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
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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
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Plotting the VTC
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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
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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
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T-Spice
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•
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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
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The VTC is not symmetric 
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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
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Setting up the Parametric Sweep
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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’
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Your T-Spice code should look like this
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Parametric Sweep Waveform
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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
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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
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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
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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
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Plotting Power and Current from the
Transient Analysis
Get this capacitor from the Devices library
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Power Consumption @
10 pF load and 10ns rise time
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Power Consumption @
1 pF load and 10ns rise time
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Power Consumption @
1 pF load and 1ns rise time
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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?
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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.
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