Download lab instructions - University of Iowa

55:041 Electronic Circuits
The University of Iowa
Fall 2014
Very Brief Introduction to Micro-Cap SPICE
Starting Micro-Cap SPICE
Micro-Cap SPICE is available on CoE machines under the Spectrum Software menu: Programs
 Spectrum Software  Micro-Cap 10 Evaluation (see the figure below).
Figure 1. Starting Micro-Cap SPICE on CoE computers.
First Circuit—Thevenin Equivalent Circuit
Follow the instructor’s instructions and build the following circuit in Micro-Cap SPICE.
Replicate the circuit as closely as possible, including the rounded box, labels, etc. Save the
circuit as Thevenin.cir.
Figure 2. A simple circuit captured in Micro-Cap SPICE.
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Author: A. Kruger
55:041 Electronic Circuits
The University of Iowa
Fall 2014
Dynamic DC Analysis
Next, perform a dc analysis to determine the current through the load using this menu sequence:
Analysis  Dynamic DC. Select OK on the popup menu. Micro-Cap SPICE calculates the
dc values and displays node voltages in small ellipse boxes as shown in the figure below.
Figure 3. Results after performing a Dynamic DC Analysis.
To determine the current through the resistors, click on the Currents icon in the toolbar,
which is the diode with an arrow above it.
Figure 4. Menu items for displaying current, power, node voltages, etc.
After selecting Currents from the toolbar, SPICE updates the display with the dc currents
through the circuit elements. Note that SPICE may place text over circuit elements. If this
happens, simply move the text boxes using the mouse.
Figure 5. Results after performing a Dynamic DC Analysis and displaying currents
and node voltages
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Author: A. Kruger
55:041 Electronic Circuits
The University of Iowa
Fall 2014
Why “Dynamic” DC?
Why is the analysis is called “Dynamic” dc analysis? It turns out that Micro-Cap SPICE
dynamically recalculates and updates values (analogous to a spreadsheet) as they are changed.
For example, while in the Dynamic dc mode, click on the 100K resistor (see (a) below) and
change the value to 10K (see (b) below). Notice how the current through the resistor is updated.
(a)
(b)
Figure 6. One can click on component values (highlighted in green) and change the values.
Micro-Cap SPICE will then recalculate the circuit’s dc values.
Post-Lab Report for First Circuit
Your Post-lab report should include the following elements
1. Section title “First Circuit Results”
2. Capture of the schematic for the circuit showing simulated currents and voltages
3. Hand calculations of a Thevenin equivalent circuit for the part shown inside the brokenline box
4. Micro-Cap SPICE simulation of the Thevenin equivalent circuit showing the load current
5. A paragraph discussing the results
Note: add text that shows your name and the date to all SPICE schematics.
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Author: A. Kruger
55:041 Electronic Circuits
The University of Iowa
Fall 2014
Second Circuit—Transient Analysis to Measure Rise Time and
Bandwidth
Build the following circuit in SPICE. Be sure to label the output node Vo as shown. Save the
circuit as LPF.cir.
Figure 7. A Simple lowpass filter (LPF)
We will first measure the circuit’s step response and determine the rise time. From the rise time
we will estimate the bandwidth using 𝐵𝑊 ≅ 0.35⁄𝑡𝑟 . Just as one would do with a real circuit,
we use a square wave generator to simulate a step function. As long as the period of the square
wave is much longer than the rise time of the circuit, one can zoom in on the transient and obtain
the step response.
Figure 8. Configuring the voltage source as a square wave. All the values are the default except
for the pulse widths (PW) and period (PER).
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Author: A. Kruger
55:041 Electronic Circuits
The University of Iowa
Fall 2014
Click on the signal generator symbol in the schematic. On the popup menu, select Square.
There are many options for changing parameters of the square wave: period, maximum- and
minimum voltages, rise- and fall times of the signal generator, etc. Change the period (PER) to
20m (for 20 ms) and the pulse width (PW) to 10 ms and leave the other values alone. You can
click “plot” to see what the resulting square wave looks like.
Next we will perform transient analysis using the menu sequence AnalysisTransient. As
a first try, perform the analysis for 100 ms by making sure 100m is in the Time Range Field.
