cascode amplifer simulation

Discussion in 'The Projects Forum' started by jzou, Jul 18, 2008.

  1. jzou

    Thread Starter New Member

    Jul 8, 2008
    3
    0
    I am currently designing a cascode amplifier that is suppose to have a gain of 97.8 and the corner frequency less than 200Hz using only BJT transistors, or more specifically, 2N3904. Ic current is to be 1ma and power supply is to be 15V-gnd. Out put is suppose to be 2V peak to peak with no distortion.

    After a whole bunch of calculations, I was able to determine the proper values for all parts, as listed in one of the files named cascode amp design calculations.

    Now you may ask: it looks fine, so what's the problem? Take a look at the scale of my simulation generated from PSpice. How on earth did I get 100V when none of my sources is more than 15V?

    The simulation is suppose to show me the input curve and the output curve, allowing me to calculate the gain. But that is a bit difficult right now.

    If anyone has any ideas, please let me know.

    The design calculations are included for your convinence, just incase I made a mistake and I missed it. I have also included the simulation profile as well.

    I used OrCAD Capture CIS 15.7, and PSpice to simulate this circuit.

    Thanks very much,
    Davy
     
  2. hgmjr

    Moderator

    Jan 28, 2005
    9,030
    214
    Can you post the schematic in a more readily accessible format?

    As it stands the only members that will be able to contribute to your thread are those with access to PSPICE.

    hgmjr
     
  3. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    Try making a screen print of your schematic, and saving it as a .PNG file.

    1) Pull up the schematic in ORCAD. Size it to fill the screen as much as possible, without clipping off parts of it.
    2) Hold the Ctrl key down, and tap PrtScreen.
    3) Start the Paint program (Start -> Programs -> Accessories -> Paint)
    4) Select Edit/Paste
    5) File/Save As...
    6) Select .PNG as the file extension, and give the file any name you wish.
    7) Upload as an attachment as you did before.
     
  4. jzou

    Thread Starter New Member

    Jul 8, 2008
    3
    0
    sorry, i am a bit new at this. is there a specific reason why you asked for .png format?

    Or does the html format just doesn't work?

    Thanks
    Davy
     
  5. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    .PNG usually produces a very compact file with minimal distortion. It isn't a "lossy" format like .JPG. It loads very quickly without having to start other application software.
     
  6. Ron H

    AAC Fanatic!

    Apr 14, 2005
    7,050
    657
    When doing AC simulations, all versions of Spice that I have used first calculate the bias point, then linearize the circuit. There are therefore no nonlinear components in the actual network that is simulated, and therefore there will be no distortion, including no clipping or limiting. If you change your input to 1 million volts, and you have a gain of 100, your output will be 100 million volts.
    If you want to see distortion, you have to run a single-frequency transient simulation on a sinewave input. I believe most versions of Spice have the capability to do an FFT on the output, showing output voltage at the fundamental frequency and all harmonics.
     
  7. jzou

    Thread Starter New Member

    Jul 8, 2008
    3
    0
    ok, never mind what I have just posted.

    the problem with the lack of a sin graph is two things: one, the AC source needs to be a Vsin source to allow me to set the frequency and amplitude. Two, the time it takes to run the transient sweep needs to be increased from the 50000ns default value to 2000 micro seconds to get a good sin wave.

    having dealt with that, how do I get pspice to show me the data pts values and show me the frequency response?

    Thanks

    Davy
     
    Last edited: Jul 18, 2008
  8. Ron H

    AAC Fanatic!

    Apr 14, 2005
    7,050
    657
    Davy, you can't easily get frequency response from a transient sim, except in a roundabout way by looking at pulse risetimes and pulse droop, and it will only be approximate. You can't see distortion in an AC simuation. You have to do both to get all that information.
     
  9. Audioguru

    New Member

    Dec 20, 2007
    9,411
    896
    Can you remove the grid lines from your schematic so we can see the wiring?
     
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