How can I calculate the W/L of these transistors?

[wjxcom]

Please look at this OPA. The specifications is:

Gain=5000
Gain bandwidth=10MHz when a 10pF load
Slew rate=10V/us when a 50pF load
ICMR=1.5-4V
Output swing=1-4V
Vdd=5V

From these parameters, I can calculate the W/L of these transistors: M12, M13, M14, M15, M7, M8, M11, M21, M18. But I do not know how to calculate the W/L of these transistors: M1, M2, M3, M4, M5, M6, M9, M10. I think there has no enough parameter to calculate the W/L of these transistors.

How can I calculate the W/L of these transistors? Help me!!

 

[wjxcom]

Hi, all: there has no people reply this topic. Maybe nobody know how to calculate?

 

[jimjim2k]

Hi

I have a general questions

In Analog:
What are the thumby rules on Transistor sizing?


In Digital:
What are the general rules on Transistor sizing?


tnx

 

[rockycheng]

wjxcom,

You've already had all the transistor sizes on the figure, hadn't you? You said you know how to calculate the size of M7. I think it's also easy to find M6, M3, M2, since they all have the same drain current, and you can assume they have the same overdrive too.

There are several places in this circuit that could be improved.
1) Decrease W18, to avoid large negative offset voltage at the output, and to keep M1, M4 in active region.
2)In this circuit, the current through cascode transistors seems three times larger than that across the input transistors. This makes the gain lower, with the same power consumption.

 

[zenisle]

w/l is a very important ratio value

 

[wjxcom]

Hi, rockycheng: in fact, I think the W/L of M7 is not correct either and I do not konw how to calculate the w/l of M7.
In addition, would you explain:
1.why decrease W18 can avoid large negative offset voltage at the output
2.why keep M1, M4 in active region?
3.why the current through cascode transistors seems three times larger than that across the input transistors will make the gain lower?

 

[rockycheng]

Could you tell me which sizes in your circuit are known, and which are unknown? And how you get these values?

As to your questiones:
1) Within the reasonable range of overdrives, the bias current currents of M21 and M18 could not be equal. And this may cause output offset.
2) In analog circuit, most of the transistors should work in the active region. If Vd4 is too low, M1and M4 might go into triode region. To make a correct bias condition is a must for a successful function.
3) You can see that Id9=1.5Id14, since Vov9 = Vov14, and W9/L9=1.5*W14/L14. And you have Id7=Id11=Id9, Id14=2Id12, so it's obvious that Id7=3Id12. When you design a folded-cascode opamp, it's usually a good idea to make the current through the input transistors larger, because the gm is larger.