Download nssc spice lab elec1

NSSC SPICE LAB ELEC1
James Morad / Ben Godfrey
June 27, 2015
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INTRODUCTION
SPICE (Simulation Program with Integrated-Circuit Emphasis) is a useful
tool for circuit analysis. By feeding the program the appropriate text file,
called a stack, we will model the DC and AC characteristics of the circuit
described in our stack. At the heart of the stack is the netlist - a convenient
way of representing your circuit in text form. When starting out in SPICE,
it will be helpful to keep these tips in mind:
• Your stack will have the extension .cir
• Every SPICE stack begins with a commented line. This is room for
you to place a title for your circuit and is not optional.
• An asterisk (*) placed at the beginning of a new line will comment out
a line
• A plus sign (+) placed at the beginning of a new line will append the
new line will append the new line to the previous line.
• Case does not matter
• Every stack must have a .END statement
• If you’re using the free version of SPICE3+NUTMEG, be careful with
tutorial focused on PSPICE. The commands are not interchangeable.
1.1
Writing your first netlist
The best way to learn (or at least in this case) is by example. Let’s take a
look at a simple circuit and its corresponding netlist. On the circuit below,
I have labeled the nodes with numbers so that building the netlist will just
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be a follow the numbers game. It will help to go through your own circuit
and label every node with a number when building a netlist.
First SPICE netlist
* The format for the DC voltage is: V[whatever name you choose] NODE1 NODE2 DCVal
* where NODE1 is the +5V rail and NODE2 is the ground rail. This is arbitrary
Vsource 1 0 5
* The format for resistor is: R[whatever name you choose] NODE1 NODE2 ResistorVal
* where the resistors terminals physically attach to those nodes
R1
1 2 4.7K
* The format for capacitors is the same. First letter must be C.
C1
2 0 1M
R2
2 3 270
R3
3 0 1.1k
C2
3 0 50m
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2.1
Example Scenarios
DC operating point aka quiescent point aka q-point
analysis
In this scenario we will use SPICE to calculate the node voltages in a resistor
network
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The netlist is a fairly simple task given the numbered nodes, so let’s throw
on the rest of the stack needed to actually perform our analysis:
EXAMPLE 2
Vin
1 0 5V
R1
1 2 4.7K
R2
1 3 330
R3
2 4 2.7K
R4
3 4 910
R5
3 0 18K
R6
4 0 1.1K
***End of netlist
***Begin control block. Everything within the control block
***can be done directly from the SPICE command line interface.
.control
op
***The OP command performs an operating point analysis of the circuit
print all
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***The print command prints on the output (terminal) what you tell it to.
***Change "all" to "V(2)" and observe the output.
.endcontrol
***End control block
.end
Now, assuming you have copied the above into a text file and saved it
is Example2.cir, fire up your version of SPICE. From the SPICE terminal,
navigate to the directory containing Example2.cir, type out the name of the
file, and hit enter. The result should appear as follows:
Spice 1 -> Example2.cir
Circuit example 2
v(1) = 5.000000e+00
v(2) = 3.418327e+00
v(3) = 4.279681e+00
v(4) = 2.509707e+00
vin#branch = -2.51931e-03
The terminal displays the node voltages (1-4) as well as vin# branch, the
branch current at the Vin node.
Verify that these results are correct by running through the nodal analysis
by hand. Assume the following branch currents
• i1 is the current running from node 1 to node 4
• i2 is the current from from node 1 to node 3
• i3 is the current from node 3 to node 4
• i4 is the current from node 3 to ground
• i5 is the current from node 4 to ground
This generates the following system of equations
1. 7.4i1 + 1.1i5 = 5
2. 0.33i2 + 18i4 = 5
3. 0.33i2 + 0.91i3 + 1.1i5 = 5
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4. i2 = i3 + i4
5. i1 + i3 = i5
Row reduce to solve this system of equations:






7.4 0
0
0 1.1
0 0.33 0
18 0
0 0.33 0.91 0 1.1
0
1
−1 −1 0
1
0
1
0 −1
5
5
5
0
0






⇒




1
0
0
0
0
0
1
0
0
0
0
0
1
0
0
0
0
0
1
0
0
0
0
0
1
0.33653
2.18279
1.94503
0.23776
2.28755






