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Any free simulation programs for transistors, FET and diode circuits?

  1. Jan 23, 2012 #1

    I've been busy in designing some circuits and I want to simulate some non linear behavior of transistors and diode circuits. I want to find a free simulation program. Does LT Spice work on these discrete components?


  2. jcsd
  3. Jan 23, 2012 #2


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    Yes, of course, but I don't know if the models are accurate enough for what you want to do.

    Download a copy and install it. I can get you started off-Forum if you like, although it is pretty easy to use.

    There is a Yahoo group for LTSpice, too. They operate the program at a much higher level and also collect models for components that are not in the standard package and adapt them if necessary for LTSpice.
  4. Jan 23, 2012 #3
    Thanks, I'll look on line tonight and see whether I can get start on that. I don't need any particular transistor or diode as long as I can get a diode, a FET and a BJT. I just want to see the trend of the output signal.

  5. Jan 23, 2012 #4


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    Go to www.linear.com
    and look in the box at the right.
    There is the program, a heap of examples and some instructions.

    The examples are mainly promoting their own products, so you might like to do your own schematic.
  6. Jan 23, 2012 #5
    I down loaded already. I managed to read the instruction and draw the schematic. Problem is I don't know how to put is a sine wave voltage source to do the simulation.

    I use the voltage source and set to 1KHz and 0.1V and hit the "run" button. But I did not get any signal.

    I cannot find any battery, so I just use the voltage source and set to DC 9V. Is this ok?

    I am still trying to get the feel of the program, I likely be able to find the answer, this is just where I am at.

    The simulation seems to want to sweep frequency but yet I did not see any generator that I can set the frequency.
    Last edited: Jan 23, 2012
  7. Jan 23, 2012 #6


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    You have found the components section. Choose a "voltage". This is a voltage generator which can be a 9 V battery or a power station or a pulse generator.

    To do a sine wave, choose "advanced" then "sinewave" then set the DC offset to zero, the amplitude to 0.1 V, the frequency to 1000 Hz then all the others to zero except the number of cycles to 5000. (for no good reason except it gives you a fair burst of signal even on RF signals)
    ALSO, over on the right set the "AC amplitude" to 0.1 volt. This is used for sweep simulations.
    Bit odd, but just do it.

    Use "edit simulation" cmd to choose TRANSIENT".
    Here you set how long you want to do a simulation. Choose about 3 periods of the sinewave, so 3 mS.
    When you finish, there will be a box attached to your cursor. Stamp this on the screen with your schematic on it. This is where the program gets its information about what you are doing.
    Bit odd, but just do it.

    Don't forget to add a ground symbol somewhere on the diagram

    Click "RUN" and you should get a black window. Click a point on the schematic and you should get a trace for that point.
    There are different symbols for voltage and current, so you just move the mouse until you get the right one.

    If you want to do a square wave, set all the rise and fall times to 1 uS. It doesn't like zero rise times.
  8. Jan 23, 2012 #7
    Thanks a lot. I guess out the first part on setting the generator for 1KHz and 0.1V. It's the second part that run the simulation that really help. I got the waveform already. You save me a lot of time.


  9. Jan 24, 2012 #8
    The program is nice. When designing linear circuits, I never have a need for simulation. But this is so useful in looking at nonlinear behavior that it is just hard to design just by theory and predict how it behave. Also I can look at the output waveform at different input level.

    With this program, I can just input anything I want and "see" the output and tailor the design to get the waveform I want!!!! And the program is very easy to learn also. I only spend a few hours and I manage to craw through it and get the waveform.......by hope or by crook. I am sure there a lot more to this program, but I got my answer!!!!


    Thanks again Vk6kro. You are of big help.

  10. Jan 24, 2012 #9


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    I'm sure I don't use it to its full capabilities either, but it is a really good program. I use it all the time.

    What is annoying is how much time I have wasted working out AC bridge circuits with series and parallel LCR circuits when this thing just solves them instantly.

