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cktdesigner Community Member Offline Posts: 38 India	how to bias constant gm rail-to-rail OTA Mar 14^th, 2012, 3:47am Hi, Can someone please explain how to choose Vb1 and Vb2 in the constant-gm rail-to-rail OTA in the attached schematic?
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loose-electron Senior Fellow Offline Best Design Tool = Capable Designers Posts: 1638 San Diego California	Re: how to bias constant gm rail-to-rail OTA Reply #1 - Mar 14^th, 2012, 4:00pm Interesting architecture - you got a source for that?
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HdrChopper

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Re: how to bias constant gm rail-to-rail OTA
Reply #2 - Mar 15^th, 2012, 10:17am

hi,

Set Vb1 to Vonp and Vb2 to Vonn, where Vonp �and Vonn are the input CM levels at which PMOS and NMOS diff pairs turn on respectively.
If you do so then,

for vicm < vonn , IM1,2 = 0. thus IM5 = 2IR2 and IM8=0 (since M3,4 sources turn M8 off)
for vicm > vonp, �IM3,4 = 0, thus IM8 = 2IR1 and IM5=0 (since M1,2 sources turn M5 off)

for vonn < vicm < vonp, both IM8 and IM5 are OFF; otherwise, since the current loop is positive, adaptive bias circuit would not be controlled. In this case IM1,2 = 2IR2 and IM3,4 = 2IR1

Hope this helps
Tosei

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RobG

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Re: how to bias constant gm rail-to-rail OTA
Reply #3 - Mar 15^th, 2012, 10:20am

You have a lot of flexibility -- you could probably connect both to mid rail so both diff pairs would only operate together at the mid-point. Whatever you do, the idea is to choose Vb1 so that M7 is still saturated when M8 is diverting all the current (i.e. Vin+ at top rail). Same thing with Vb2. You might, for example, create Vb2 with a diode connected mosfet so that M5 acts as a low-voltage cascode when it diverts the current.

[edit, Tosei posted while I was typing... ]
rg

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HdrChopper Community Fellow Offline Posts: 493	Re: how to bias constant gm rail-to-rail OTA Reply #4 - Mar 15^th, 2012, 10:25am Rob, I think if you choose mid rail for Vb1 and Vb2 then M8 and M5 will both be on and there is a +ve loop gain in your adaptive biasing circuit with those two guys on. I think you are losing control of such loop if both are on Tosei
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RobG

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Re: how to bias constant gm rail-to-rail OTA
Reply #5 - Mar 15^th, 2012, 12:16pm

HdrChopper wrote on Mar 15^th, 2012, 10:25am:

Rob,

I think if you choose mid rail for Vb1 and Vb2 then M8 and M5 will both be on and there is a +ve loop gain in your adaptive biasing circuit with those two guys on. I think you are losing control of such loop if both are on

You may very well be correct as I haven't tried it. I don't think there's a problem with them both being on. It looks to me like there will be a steady state operating point where the "catch" devices have the same current density as the diff-pair devices. For example, if the gates of M5, M8 and M1-M4 are all tied to midrail, then Vgs5+Vsg8 = Vgs1+Vsg3 so all of the devices will be biased at 2*I_R2/3. -- I guess that would make the gm not-quite constant if both M5 and M8 are on, but other than that I think the operating point is defined.

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HdrChopper

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Re: how to bias constant gm rail-to-rail OTA
Reply #6 - Mar 15^th, 2012, 12:54pm

Rob,

I think you are correct. I've seen this one with ratios higher than 1:1 in the adaptive biasing loop, in which case then I think I would be correct. But that ration being 1:1 I guess is safe and would operate as you pointed out.
Not sure how mismatches would play a role here if you bias everything at mid rail though. I'm think what if, instead of the ideal 1:1 ratio, you get 1:1.01...slowing drifting to rails?

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