# Finding exit resistance of a MOSFET [closed]

Two MOSFET-s $M_1,M_2$ are in saturation. Canal lengths: $L_1=2L_2$, Drain currents: $I_{d1}=I_{d2}$, $V_{ds1}=V_{ds2}$ and $\Delta L$ is same for both of these MOSFET-s.

So, I don't know what exit resistance is. Is it $r_{ds} = v_{ds}/i_d$ ?

## closed as off-topic by Bimpelrekkie, placeholder, Daniel Grillo, stefandz, duskwuffMay 27 '16 at 8:18

• This question does not appear to be about electronics design within the scope defined in the help center.
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• I'm voting to close this question as off-topic because this question looks like homework or assignment with no shown effort to find a solution. – Bimpelrekkie May 24 '16 at 18:42
• @FakeMoustache while it clearly originates with homework, the question being asked seems more to be about one step along the way to figuring out how to solve it, which is allowed, versus asking for the solution which is not (in fact, the nature of the ultimate answer required by the assignment doesn't even appear to be part of the posted question at all). However, the poster could definitely do a better job of explaining the specific question they are asking here. – Chris Stratton May 24 '16 at 18:47

With the provided information it is not possible to calculate the output resistance $r_{ds}$.
However, since $$r_{ds} \approx \frac{1}{\lambda I_d}$$ and $$\lambda = \frac{1}{V_E} \frac{\Delta L} L$$
it is possible to determine that $$\frac{r_{ds1}}{r_{ds2}} = \frac{\lambda_2}{\lambda_1} = \frac {L_1}{L_2} = 2$$