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The George Washington University
School of Engineering and Applied Science
Department of Electrical and Computer Engineering
ECE122
Lab 4: VTC & Power Consumption
Jason Woytowich
Ritu Bajpai
Last modified on September 26, 12007
Voltage Transfer Characteristic
• Vin on the X-Axis and Vout on the Y-Axis
5V
Vout
0V
0V
Vin
5V
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Voltage Transfer Characteristic
• A symmetric VTC is one where the Vin vs
Vout curve crosses through the dead
center of the graph.
• Using 5V inputs and outputs this point is
2.5V in and 2.5V out
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Lab activity
• First we will plot VTC for an inverter.
• Check if the VTC is symmetric or not.
• If VTC is not symmetric we will find
Wp/Wn such that the VTC for an inverter
is symmetric.
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Step 1: Plotting the VTC
• The VTC curve gives us all the output
values of the inverter corresponding to all
the input values within the range of Gnd
and Vdd.
• The input varies from 0 (Gnd) to 5V (Vdd)
in our case and so does the output.
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Replace the pulse input by a DC
source in the inverter test circuit.
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Select DC transfer sweep
analysis and select sweep 1
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DC sweep analysis
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Choose DC results
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Voltage Transfer Characteristic
of the inverter
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Step 2: Is the VTC symmetric?
• The obtained VTC plot is not symmetric.
• For symmetric VTC, at intersection of input
and output curve, both input and output
should be equal to half the maximum
possible value.
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Step 3: Obtaining symmetric VTC
• Keeping the length fixed and the width of
NMOS fixed we vary the width of PMOS to
obtain a symmetric curve.
• That means that we will perform DC
sweep that we performed earlier along
with the parametric sweep.
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Insert command=>analysis=>parametric sweep=>sweep1
Retain the DC sweep command
From analysis types
choose parametric
sweep and sweep for
width.
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Defining pMOS width as a parameter
In the T-Spice code write the following command
.param width=35u
And in pMOS properties change
W=‘28*l’ to W=‘width’
• Thus PMOS width is now defined by
parameter width while NMOS width remains
unchanged.
• The following slide shows the changes
incorporated in the T-Spice code.
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T-Spice code
Command to sweep input voltage: DC SWEEP
Parameter initial value set to 35u
Command to sweep parameter
width from 35u to 100u linearly
in steps of 5u: PARAMETRIC
SWEEP
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Parametric sweep analysis waveform
Increasing
width
Each output plot corresponds to different width, width is varied in the steps of
5u.
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Double click on the symmetric VTC
to obtain trace characteristics.
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Designing for symmetric VTC
• Record the width of the pMOS
corresponding to symmetric operating
point.
• In this case width = 80u
• In S-Edit substitute this width for the
pMOS and perform transient analysis.
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Rise time at symmetric operation
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Fall time at symmetric operation
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Power Consumption
• Next we will use Tanner Tools to estimate
the power consumption of a design.
• We will also identify the sources of power
consumption.
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Power Consumption
• You already have the following test-bench:
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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 output from transient results
Identify the name
of your voltage
source from the TSpice code.
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Plotting current output from transient results
Identify the name
of your capacitor
from the T-Spice
code.
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Power Consumption
10pF Load & 10ns Rise and Fall Times
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Power Consumption
• Lower the value of the capacitor to 1pF
and resimulate
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Power Consumption
1pF Load & 10ns Rise and Fall Times
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Power Consumption
• Decrease the rise and fall times of the
pulse source to 1ns.
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Power Consumption
1pF Load & 1ns Rise and Fall Times
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Analysis and Results
• 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 and Result
• Report numerical values of your results in
tabular form.
• Do you obtain different values of power
consumed on varying the load and rise
and fall time of the pulse? Compare and
analyze your results.
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Home work
• Draw (on paper) a schematic of a half
adder and a full adder (using half adder).
Use minimum possible gates.
• You will do the schematic design and
extract the layout for the above on next
turn.
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