follower at VDD = 5V
28. EXERCISE 24: BiCMOS Circuit Comparisons 28 Figure 30: Time Domain Analysis Results of BiCMOS Common Emitter at VDD = 5V Figure 31: Time Domain Analysis Results of BiCMOS Gated Diode at VDD = 5V
29. EXERCISE 24: BiCMOS Circuit Comparisons 29 Figure 32: Time Domain Analysis Results of CMOS at VDD = 3.3V Figure 33: Time Domain Analysis Results of BiCMOS Emitter Follower at VDD = 3.3V
30. EXERCISE 24: BiCMOS Circuit Comparisons 30 Figure 34: Time Domain Analysis Results of BiCMOS Common Emitter at VDD = 3.3V Figure 35: Time Domain Analysis Results of BiCMOS Gated Diode at VDD = 3.3V
31. EXERCISE 24: BiCMOS Circuit Comparisons 31VDD Parameter Ideal CMOS Emitter Common GatedCase Follower Emitter Diode5V tR 0s 10.914us 265ns 525ns 386ns5V tF 0s 12.294us 2.27us 346ns 367ns5V tPLH 0s 4.723us 135ns 387ns 203ns5V tPHL 0s 5.691us 947ns 256ns 202ns5V tP 0s 10.414us 1.082us 643ns 405ns5V fMAX Inf. Hz 96k Hz 924k Hz 1.555 MHz 2.469MHz3.3V tR 0s 20.745us 303ns 491ns 505ns3.3V tF 0s 27.522us 4.346us 396ns 616ns3.3V tPLH 0s 7.324us 146ns 491ns 261ns3.3V tPHL 0s 14.251us 1.887us 291ns 344ns3.3V tP 0s 21.575us 2.033us 782ns 605ns3.3V fMAX Inf. Hz 46k Hz 492k Hz 1.279M Hz 1.653M Hz1.8V tR 0s 33.158us 3.097us 662ns 1.421us1.8V tF 0s 39.754us 9.227us 455ns 1.533us1.8V tPLH 0s 14.732us 146ns 457ns 347ns1.8V tPHL 0s 19.033us 2.813us 323ns 376ns1.8V tP 0s 33.765us 2.959us 780ns 723ns1.8V fMAX Inf. Hz 30k Hz 338k Hz 1.282M Hz 1.383M Hz Table 7: Corner Frequency Results From the time domain analysis results above the CMOS has the slowest switching speedof the four circuits. The BiCMOS Emitter Follower was also slower than expected as thefrequency analysis showed that it would be the fastest of the four. However, with the new data itappears that the BiCMOS Gated Diode offers the best combination of switching speed and lowpower usage of the four devices.
32. EXERCISE 24: BiCMOS Circuit Comparisons 32 Conclusion From all of the simulation results comparing the four devices as device sizes shrink the1.8V BiCMOS Gated diode inverter offers a 1.4MHz switching speed at a cost of 5nW of powerduring switching. However it suffers from extremely large output impedance and poor noisemargins. The 3.3V BiCMOS Gated diode also offers the fast switching speeds and low powerusage (57nW of power during switching). The BiCMOS Common Emitter has a 180 degreephase shift preventing its use as configured for inversion and requires a massive amount ofpower (when compared to the other devices) making it a poor choice for small digital logicdevices. The CMOS inverter offers the best noise margin performance and low power utilizationof the four devices but suffers from the slowest switching speed making it an impractical optionfor high speed electronics. Finally, the BiCMOS Emitter Follower circuit performs goodinversion at speeds lower than the Gated Diode, yet fast when compared solely to the CMOSdevice. The BiCMOS Emitter Follower also had the lowest output impedance allowing for alarger fanout than the other three technologies, consumes less power than the Common Emitterand Gated Diode when not inverting. Of the three BiCMOS device sizes compared the 1.8Vsized device seems to have the greatest benefits offering a small device size with low powerutilization with only a small decrease in switching frequency speed.
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