Transcript ECE 3130 * Digital Electronics and Design

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 1st 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 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
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
Get this capacitor from the Devices library
Allan Guan
Power Consumption @
10 pF load and 10ns rise time
Allan Guan
Power Consumption @
1 pF load and 10ns rise time
Allan Guan
Power Consumption @
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