Miller OTA Design - questions about Miller methodology

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dtzounakos

Joined: 16 Mar 2007
Posts: 36

05 Nov 2009 10:14 Miller OTA Design

I uploaded af paper about Miller OTA based Gm/IDS Methdology!It's short and simple.

I understood everything except the concept of finding the Early Voltage VA3,VA4

through equation 1! From design specifications we know the overall gain Av(dB)> 80,

not Av1. How do we know and calculate Av1 dc gain? I would appreciate if someone

could spend a few minutes and answer my question!

Thanks!!

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erikl

Joined: 09 Sep 2008
Posts: 735
Helped: 138
Location: Germany

06 Nov 2009 14:05 Re: Miller OTA Design

dtzounakos wrote:

... I understood everything except the concept of finding the Early Voltage VA3,VA4 through equation 1!
From design specifications we know the overall gain Av(dB)> 80, not Av1.
How do we know and calculate Av1 dc gain?

I think the statement

"considering the DC gain spec, L1 and VA1, the current mirror VA3 can be obtained through equation 1 → VA3 = VA4 = 10.2;"

is wrong: Like VA1 = VA2 (s. item above the a.m. statement), VA3 = VA4 = 10.2 is obtained from the gm/Id vs. VA curves (with L as parameter, s. item below the a.m. statement), not through equation 1 . Then, with all values introduced into this equation, Av1 can be calculated (Av1 ≈ 100 = 40dB), then similarily Av2 .

It's not the only error in this paper: M1-M2 is a PMOS differential pair, not an NMOS (p. 3), and L(M8 ) = 1 and not 1.5µm (p. 4).

HTH! erikl

dtzounakos

Joined: 16 Mar 2007
Posts: 36

06 Nov 2009 15:33 Re: Miller OTA Design

Thanks for interesting to this paper.
But i still can't uderstand how calculates VA3,VA4 without prior knowing the right

value of L3=L4! At first statement, we choose L1=L2=3um and we find value

VA1 and VA2 through plot VA=f(Gm/IDS)! At second statement, we don't know L3,L4

and VA3,VA4.In order to estimate one value of these variables, we must prior know

the other value!

Unless if we prior choose (arbitrary) Av1=40dB!

erikl

Joined: 09 Sep 2008
Posts: 735
Helped: 138
Location: Germany

06 Nov 2009 17:19 Re: Miller OTA Design

dtzounakos wrote:

... i still can't uderstand how calculates VA3,VA4 ...

No, it's not calculated, it's obtained from the gm/Id vs. VA curves (with L as parameter), s. my previous posting.

dtzounakos wrote:

... without prior knowing the right value of L3=L4!

gm/Id is already known. In the gm/Id vs. VA family of curves (L is the family parameter) a parallel line to the x-axis is drawn with the known ordinate value gm/Id. It meets the array of curves at several points which mean L-VA pairs. Now you can choose a convenient L-VA combination, e.g. 0.6µm - 10.2V for M3-M4 in this case, which will achieve a reasonable Av1 .

dtzounakos wrote:

At first statement, we choose L1=L2=3um and we find value VA1 and VA2 through plot VA=f(Gm/IDS)!

Yes; same method as mentioned above. The L-value is governed by considering the required GBW, as well as by noise & matching (means: offset) requirements. The chosen relatively high L=3µm value achieves an appropriately high VA1=VA2 value.

dtzounakos wrote:

At second statement, we don't know L3,L4 and VA3,VA4.
In order to estimate one value of these variables, we must prior know the other value!

That's true, but you can always choose between several L-VA pairs, s. above. Higher L-VA pair values result in larger gain & GBW, however need more real estate area.

dtzounakos wrote:

Unless if we prior choose (arbitrary) Av1=40dB!

That's another possibility. In this case, the VA value can be calculated, and the point of intersection between its value and the known Gm/IDS value on the VA=f(Gm/IDS) array of curves gives the L value.