# Nodal Analysis with a FET

Discussion in 'The Projects Forum' started by Engin_Nerd, May 1, 2011.

1. ### Engin_Nerd Thread Starter New Member

Oct 22, 2010
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I'm trying to do nodal analysis of a DC circuit that uses a mosfet and want to compare my results to what I would get using PSPICE....how do I do this?

If i was to remove the FET from the schematic what would I use to replace it for the circuit to behave the same (for the analysis). BTW the fet i'm using is a 2n7002.

2. ### Jony130 AAC Fanatic!

Feb 17, 2009
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But where is the diagram?

3. ### Engin_Nerd Thread Starter New Member

Oct 22, 2010
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Heres the diagram in a jpg, hope thats ok.

Also, this might be a seperate topic, but I'm not sure how I would calculate the rise time and the fall time at the load??

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4. ### hgmjr Moderator

Jan 28, 2005
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Is this a homework problem?

hgmjr

5. ### Engin_Nerd Thread Starter New Member

Oct 22, 2010
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project i want to do to learn about fets. did not really cover them in my electronics class.

6. ### Jony130 AAC Fanatic!

Feb 17, 2009
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For your MOSFET circuit when the Micro 5V signal is in high state (5V).
You can treat MOSFET as a short circuit.
As long as the drain current does not exceed 75mA you can treat your MOSFET as switch with Ron = 3.5Ω.

And when the Micro 5V signal is in low state then you can remove the MOSFET form your circuit.

Last edited: May 3, 2011
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7. ### Engin_Nerd Thread Starter New Member

Oct 22, 2010
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Thanks for the

When you treat it like a switch, do you need to consider the 5v from the micro? that was confusing me...

Also when I want to know the rise time and fall time do I use this equation:
V(t) = Vo*(1-(e^(t/T))

When I used this equation i got a rise time in nano-seconds, but pspice gave me a rise time in micro-seconds..i'm a bit lost about how to do this.

8. ### Jony130 AAC Fanatic!

Feb 17, 2009
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Yes, you can ignore the 5v from the micro.
Yes
Hmm, the rise time is defined as
tr 2.2RC
So for your circuit
tr 2.2* 770Ω*26nF ≈ 45μs for load
And Tr 2.2 *740Ω*13nF ≈ 22μs for ADC input.

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