ECE 3130 – Digital Electronics and Design

1. ECE 3130 – Digital Electronics and Design Lab 4 VTC and Power Consumption Fall 2012 Allan Guan

2. Today’s Lab • Plot VTC for an inverter • Check if VTC is symmetric • If VTC is not symmetric we will find Wp/Wn such that the VTC for an inverter is symmetric Allan Guan

3. What is VTC? • Voltage Transfer Curve • Plots output voltage vs. input voltage • Symmetry – when a line plotted through the origin and Vdd/2 intersects the VTC at Vdd/2 Allan Guan

4. Plotting the VTC • Open your inverter test bench from the 1st lab • Replace the pulse input with a DC source • Use the net label to label “in” and “out” of the inverter Allan Guan

5. Simulation Settings • Select DC sweep analysis • Set the source name to the name of your inverter’s input source (IMPORTANT: add a ‘v’ in front of the name!) • Click OK, do NOT simulate Allan Guan

6. T-Spice • Click the “Open in T-Spice” button (T-icon to the right of the green play button) • Add the following lines of code • Hit the green play button That vertical line is just the cursor, ignore that Allan Guan

7. The VTC is not symmetric  Allan Guan

8. Obtaining a Symmetric VTC • Keeping the length and width of the NMOS fixed we can vary the width of the PMOS to obtain a symmetric curve • To do so, we will perform a DC sweep like before but with the addition of the parametric sweep Allan Guan

9. Setting up the Parametric Sweep Allan Guan

10. Defining the pMOS width as a parameter • In the T-Spice code, write .param width=3u • In the pMOS properties, change W=3u to W=‘width’ • Now, the pMOS width is defined by parameter ‘width’ Allan Guan

11. Your T-Spice code should look like this Allan Guan

12. Parametric Sweep Waveform Allan Guan

13. Designing with Symmetric VTC • Click the trace to determine the width required for the symmetric VTC • Record the width of the pMOS corresponding to the symmetric operating point (you should get 3.2u) • Replace the inverter input with the original Pulse source • Go back to simulation settings and uncheck the DC and parameter sweep and select Transient Analysis • Open up the T-Spice command window and substitute this width for the pMOS and simulate Allan Guan

14. Rise/Fall Times @ Symmetric Operation • In the W-Edit window, go to the waveform calculator • Click “Measures…” and select “rise time” • Type in a trace name and press “Measure” • With the same trace, measure the “fall time” • Since we changed the pMOS width to obtain a symmetric VTC, the rise and fall times should be the same Allan Guan

15. Power Consumption • Now, we will use Tanner Tools to estimate the power consumption of a design • We will also identify the sources of that consumption Allan Guan

16. Power Consumption • Simulate the circuit over 2 periods with fine resolution (2ns) • Show the waveforms for: • The input and output voltages • The power provided by the power supply • The currents drawn from the power supply and the capacitor Allan Guan

17. Plotting Power and Current from the Transient Analysis Get this capacitor from the Devices library Allan Guan

18. Power Consumption @10 pF load and 10ns rise time Allan Guan

19. Power Consumption @1 pF load and 10ns rise time Allan Guan

20. Power Consumption @1 pF load and 1ns rise time Allan Guan

21. Analysis • Report numerical values of your results in tabular form. • Can we vary the width of NMOS instead of PMOS in order to obtain symmetric VTC? If yes, should we increase or decrease it’s value keeping PMOS width fixed? • On the VTC of the inverter, show the triode, saturation, and cut-off region. Which region is used for digital design and which one is used for analog design? Allan Guan

22. Analysis (Continued) • Do you obtain different values of power consumed on varying the load and rise/fall time of the pulse? Compare and analyze your results. Allan Guan