Be sure to select AutoAlways for X Range and Y Range. This will allow you to see the
complete waveform. Also, make sure there is only one plot, namely V(VO) versus T. The
notation V(VO) means the voltage at node VO.
Figure 9. Transient Analysis popup menu.
Once all the fields are filled in, click on “Run”. SPICE will then perform the transient analysis
and plot the results, which should resemble something similar to Figure 10. Note that while the
default graph settings are adequate for on-screen viewing, they are insufficient for copying to a
Word document—lines are too thin and the text too small.
Figure 10. Output from Transient Analysis—poor for using in reports.
One can easily fix this by clicking on the plot and then making appropriate adjustments on the
popup menu and its various tabs. Below is the same plot, but dressed up a bit by making the
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Author: A. Kruger
55:041 Electronic Circuits
The University of Iowa
Fall 2014
fonts bigger and lines thicker. Also, note that one can annotate the plot with text boxes, arrows,
and so on.
Figure 11. Improved version of Transient Analysis output.
As indicated in the figure, the lines are jagged. This is because SPICE uses its default time step
during the simulation. For a smoother plot, we should adjust the time step. The figure below
shows the settings for our next transient analysis. The Time Range is now 10m which will
effectively zoom in to the first 10 ms, and we set the maximum time step to be 100 ns to get a
smooth plot.
Figure 12. Modifying Transient Analysis Limits to zoom into the first 10 ms and also to
produce a smoother plot.
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Author: A. Kruger
55:041 Electronic Circuits
The University of Iowa
Fall 2014
Figure 13. Zoomed-in and smoothed output from Transient Analysis.
The rise time is defined as the time for the waveform to go from 10% to 90%. The
amplitude of the signal generator is 5 V, so for this waveform the rise time is the time to
go from 0.5 V to 4.5 V. Making such measurements is easy using Micro-Cap SPICE
using the Horizontal Tag Mode tool.
Figure 14. Zoomed-in and smoothed output from Transient Analysis with the 10–90% rise
time indicated. The Horizontal Tag Mode tool was used for the annotation.
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Author: A. Kruger
55:041 Electronic Circuits
The University of Iowa
Fall 2014
Second Circuit—AC Analysis Bandwidth
One can also use SPICE’s ac analysis capabilities to sweep the frequency over a range of
frequencies and plot the output. We will use the same RC circuit from before and determine the
bandwidth using an analysis this was and then compare the results. Run the ac analysis:
Analysis  AC. Edit the popup menu so that it matches the figure below. Remember to set
ranges to AutoAlways. Note how one specifies frequency ranges in SPICE: the higher
boundary first. For example, to plot the frequency response of a range 10 Hz to 10 kHz, we use
10K,10 in SPICE.
Figure 15. AC analysis options.
The resulting plot is shown below. The font sizes were enlarged, the plot- and grid lines were
thickened, and we used the cursor to move along the data points until we reached the -3dB point.
This is shown in the yellow box as 157.549,-2.966. The interpretation is that the output
voltage is -3 dB down from the low frequency value of 0 dB at a frequency of 157.549 Hz.
Thus, we will take the 3-dB bandwidth to be 158 Hz.
Figure 16. Output from the AC Analysis. The cursor tool was used to find the 3-dB frequency.
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Author: A. Kruger
55:041 Electronic Circuits
The University of Iowa
Fall 2014
Referring to the graph and the rise time graph above, complete the following table.
𝑡𝑟
1
Hand calculation
BW (Hz)
Transient analysis
AC analysis
1
—
Complete and include in Post-Lab Report.
Table 1. Bandwidth results.
Post-Lab Report for Second Circuit
Your Post-lab report should include the following elements
1. Section title “Second Circuit Results”
2. Capture of the schematic for the LPF. Enlarge fonts, make lines thicker etc. so you have
a professional-looking schematic
3. Capture of the transient analysis (step response) output
4. Hand calculations of the theoretical bandwidth for the circuit
5. A completed version of Table 1 above
6. A paragraph discussing the results
Note: add text that shows your name and the date to all SPICE schematics.
Third Circuit—Stabilizing MOSFET Q-Point
Build the circuit shown in Micro-Cap SPICE. For the MOSFET, use the 2N6660 NMOS part.
