Trouble with simulation software -Msim Blue and LTSPICE

Discussion in 'General Electronics Chat' started by GiovaGiov, May 26, 2015.

  1. GiovaGiov

    Thread Starter Member

    Apr 3, 2015
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    Hi, just for fun and educational purpose, I'm doing some schematic simulation on LTSpice and Multisim Blue. Well, thanks to these software I am able to simulate and then build some basic and simple circuit. But I encountered some trouble:

    Take a look to the attached image:
    LtSpice_Simulation.png

    This is a simple triangle wave generator based on an NE555: in the simulation I attempted to insert a transistor to amplify/boost the output signal, but indiscriminately by the value of the components (resistors and the kind of transistor) I got a weird output, which go from 41mV to 65mV. All is fine and acts as expected if I remove the transistor. I encountered this trouble also using Multisim, but the strange result is always the same also if I change the kind of transistor and the base resistor.

    Some hints? I'm just doing something's wrong?

    Thank you.
     
  2. Dodgydave

    AAC Fanatic!

    Jun 22, 2012
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    Capacitor is wrong way round.
     
  3. GiovaGiov

    Thread Starter Member

    Apr 3, 2015
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    Thank you for the advice. Which value would be better?
    Do you also know the trouble about transistor?

    Thank you.
     
  4. wayneh

    Expert

    Sep 9, 2010
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    Is there anything connected to the output?

    Please also plot the threshold pin voltage, or the voltage on the base of the transistor.
     
  5. GiovaGiov

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    Apr 3, 2015
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    In the simulation, there's nothing connected to the output (with transistor and without transistor).
     
  6. djsfantasi

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    Apr 11, 2010
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    It's not the value; it connected backwards! The capacitor lead labelled "+" is connected to Gnd. ...The wrong way round, like Dave said.
     
  7. GiovaGiov

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    Apr 3, 2015
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    Ok, thank you: now is placed correctly :)
    But I still can't figure out the other issue.
     
  8. MikeML

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    Oct 2, 2009
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    The other issue is "what are you trying to do with the transistor?"

    The trouble is not with the simulation software; it is showing you the error of how you have connected the circuit.

    In your plot, V(out2) is not the collector of the transistor,which is V(out).
     
  9. crutschow

    Expert

    Mar 14, 2008
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    Wrong is right. The problem with indiscriminate (random) selection of parts value is that you get indiscriminate results, otherwise known as "garbage-in garbage-out". :eek:

    Your low value of transistor base resistor causes the transistor to fully turn on and be near saturation so it can't amplify the signal.
    Try a base resistor value of about 125kΩ.

    In the future please put a little more thought into the design before asking us to determine what's wrong. :rolleyes:

    And there's no problem with the software, it's with the humanware.
     
  10. GiovaGiov

    Thread Starter Member

    Apr 3, 2015
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    Thank you for the reply.

    Please consider that I am a novice :)

    Well, with the transistor on the output I'm trying to see the triangle wave which could reach the same Vout as Vin (15v), so the max amplification.
    Something as I've seen here (this is not from me):
    NE555amplif_Max.JPG

    Can you explain me the "error of how you have connected the circuit"?
     
  11. GiovaGiov

    Thread Starter Member

    Apr 3, 2015
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    You're totally right, but as I've said I am a novice. Obviously I'm sorry for that garbage :)
     
  12. MikeML

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    Oct 2, 2009
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    You do not need a simulator to see what is wrong with your circuit. The switching points for a 555 are at 1/3Vcc and 2/3Vcc. Vcc is 15V, so 5V and 10V, respectively.

    What is the current through R2 (base current into Q1)? The minimum current is (5V-Vbe)/4.7K = 4.3/4.7K = 0.91mA. So since Hfe of Q1 is a minimum of 100, 0.91mA is enough to saturate it, meaning that the collector voltage is always ~Vcesat.

    I ask again, what are you expecting the transistor to do?
     
  13. crutschow

    Expert

    Mar 14, 2008
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    Nothing wrong with being a novice. Everybody was one at some time.
    But the point I was trying to make was that, when you do an electronic design, no matter how simple, you need to learn the basics of the circuit you are working with.
    If you want to add a transistor then learn how a transistor works and how to bias them for what you want to do.
    That way if the simulation doesn't give the results you expect you have an idea how to troubleshoot the problem.
    Randomly selecting part values and then not knowing why the circuit isn't working is, at best, a slow, frustrating, and ineffective way to learn electronics. ;)
     
  14. DickCappels

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    Aug 21, 2008
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    I don't see how your power source can get back to the emitter of the transistor (ground) except through the capacitor. Maybe if you ground the negative end of the voltage source...
     
  15. djsfantasi

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    I think the negative end of the voltage source IS grounded. Look at the upper left of the diagram a ground symbol is superimposed on the wire. Hard to see.
     
    DickCappels likes this.
  16. Bordodynov

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    May 20, 2015
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    Change npn transistor on pnp.
     
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  17. MikeML

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    He wants to amplify the sawtooth by three so that it swings from oV to 15V at the collector of the transistor.
     
  18. Bordodynov

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    May 20, 2015
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    To get the full scope of the triangular signal is necessary to use a more complicated amplifier. One transistor is not enough. It is better to develop a scheme of such a signal.
     
  19. crutschow

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    Mar 14, 2008
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    Certainly one transistor is enough. It just needs to be properly configured and biased.
     
  20. MikeML

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    I thought about this for a bit. I had a hard time coming up with a single transistor circuit whose output swings from near 0V to near 15V without a lot of distortion or by adding power supplies. My suggestion would be to use a rail-to-rail op-amp (i.e. lots of transistors);)

    Maybe you could show us what you have in mind?
     
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