# Problem with JFET/MOSFET

Discussion in 'Homework Help' started by PsySc0rpi0n, Jul 2, 2015.

1. ### PsySc0rpi0n Thread Starter Well-Known Member

Mar 4, 2014
1,184
3
Hi...

I have started a new problem to try to solve.

The circuit is attached and we are supposed to calculate Q-point for both transistors.

So the first thing I did was to use Thevenin theorem and calculate Vth and Rth for the Voltage divider at the JFET gate.

I found that Vth = 2V and Rth = 90kΩ

Then I've used the device Id equation and the gate net equation to find Id and Vgs.

Assuming that the JFET is working at it's active zone

Id = Idss(1-(Vgs/Vpoff))^2

and

Vth + Rth*Ig + Vgs + Rs*Id = 0

From the FET's theoretcial characteristics we are know Ig to be 0A, so:
Rth*Ig = 0V

So I got 2 solutions from the quadratic Id equation:

Vgs = -9.26V and Id = 7.26mA

and

Vgs = -3.24V and Id = 1.24mA

Now, I'm supposed to find out Vds to confirm or not the initial assumption. It's here that I have a question.
Because there is no voltage drop at the MOSFET (transistor at the right side), can I simply write the JFET output net equation as:

20 = Vds + Rs*Id ????

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2. ### PsySc0rpi0n Thread Starter Well-Known Member

Mar 4, 2014
1,184
3
Ok, I think I got the final result even without any help from here!