I am trying to solve a Sedra/Smith problem involving NMOS transistors. Given that both transistors are biased at the same point. Question is to find \$g_{m2}, i_d, v_{d1}\$ and the value of \$R_D\$ for which \$v_o\$ is pulses of amplitude 1V.

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However I can't manage to find the value of the voltage pulses at the drain of \$Q_1\$ I actually found the solution manual for the corresponding text book, however I think that the solution is not well implemented as it suggest that the solution would be determined by: $$V_{D_1}=i_{D_1}50\Omega$$ which have no sense.


We have to find out \$g_{m2}, i_d, v_{d1}\$ and \$R_D\$.

Since \$R_{i2} = 1/g_{m2} = 50\Omega,\$ \$g_{m2} = 0.02\mho\$.

Since both transistors are biased at the same point, \$g_{m1} = g_{m2}\$.
$$i_d = g_{m1}v_i = 0.1 mA$$ now, $$v_{d1} = i_d\times 1/g_{m1}=i_d\times 50\Omega$$

Now the equation in solution manual makes sense. I think you can do the rest your own.

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  • \$\begingroup\$ I was actually reading about it, but, one more thing, it says that it is 0.5V (in the solutions manual), however I obtain 5mV, which one is correct?. \$\endgroup\$ – Hans May 19 '14 at 6:37
  • \$\begingroup\$ Is it? I am also getting 5mV. What is the value of \$R_D\$ given in the manual? Is it \$1k\Omega\$? \$\endgroup\$ – nidhin May 19 '14 at 6:41
  • \$\begingroup\$ Well, assuming we are getting 1V at \$V_O\$ I obtain \$10\text{-}k\Omega\$ (the manual as well). I use the T-Model to explain that, I guess is the best way to do it. \$\endgroup\$ – Hans May 19 '14 at 6:51
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    \$\begingroup\$ the same \$i_d\$ flows through both the transistors and hence \$v_o = 0.1mA\times R_D = 1V\$ or \$R_D = 10k\Omega\$. \$\endgroup\$ – nidhin May 19 '14 at 7:04

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