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

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

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

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

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

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

The VTC is not symmetric  Allan Guan

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

Setting up the Parametric Sweep Allan Guan

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

Your T-Spice code should look like this Allan Guan

Parametric Sweep Waveform Allan Guan

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

Rise/Fall 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

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

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

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

Power 10 pF load and 10ns rise time Allan Guan

Power 1 pF load and 10ns rise time Allan Guan

Power 1 pF load and 1ns rise time Allan Guan

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

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