    Notice on the schematic window, at the bottom left, there is a little window that tells you voltages and currents in text depending on where you put your mouse.

    In the same position on the graph screen, there is a useful readout of the mouse position coordinates. Handy for taking accurate readings.

    The DC sweep is a good trick too. You can pick one voltage or current source and change its value between limits. Then watch what this does to the operating conditions of the circuit.
  11. Jan 24, 2012 #10
    Yes, able to read the current and voltage at any point is very useful. So far, I can only see it when I put the current or voltage into the graph. Then I have to delete afterwards.
    Thanks for helping.
    Last edited: Jan 24, 2012
  12. Jan 24, 2012 #11


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    Actually, it is the same line just above the "start" button, not two windows.

    You get to it by clicking on the top window (the graph one) or the bottom one (the schematic one) and then moving your mouse to where you want to do the measurement.

    Try this:
    hold down the ALT key and click on a component. This gives a temperature symbol and a graph of power dissipation in the component.
  13. Jan 25, 2012 #12
    There is nothing right above the "start" at the lower left corner either I have the schematic on full screen or the graph.
  14. Jan 25, 2012 #13


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    There should be a white bar above the toolbar at the bottom of the screen.

    The writing is put in the left of this bar.

    Try taking LTSpice out of full screen mode. Maybe it is under the toolbar. Look for the "two squares" symbol at the top right of the screen.

    This is using Windows XP.
  15. Jan 26, 2012 #14
  16. Feb 2, 2012 #15
    I have a funny encounter in the LTspice. I want to do simulation using opamp circuits. I don't need any particular opamp as I am not pushing the limits of the opamp at all. I just picked the first opamp in the list and did the simulation. The simulation seemed to get stuck and I had to halt the simulation. Upon tinkling a bit, I realize it simulated the opamp in oscillation!!! I can see the a smear of signal riding on the 2KHz I put in!!!! The simulation was slow because it was trying to plot out the HF oscillation and it took a long time. Can't get away being lazy even on the simulation!!! I didn't realize the opamp I picked was a high speed opamp and it did not like the high value resistors at the input and feedback loop.....Just like the real circuit!!!! I was too lazy to read the data sheet and just picked pick one!! Change an opamp and fixed everything.

    I have been having a ball with the LTspice, it really help me in visualizing the wave form in different designs. Now the next challenge is to add the vacuum tube models in.
  17. Feb 2, 2012 #16

    jim hardy

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  18. Feb 2, 2012 #17
    I think it was LT1001. I never did read the data sheet, it just labelled high speed. My stuff don't involve any particular opamp. I just choose another that don't oscillate!!! I am working at audio frequency under 5KHz, so almost any cheap opamp will do.
  19. Feb 2, 2012 #18


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    This software is better than a lot of commercial software that you have to pay for, yet it is free.

    Best of all, though, is that it is actually fun to use. That schematic editor is brilliant.

    I don't see why any 14 year old anywhere couldn't use it.

    It can go into a microstep mode if you give it a weird circuit. It doesn't like square waves with zero rise and fall times.
    It also doesn't like reverse biased diodes if the diode resistance is the only current path.

    I imported a couple of common opamps into it. A LM741 and a LM324.

    I also set up a couple of 2N2222's with different current gains. You just modify the BF= part of the parameters.

    Occasionally, click the "sync release" option in the "tools" pulldown. This goes and gets the latest version, although if you leave it long enough, it will suggest this itself.
  20. Feb 2, 2012 #19
    I like this guy a lot for fast and fun simulations:


    It's very, very easy to use, and has most of the goodies. It even has tubes, transmission lines, spark plugs, memristors and other exotic stuff.
  21. Feb 2, 2012 #20


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    That is a nice demo site, but LTSpice lets you put your own values in as the components.

    It then calculates the currents and voltages in the circuit according to what you draw.

    Try it. You might like it.
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