Sedra Smith, Ch6, ex 6.5

Thread Starter


Joined Jun 27, 2010

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 :(




Joined Mar 8, 2010
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.