I'm working on designing a cascode current mirror, depicted below:
Constraints: all Von needs to be above 200 mV, all Vds neds to be at least double Von, L is 0.35 um.
Derived from the SPICE file, k' = 181.47 uA/V^2, and the given bias current is 25 uA. Rearranging the equation for Von to solve for W gives:
$$
W=(2I_{D}L)/(k'(V_{ON})^2)
$$
I input my given values and find a value for W of 2.41 um. To make them have a slightly happier ratio, I bump that down to 2.3625 um, which makes W/L 6.75.
I then make all of my Ws that value, run a bias simulation, and investigate; this is what I find. All voltages are in mV (I apologize, there seems to be no way to make a table):
MOS – Vgs – Vth – Von – Vds – Region
M1 — 776 – 585 – 191 – 3000 – Saturation
M2 — 776 – 558 – 218 – 776 – Saturation
M3 — 905 – 585 – 320 – 152 – Triode
M4 — 905 – 585 – 320 – 2850 – Saturation
M5 — 776 – 582 – 194 – 102 – Triode
M6 — 803 – 587 – 216 – 803 – Saturation
M7 — 905 – 582 – 323 – 102 – Triode
M8 — 803 – 587 – 216 – 803 – Saturation
Ignoring for a moment M1 and M4, why are M3, M5, and M7 in triode? I've tried changing the W up and down, and I can't get any of those Vds values to go above 200, let alone 400 mV. How do I set my Vds higher? What's wrong with the Von of M3, M4, and M7?
Best Answer
you don't have a bias, therefore an incomplete current mirror, so electrically it will act as a class A stage with a ccs
Stick a resistor in where Ibias would be so you can bias the bottom fets
Here is another example:
I recommend you look at this guy's page, since you are learning the circuit: http://cmosedu.com/jbaker/courses/ee420L/s17/students/garrod/Lab%209/Lab%209.htm