incorrect DC analysis of JFET at university??

[gcgrant]

Can somebody please check the sums below, I have a feeling our lecturer has made a mistake and we have an assignment due with similar questions this weekend need to know that I am doing it right. Here's the question I posted to the university forum:


Cheers,
G.

[gcgrant]

Here is a better screenshot of the lecture slide:

[Andy Watson]

Assuming you want maximum signal amplitude without clipping, the Q-point is not half of the supply voltage (I make it to be about 5.62V).

[LvW]

The shown circuit must fulfill two conditions at the same time: (a) the FET condition (shown quadratic relation): ID=f(VGS) and (b) the ohmic rule for the source resistor: VS=ID*RS.
From both relations the corresponding value for the source resistor can be computed. Hint: Always start with the non-linear equation.

[gcgrant]

Thanks, so I am right that based on the shown resistor values and the V_DD value given, the example is incorrect in the way the V_GS and I_D have been calculated?

[LvW]

Thanks, so I am right that based on the shown resistor values and the V_DD value given, the example is incorrect in the way the V_GS and I_D have been calculated?

Yes - I think so.
When (a) the FET characteristic is completely specified (as in your case) and (b) the value of Rs is fixed, you have not the freedom to select the drain current.
That means: If the current is required to be Idss/2 there is only one solution for the resistor Rs allowing this current.
The quadratic FET equation gives you the correspondig VGS value - and Ohms law gives you the Rs value.

[gcgrant]

Excellent,  thanks,  that's exactly how I see it.  Now I'm concerned if I do it the right way I'll be marked wrong 😯😧

[tec5c]

If I were you, I would do it the right way (why practice doing things incorrectly? It's nonsense...)

However, I think you should speak with your lecturer about this and see what he/she has to say.

If you don't mind me asking, which Uni are you at?

[gcgrant]

I don't think it's right to name publicly, plus they'll most likely throw some policy at me. But yes I will be definitely doing it right. Have been having quite a few mistakes from the lecturer in this unit...

[tec5c]

plus they'll most likely throw some policy at me.

Interesting...