TL:DR How do I know how to size transistors?
This is the same problem as this one.
Now knowing that I can change L, I know that my W/L ratio to make Von = 200 mV is roughly 6.75. Also knowing that M5 is supposed to be operating in triode, here's my thought process:
Von > 200 mV; this is set by W/L = 6.75 (math done elsewhere).
Vds (for devices operating in saturation) > 2Von (= 400 mV)
Vd5 = 400 mV
Which means that Vd5 = 800 mV
Which means that Vg5 = 800 mV
Therefore Vth <= (less than or equal to) 600 mV.
Assuming that Vth are relatively similar:
Vgs6 = Vg6 - Vs6 = Von + Vth
Vg6 = Von + Vth + Vs6
Vg6 = 200 mV + 600 mV + 400 mV = 1.2v = Vg8 = Vd8 = Vg7.
The maximum that Vs8 can be is 800 mV for M8 to meet the Vds spec. However, since Vgs7 = 1.2 V and Vth = 0.6 V, then:
Vds7 < Vgs7 - Vth = 600 mV.
And that's where my train of thought stops. The only pertinent equations for MOSFETs that I can think of involve W/L, not one or the other, so I don't know how to set the sizes of these transistors. I was told that M5, M6, and M8 should all be the same size, and the Gray et. al. book Analysis and design of Analog integrated circuits says that W/L7 should be 1/3 of the other W/L's, but I don't know if that's meant for unity gain current mirrors or all current mirrors in this configuration.
If I don't want to keep that ratio that I calculated earlier...then I'm really lost and don't know where to begin.
I'm trying to figure out how to find out as much as possible from hand calculations before I go to simulation. I've got a lot of different knobs to turn: W7, L7, M7, W8 = W5 = W6, L8 = L5 = L6, M8 = M5 = M6 (multiplier factor, not transistors themselves). The smallest number of knobs I can turn (if M8, M5, and M6 are identical and I keep the initial calculated ratio AND if the ratio of M7 is supposed to be 1/3 of the others) is 4; that number quickly gets out of hand if I started changing them relative to each other.
What's the next step? How do I size these transistors?
Edit: Walking through simulations. Will be updated periodically
2018 Feb 18
L1,3,5,6 = 0.35 µm
L2&4 = 0.7 µm
W1&3 = 2.3625 µm
W2&4 = 4.725 µm
W5 = 0.35 µm
W6 = 0.76 µm
M1-2 = 4
M3-6 = 1
V2 = 795.3 mV sets the output current equal to 100 µA, and the bias voltages are as follows:
-------------------------M1 ---- M2 ----- M3 ---- M4 ---- M5 --- M6
Vgs (mV) ---------- 798.3 --- 782.7 --- 798 --- 783 --- 1182 -- 917
Vth (mV) ----------- 571 ----- 558 ---- 571 --- 558 --- 385 --- 480
Von(mV) ----------- 227.3 --- 227.8 --- 227 --- 225 --- 797 --- 488.5
Vds (mV) ----------- 399.3 --- 396 ----- 399 --- 399 --- 266 --- 917
Vds % Difference - -0.17% - -1.01% - -0.25% - -0.25% -- N/A -- N/A
Ro = 2 M-Ohm (Can't get the Greek letter to display).
Still, no matter what I do, Vd4 is constant at less than 800 mV. What sets this level? Is it M5?