Solving for the node voltages gives the same values as produced in Spice.
Now try to comment out the entire control block from Example2 and save
the stack. Run Example2.cir from the SPICE command line and enter in the
commands OP and PRINT by hand. Verify that the results are expected.
2.2
Transient analysis
Transient analysis allows us to follow the voltage at the nodes of our system
over time. We will start with this section by looking at the voltage decay
in an LRC network with some initial condition. Before SPICE, solving for
the time dependence of the voltage in an LRC network meant setting up a
differential equation and doing all sorts of busy work. With SPICE, you can
plot a graph of the voltage or current at any node within a matter of seconds
(depending on how fast you type)!
In the following diagram, take the value of the inductor (L1 ) to be 100
Henries.
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EXAMPLE 3 LRC
***Begin netlist
Vin 1 0 PULSE (0V 5V 0 .1s .1s 10s)
R1 1 2 100
C1 1 0 1m
L1 2 0 100
***End netlist
***Specify initial condition (5V at node 1) with .IC command
.IC V(1) = 5
***Perform transient analysis
.TRAN .1s 20s
***Begin control block
.control
run
plot V(1) V(2)
.endcontrol
***End control block
.end
The output should show up on your monitor as two plots on the same set
of axes, as shown below
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Now, we will look at the same network with no initial condition and pulsed
voltage from a source connected to node 1.
EXAMPLE 3 LRC
***Begin netlist
***Voltage source is pulsed by using the pulse modifier
***PULSE([initial_V] [final_V] [start_time] [rise_time] [fall_time] [length_pulse
***a missing modifier that I did not include in my code comes after the length_pu
***adding the modifier will allow you to specify the period of your pulse if you
***wish your pulse to be periodic
Vin 1 0 PULSE (0V 5V 0 .1s .1s 3s)
R1 1 2 100
C1 1 0 1m
L1 2 0 100
***End netlist
***Specify initial condition (5V at node 1) with .IC command
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.IC V(1) = 5
***Perform transient analysis
.TRAN .1s 20s
***Begin control block
.control
run
plot V(1) V(2)
.endcontrol
***End control block
.end
The output from this stack is shown below.
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2.3
AC analysis
Our last sections will show you how to perform an AC analysis of your circuit
and, more importantly, how to plot the frequency response of the circuit.
Let’s start by taking a look at the stack we’re going to use for the following
low-pass filter.
Low Pass filter
***Begin netlist
Vin 1 0 AC 1
R1 1 2 4.7
C1 2 0 1u
***End netlist
***Begin control block
.control
***The AC command performs an AC analysis on the circuit
***DEC tells AC to output in decades, i.e. logarithmic plot
***10 tells us to plot 10 points per decade
***100 is the starting frequency on our plot
***100k is the final frequency on our plot
ac dec 10 100Hz 100KHz
***We can plot a Bode plot by telling the PLOT command to
***plot V(2)/V(1) on a decibel scale with the DB() command
plot db(V(2)/V(1))
.endcontrol
***End control block
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.end
The output is below:
Now try to build a high-pass filter and verify that the plot is correct by
calculating the corner frequency. Finally, let’s use what SPICE was for and
introduce an integrated circuit element: the ideal operational amplifier. The
ideal op-amp can be modeled in the following way
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We can build a simple low-pass filter with gain and check out its frequency
response.
This system has four nodes. One at Vin, one at the inverting input, one
at the Vout, and the ground. Try drawing them in yourself by following the
numbering in the netlist.
Active Low Pass filter
***Begin netlist
Vin 1 0 AC 1
R1 1 2 1
R2 2 3 4.7
C1 2 3 1u
***Notice the gain is set VERY high.
E1 3 0 0 2 1E10
***End netlist
.control
ac dec 10 100 100K
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plot db(V(3)/V(1))
.endc
.end
The output plot should be identical to that of the passive low-pass filter
with one exception: we now have gain.
Confirm that the corner frequency makes sense by using an ideal op amp
model and calculating the corner.
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3
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LUX Amplifier
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Assignment
Model the Schmitt trigger above using the NPN model with parameters
shown below using the following model:
.MODEL QSTD NPN(is=1.0e-16
+ bf=100
+ br=0.1
+ rb=50
+ rc=10
+ tf=0.12ns
+ tr=5ns
+ cje=0.4pf
+ pe=0.8
+ me=0.4
+ cjc=0.5pf
+ pc=0.8
+ mc=0.333
+ ccs=1pf
+ va=50)
See Getting Down with Spice 2 for how to add a BJT
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Create a Sallen-Key low-pass filter based off of the following design with a
corner frequency of 20 [kHz]
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