# Sedra Smith, Ch6, ex 6.5

#### yitch

Joined Jun 27, 2010
8
Hi,

I'm lost as to how to begin, The VGS is not given, how would one be able to solve for the W?

And I subbed in the value of W1 into the current steering equations, I2 and I3 are a magnitude of .1 from the answer and I5 is totally off the mark.

I have attached the images, pls help

Thanks!

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#### Ghar

Joined Mar 8, 2010
655
You find Vgs from the requirement of swing.

For saturation, which is the requirement for a MOSFET acting as a current source:
Vds > Vgs - Vth

You want drain of Q2 to go within 0.2V of -Vss, meaning you want the transistor to stay in saturation even with a Vds of 0.2V.

This gives:

Vgs - Vth < 0.2V
Vgs < 0.8V

Plug that into the current equation:

$$I_D = \frac{1}{2}k_n'\frac{W}{L}(V_{gs} - V{tn})^2\\ 10\mu = \frac{1}{2}200\mu\cdot\frac{W}{1\mu}(0.8 - 0.6)^2\\ 0.1 \mu= W 0.04\\ W = 2.5\mu$$

Similarly you do that for the PMOS to get W4.