LTSpice Floating Node

Discussion in 'Analog & Mixed-Signal Design' started by jsthomps, Jul 16, 2017.

  1. jsthomps

    Thread Starter Active Member

    Mar 30, 2010
    30
    4
    Hi All,
    I'm trying to simulate a circa 1960's circuit that creates a noise based sound effect (torpedo launch "Swoosh"). The circuit uses a PNP transistor with an open collector. I assume this transistor generates noise which is then conditioned by the rest of the circuit. LTspice complains the the node is floating and will not continue with the simulation. Is there a way to tell LTspice to ignore this floating node? See cicuit below:
    Screenshot 029.JPG
    Thanks in advance,
    Scott
     
  2. Bordodynov

    Active Member

    May 20, 2015
    930
    286
    Add a resistor to the "OUT" output, for example 47 kΩ. The second pin of the resistor is connected to ground GND. Then there will be no floating node.
     
  3. RichardO

    Well-Known Member

    May 4, 2013
    1,549
    524
    Connect a very high value resistor from the collector of Q1 to its emitter. I would use something like 100Meg. (You must spell out "Meg" since 100M would be interpreted as 100 mil ohms.)
     
  4. ian field

    AAC Fanatic!

    Oct 27, 2012
    5,317
    948
    Probably the best idea - LTspice goes on strike if there's loose ends.

    I'm guessing that's a germanium transistor used as a noise generator - there were germanium Zeners, but not abundantly.
     
  5. RichardO

    Well-Known Member

    May 4, 2013
    1,549
    524
    My suggestion of adding a high value resistor to the collector of Q1 may not be valid. I just did a quick test and LTspice did _not_ complain about a floating collector of a transistor. :oops:
     
  6. Bordodynov

    Active Member

    May 20, 2015
    930
    286
  7. WBahn

    Moderator

    Mar 31, 2012
    19,523
    5,399
    How would that solve a problem with the collector of Q1 floating?
     
  8. WBahn

    Moderator

    Mar 31, 2012
    19,523
    5,399
    If your circuit relies on a component acting as a noise generator, then a simple LTSpice transient simulation won't do much for you.

    You need to model the noise source as a suitable voltage/current source and use an appropriate math function to get the desired signal properties.
     
  9. Bordodynov

    Active Member

    May 20, 2015
    930
    286
    See
    Draft514.png
     
    jsthomps likes this.
Loading...