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Find the MOSFET bias point

Daniel San

Dec 27, 2013
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Dec 27, 2013
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  1. The problem

    Given the following circuit find the bias point (VDS,IDS).

    30b2pad.png


  2. The attempt at a solution:

    The first thing I did was find the value of VG, the voltage from the gate of the mosfet to the -VDD source.

    VG =2 VDD * R2/(R1+R2)

    To find the current IDS I needed the voltage from the gate to the source, VGS, which is:

    VGS = VG - RsID

    Since we're operating in the saturation region I can replace ID by:
    VGS = VG - Rsk(VGS-VT)^2 (1+λVDS)

    where VDS is the voltage between the drain and the source.
    I also know that:
    VDS = 2VDD - (Rd+Rs)ID

    This is as far as I can get. I'm a little confused regarding what I should do next.

    If anyone could help I'd appreciate.

    Thanks.
    Daniel.


 
Last edited:

(*steve*)

¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
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Jan 21, 2010
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Express both equations in the form Vds = ...

Then solve them for Id

Once you know Id, everything else will fall out.
 

Laplace

Apr 4, 2010
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I tried solving for Id algebraically by plugging your source equations into MathCad and ended up with an out-of-memory error on a 64-bit virtual machine that had been allocated 8 GB of RAM. Then tried solving numerically and got a real solution for Id near 0.5 mA, plus two imaginary number solutions. So I would recommend setting this up in a spreadsheet form then try to get the solution to converge somewhere near 0.5 mA.
 
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