Nonlinear Devices -- Diodes

All of the previous circuits have been linear. This is to say that all the devices (voltage sources, current sources, dependent source and resistors) are linear devices and the overall “shape” of the problem does not change as the values of the system are scaled up or down. For example, if a circuit is solved once, then after that all of the voltage sources in the circuit are doubled, the circuit doesn't need to be solved a second time because all the node voltages will merely be double those of the first solution. Try it yourself if you disbelieve.

Linear circuits also obey the principle of “superposition” which is to say that the circuit can be solved for each source separately and then all of those solutions can be added up to get the solution of a circuit containing many sources. A textbook in basic circuit theory will explain superposition in linear circuits and you can try working through the textbook examples on the simulator using what has been explained so far.

At this point, we take the step into nonlinear circuits which do NOT obey superposition and do NOT scale. The most elementary nonlinear component is a diode.



.model 1N414 D IS=2e-14

Vcc  1   0   5
Dx   1  10   1N414
Dy  10  20   1N414
Dz  20  30   1N414
Rd1 10   0   1k
Rd2 20   0   1k
Rd3 30   0   1k

.print dc v(10) v(20) v(30)
.dc Vcc 0 5 0.5 >eg6.dat

You can run this example and look at the results like so:

gnucap -b eg6.ckt
set style data lines
plot 'eg6.dat' using 1:2, 'eg6.dat' using 1:3, 'eg6.dat' using 1:4

You may not like using gnuplot and may prefer some other plotting program such as gwave or gle. Gnucap output can be used by most plotting programs in much the same manner as above by using the redirection arrow on the command that runs the simulation (“dc” in this case). Note that it usually won't work to redirect the normal output to a file using your shell and then cut and paste that output into your plotting program because the normal output does not use standard scientific notation, using the internal redirection option provided also guarantees you get a nice, portable data file in standard exponential notation.

If the above did work you should have been able to see the node voltages as a function of supply voltage and see the diodes move into their conductive band one by one. And see the traditional 0.7 volt drop across each diode. However, various diodes behave differently so gnucap needs to know what sort of diode you are using. That is what the ”.model” command line is doing for you – it associated parameters in the diode model with a name that you choose to assign to your diodes. (By the way, I have no idea what the true measured parameters are for a real 1N414).

gnucap/manual/examples/nonlinear_devices_--_diodes.txt · Last modified: 2009/06/24 21:58 by aldavis
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