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I need explanation of circuit gains I'm seeing

Discussion in 'General Electronics Discussion' started by Trent, Jun 30, 2011.

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  1. Trent


    Apr 17, 2010
    I've been designing a simple dual FET Wheatstone bridge using the website "" which allows one to draw a circuit very easily and it shows how it runs and does all the calculations. It's very simple to use.

    I've attached a pic of a typical circuit.

    The transistors used in the simulation are supposed to be MPF102 n-JFETs, with Vp= -8V, and a 20mA limit.

    By playing with bias resistor values, I've gotten some unusual results.

    The attached graph and table of my data shows a range of values for FET gain, gm, and the signal amplitudes I got for each circuit I tried.

    In the very high signal amplitude circuits, the resistor settings are extremely touchy. Small changes can send the amplitude from the mV range down to the uV range.

    My questions are:
    1. These high signal amplitudes don't seem possible, are they a fluke of the calculations?
    2. Why should circuits with low FET gain, gm, have high signal amplitudes and vice-versa?
    3. Why don't the circuits with the highest gm have the highest signal amplitudes?
    4. Looking at the chart, which values should I strive for? High gm but lower signal amplitude (assuming these amplitudes are actually possible)? Or, low amplitude, with high gm value?
    Or, should I look at some combination of the other FET parameters?


    Attached Files:

  2. poor mystic

    poor mystic

    Apr 8, 2011
    I wonder whether it behaves the same way in the real world? Or you could try it in SPICE.
  3. Trent


    Apr 17, 2010
    The only spice program I have is "5Spice" and I haven't learned how to use it fully yet. The website mentioned is easier to use.
    I haven't tried this particular circuit in real life. I was hoping the simulation would help me tune it, but now I don't know which parameters to tune it to.
    I don't have a signal generator so I can't measure my input vs output signals in real life.
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