# Help with NCEES #129 (Electrical and Electronics)?



## Angstrom (Apr 10, 2010)

This problem is very frustrating to me, because it should be a simple, systematic problem and I want to understand it. When I set up the KCL at the drain, it is very similar to, but not quite the same as, the solution provided in the NCEES book.

Here is what I get for my KCL equation:

0 = (Vout-Vin)/Rg + gmVin + (Vout-15)/Rd + Vout/Rl

And here is what is provided in the NCEES book:

0 = (Vout-Vin)/Rg + gmVin + Vout/Rd + Vout/Rl

So you see, the _only_ difference between my KCL equation and the book's is that my equation includes the 15 V in the (Vout-15)/Rd term. The 15 V seems to disappear in the NCEES book. The NCEES solution doesn't explain how this equation was derived; it just presents it.

What am I missing? I'm sure it's some very basic, important assumption that I'm forgetting, but I have looked at this problem for a long time and I can't understand why the 15 V disappears. This is so frustrating, because I am _so_ close. Somebody please, please help me, for my sanity's sake.

Thank you in advance.


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## benbo (Apr 10, 2010)

Angstrom said:


> This problem is very frustrating to me, because it should be a simple, systematic problem and I want to understand it. When I set up the KCL at the drain, it is very similar to, but not quite the same as, the solution provided in the NCEES book.
> Here is what I get for my KCL equation:
> 
> 0 = (Vout-Vin)/Rg + gmVin + (Vout-15)/Rd + Vout/Rl
> ...


I don't have the book anymore, apparently. Can you state the question and describe the circuit. I imagine it's some sort of FET. Maybe a small signal model. Is this an AC input? The 15V sounds like a supply voltage, sometimes they consider those as ground to an AC signal, but I'm not sure that's it. Even if I can't figure it out, at least this will bump it up.


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## rshankle PE (Apr 11, 2010)

benbo said:


> Angstrom said:
> 
> 
> > This problem is very frustrating to me, because it should be a simple, systematic problem and I want to understand it. When I set up the KCL at the drain, it is very similar to, but not quite the same as, the solution provided in the NCEES book.
> ...


Hi,

I need to look at the problem when I get home, but I believe it is part of small signal analysis. Basically the 15v is now shorted, so Rd and Rl are in parallel as seen from Vout.

I'll try to check later this evening and post if I think otherwise.

GL


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## jassiinpublic (Apr 11, 2010)

Do you have a copy of Sedra &amp; Smith? Pg 293 has the exact same configuration with the equivalent model (expands on the hint given in the problem). I didn't have time to read your entire post, I'll take a look tomorrow. Gotta jet.



Sparrow said:


> benbo said:
> 
> 
> > Angstrom said:
> ...


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## Angstrom (Apr 11, 2010)

Thank you very much, guys. That absolutely is what I was forgetting. It is small-signal analysis, so the 15 V source needs to be shorted. There were also two capacitors, but I remembered to short those. It makes perfect sense, now. Ugh, I should have remembered that; I haven't worked these types of problems for so long until studying for this exam. I think I understand it pretty well, now, so I should be prepared for any of these kinds of problems that appear on the test.

Thank you again for your help.


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## rshankle PE (Apr 11, 2010)

Angstrom said:


> Thank you very much, guys. That absolutely is what I was forgetting. It is small-signal analysis, so the 15 V source needs to be shorted. There were also two capacitors, but I remembered to short those. It makes perfect sense, now. Ugh, I should have remembered that; I haven't worked these types of problems for so long until studying for this exam. I think I understand it pretty well, now, so I should be prepared for any of these kinds of problems that appear on the test.
> Thank you again for your help.


No problem, I needed the reminder also.


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