# Vgs on Shockley Curve/ Vgs calculation?

Discussion in 'Homework Help' started by no12thward, Feb 13, 2011.

1. ### no12thward Thread Starter Member

Jan 10, 2011
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0
For this transistor circuit, I have drawn in my points and estimated Vgs on the Shockley Curve; But when calculating Vgs mathematically (Vgs= Vg-Vs) to check, I get 0V-5.5V= -5.5V (Vg always=0V). That is terribly off from my -1.54V Vgs value estimated at the bottom of my Shockley curve. I though the Vgs values from curve and calculation are supposed to be slightly the same? Any help?

Thanks guys!

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2. ### no12thward Thread Starter Member

Jan 10, 2011
30
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also, if Vds= Vd-Vs, and is to always be positive, why am i getting 3.5v-5.5v= -2?

3. ### t_n_k AAC Fanatic!

Mar 6, 2009
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I have Id=2.74mA and Vgs=-2.48V

4. ### no12thward Thread Starter Member

Jan 10, 2011
30
0
i guess im just not seeing how you derive that

5. ### t_n_k AAC Fanatic!

Mar 6, 2009
5,448
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$I_D=I_{DSS}(1-\frac{V_{GS}}{V_P})^2$

Must satisfy

$V_{GS}=3-2000I_D$

Where ID is in ampere.

I substituted the second relationship into the first and solved for ID, given IDSS and VP.

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