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<body>
<h1> 16_opamp_dc: dc sweep </h1>
<h2> Scope </h2>
<p>
In this simulation we are going to map a simple opamp circuit's gain
at different voltages (dc sweep).
<h2> The schematics </h2>
<p>
The single-sheet schematic contains the opamp, the voltage sources for
input and two more voltage sources for the power supply (positive and
negative) for the opamp and spice command symbol.
<p>
<center>
<a href="16_opamp_dc.rs"><img src="16_opamp_dc.svg" width=400px></a>
<br>Click the image to get the sch-rnd sheet</center>
<p>
<h2> Opamp model </h2>
<p>
This example uses the lm358 macromodel from sch-rnd's stock spice
library. This model is a subcircuit of the amplifier and simulates a
lot of limiters and parasitics.
<h2> Preparing for simulation </h2>
<h3> Q1 </h3>
<p>
The model uses the standard opamp pinout so the hardwired spice/pinnum
attributes on the terminals will simply work.
<h3> V1 </h3>
<p>
V1 is generating the input dc voltage. We can leave it 0 here, since
the dc sweep command for the analysis will provide the change over time
<h3> V2 and V3</h3>
<p>
Both are 5V DC rails for powering the opamp with the required positive
and negative supplies.
<h3> Raw spice commands </h3>
<p>
It contains the following script:
<pre>
dc V1 -50m 60m 2m
plot v(in) v(out)
</pre>
<p>
which runs a DC sweep analysis from -50mV to +60mV on the input, increasing
voltage by 2 mV steps. At the end the input and output voltages
are plotted.
<h2> Export and run ngspice </h2>
<p>
Running ngspice the usual way on the export yields the following graph:
<p>
<img src="16_opamp_dc.png">
<h2> Using other implementations </h2>
<h3> gnucap </h3>
<p>
<b>Gnucap throws an error: <b>open circuit: internal node 14</b>
<p>
Gnucap uses a different command syntax. Modify the spice command symbol's
spice/command attribute to:
<pre>
print dc v(in) v(out)
dc V1 -50m 60m 2m > plot.txt
</pre>
<p>
After the export, write the single word <b>spice</b> in the first line of the
file (e.g. using a text editor), otherwise gnucap won't know the file is in spice
syntax. Then run <i>gnucap 16_omapm_dc.cir</i> and it will dump a text
table to plot.txt that can be plotted using a suitable utility, e.g. gnuplot.
<p>
The <a href="gnucap/16_opamp_dc.rs">gnucap-modified schematic is also available.</a>
<h3> xyce </h3>
<p>
TODO
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