Be sure to connect the substrate terminal to the source. Save the circuit as MOSFET.cir.
Figure 17. MOSFET circuit for exploring how variations in the FET’s 𝑉𝑇𝑁 affects the Q-point.
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Author: A. Kruger
55:041 Electronic Circuits
The University of Iowa
Fall 2014
First, set 𝑅2 = 180K and 𝑅𝑆 = 0, and perform a Dynamic DC Analysis, and measure 𝑉𝐷𝑄
and 𝐼𝐷𝑄 and fill in the values in the table. Next explore how 𝑉𝐷𝑄 and 𝐼𝐷𝑄 change if we were to
replace this MOSFET with another 2N6660, but the second transistor has 𝑉𝑇𝑁 = 1.5 V rather
than the nominal 𝑉𝑇𝑁 = 1.69 V that is built into Micro-Cap SPICE.
Double-click on the MOSFET and on the popup menu, and change the VTO parameter (in the
lower right corner) to 1.5 V. Micro-Cap SPICE recalculates the dc values. Record the values in
the table. Clearly, changing 𝑉𝑇𝑁 affects the Q-point.
Next we will explore how 𝑉𝐷𝑄 and 𝐼𝐷𝑄 and change for the same change in 𝑉𝑇𝑁 if the circuit has a
source stabilization resistor. Set VTO parameter back to its original value (1.69 V), then set
𝑅2 = 270K and 𝑅𝑆 = 220 Ω, and perform a Dynamic DC Analysis and record the 𝑉𝐷𝑄 and
𝐼𝐷𝑄 . Next, change VTO to 1.5 V, thus simulating replacing the transistor. Note the resulting 𝑉𝐷𝑄
and 𝐼𝐷𝑄 values.
𝑉𝑇𝑁 (V)
Hand Calculation, 𝑅𝑆 = 0, 𝑅2 = 180K
SPICE Default = 1.69
1
SPICE, 𝑅𝑆 = 0, 𝑅2 = 180K
SPICE Default = 1.69
SPICE, 𝑅𝑆 = 0, 𝑅2 = 180K
1.5 V
Hand Calculation, 𝑅𝑆 = 220 Ω, 𝑅2 = 270K
SPICE Default = 1.69
SPICE, 𝑅𝑆 = 220, 𝑅2 = 270K
1.5 V
1
SPICE, 𝑅𝑆 = 220, 𝑅2 = 270K
1
𝑉𝐷𝑄 (V)
𝐼𝐷𝑄 (mA)
SPICE Default = 1.69
Complete and include in Post-Lab Report. Use 𝑉𝑇𝑁 = 1.69 V and 𝐾𝑛 = 41.88 mA⁄V 2.
Table 2. Source resistor stabilization results.
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Author: A. Kruger
55:041 Electronic Circuits
The University of Iowa
Fall 2014
Post-Lab Report for Third Circuit
Your Post-lab report should include the following elements
1. Section title “Third Circuit Results”
2. Capture of the schematic for the circuit showing simulated currents and voltages for
𝑅𝑆 = 220 Ω, 𝑉𝑇𝑂 = 1.69 V, 𝑅2 = 270K.
3. Complete the following two sentences (show your calculations)
o A ____% change in 𝑉𝑇𝑁 resulted in a ____% change in 𝐼𝐷𝑄 and a ____% change
in 𝑉𝐷𝑄 when there is no source stabilization resistor (i.e., 𝑅𝑆 = 0).
o A ____% change in 𝑉𝑇𝑁 resulted in a ____% change in 𝐼𝐷𝑄 and a ____% change
in 𝑉𝐷𝑄 when there is a source stabilization resistor (i.e., 𝑅𝑆 = 220 Ω).
4. Completed Table 2
5. Hand calculations for the indicated values in Table 2
6. A paragraph discussing the results
Note: add text that shows your name and the date to all SPICE schematics.
Next Steps
The preceding barely scratches the surface of Micro-Cap SPICE. There are whole books on
SPICE and one could teach a complete course on using SPICE. Micro-Cap SPICE has extensive
built-in help and really nice demonstrations available through Help  Demos. There are also
many sample circuits available through Help  Sample Circuits. Students are
encouraged to take a look some of the demos as they are very well-crafted and quite extensive.
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