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 Identical circuits BUT completely different device behavior 

Last post Sat, Dec 1 2012 7:17 AM by Shiraz. 2 replies.
Started by Shiraz 30 Nov 2012 08:49 PM. Topic has 2 replies and 579 views
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  • Fri, Nov 30 2012 8:49 PM

    • Shiraz
    • Not Ranked
    • Joined on Sun, Sep 23 2012
    • Posts 15
    • Points 195
    Identical circuits BUT completely different device behavior Reply

    Ok, so somehow magically (why the calculations and theory does not match with the pspice model is another post) a circuit begins to work in a project. So I copy this circuit over to a new project (to preserve the parameters and settings of the devices because one is becoming increasingly unsure of reproducing the same results in pspice given a set of parameters).

     The circuits I am attaching are IDENTICAL very basic common source amplifiers with active loads. 

     1)

    https://docs.google.com/open?id=0BwG45lg3wcwnU3ZDQ3NtNjZCM2s

      Project_Working with 1000 Gain-Nov-30-2012.zip is a file that has the original circuit which gives an input signal of 1mv an amplification of 1000 (to around 1V).

    2) 

    https://docs.google.com/open?id=0BwG45lg3wcwnWml3UFVMQ0dxekE

    P2_First_Stage_Working-Nov-30-2012.zip is simply a new project into which I have copied and pasted the circuit from above (nothing changed except adding the .lib files in configuration. BUT the amplifier mosfet (the one at the bottom) is now displaying completely different device traits. 

     

    3) If I force a drain current into an n channel mosfet which forces the mosfet into saturation I should NOT have to bias the mosfet further right ? YET I have to do this here where I supply a dc voltage and get an n channel mosfet into the saturation region by observing the VTC traits (Vds vs Vgs graph). Then I go back and give the right dc voltage to bias the amplifier in the right region. I am fairly new to CS amplifiers but my professor was surprised that it needed a bias voltage + an active load.

     p.s if someone could also explain why pspice is prone to magical charms that would be very cool even though it SHOULD not be because one happens to encounter some bad people (and one has lived a very long time to have met such misfortune) who somehow can affect the operation of a model (does this make sense)? one is totally embarassed about mentioning this but for a long time one suffered at the hands of such people without knowing it and now (only lately) has one begun to recognize this odd and terrible thing.One sincerely hopes one is spared the proximity of such a people (is this a professional hazard ; if so which profession would enable one to be independent and safe from such practices)

    • Post Points: 20
  • Sat, Dec 1 2012 4:23 AM

    • oldmouldy
    • Top 10 Contributor
    • Joined on Tue, Jul 15 2008
    • Woking, Surrey
    • Posts 1,382
    • Points 23,570
    Re: Identical circuits BUT completely different device behavior Reply

    No conspiracy here bud. They might be the same circuit but the simualtions are not using the same libraries! The I / V Traits includes a library that has a Lambda for the MbreakN and the First Stage Gain does not.

    **** INCLUDING Project_I_V_Traits_profile.inc ****

    .LIB ".\pwrmos.lib"

    **** RESUMING Project_I_V_Traits.cir ****

    .LIB "../../../project_i_v_traits-pspicefiles/project_i_v_traits.lib"

    * From [PSPICE NETLIST] section of C:\OrCAD\OrCAD_16.5_Lite\tools\pspice\PSpice.ini file:

    .lib "nomd.lib"

    VS:

    **** INCLUDING P2_First_Stage_2000_Gain_profile.inc ****

    .LIB ".\pwrmos.lib"

    **** RESUMING P2_First_Stage_2000_Gain.cir ****

    .lib "nomd.lib"

    • Post Points: 20
  • Sat, Dec 1 2012 7:17 AM

    • Shiraz
    • Not Ranked
    • Joined on Sun, Sep 23 2012
    • Posts 15
    • Points 195
    Re: Identical circuits BUT completely different device behavior Reply

    Thanks for calling me  buddy I really appreciate that,  I prefer being loved and also loving back and being good and kind and gentle ... (though I am dreading the forgiving thing because I can't take more trouble). 

    I knew there was something to do with that strange library I never added.

    But I never  thought I would need to add other libraries. I had specified the specs so :

    1) by editing properties, adding property, making them visible (name and value).

    2) I then copied and pasted that component to the other project.

    Ok I did "edit pspice model" for both projects and I see that in one case some parameters are specified and in another the parameters did not get copied over (and hence that library and I recall you may have mentioned that if I edit a model a new library is created specific to the project).

    Ok cool, thanks!

    • Post Points: 5
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Started by Shiraz at 30 Nov 2012 08:49 PM. Topic has 2 replies.