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neoflash Community Fellow Offline Mixed-Signal Designer Posts: 397	Design of Rail-Rail OTA in 45nm Process Apr 02^nd, 2008, 6:39am It finally comes. Process scaling is so aggressive that designing a simple op-amp becomes so hard. In 45nm process, Vth of transistor is as high as 450mV-500mV while the voltage supply is as low as 0.95v. Any good idea to design a rail-rail OTA in this kind of process?
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Berti

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Re: Design of Rail-Rail OTA in 45nm Process
Reply #1 - Apr 2^nd, 2008, 11:36pm

Hi Neoflash,

I also encountered the problem of high vt at low supply. I think multi-stage (e.g. nested-miller) amplifiers
are a very good option for continuous-time circuits. Of course, the design is more complex than for a single-stage
OTA, but you can achieve high gain and bandwidth without the need for stacking more the 3 devices (take a look at the
paper from G.Mitteregger from ISSCC 2006).
For discrete-time (switched-capacitor) I haven't found a good solution, yet.

I am just interested: Does the technology your are using also offer low-vt or even analog process options (which are not available for you)?

Regards

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neoflash Community Fellow Offline Mixed-Signal Designer Posts: 397	Re: Design of Rail-Rail OTA in 45nm Process Reply #2 - Apr 3^rd, 2008, 6:09am Actually there is low-vt device, however, the threshold voltage is just 20-30mv lower than normal Vt device.
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ACWWong Community Fellow Offline Posts: 539 Oxford, UK	Re: Design of Rail-Rail OTA in 45nm Process Reply #3 - Apr 4^th, 2008, 4:42am You could also try to exploit RSCE and back gate biasing to lower threshold voltage.
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neoflash Community Fellow Offline Mixed-Signal Designer Posts: 397	Re: Design of Rail-Rail OTA in 45nm Process Reply #4 - Apr 4^th, 2008, 7:04pm ACWWong wrote on Apr 4^th, 2008, 4:42am: You could also try to exploit RSCE and back gate biasing to lower threshold voltage. Hi, Wong: Thanks for the head up. I was always trying not use that technique because they look risky. Is this inevitable choice?
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loose-electron

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Re: Design of Rail-Rail OTA in 45nm Process
Reply #5 - Apr 10^th, 2008, 12:44pm

Typical threshold setting is 20% to 40% of the Vpower.

Thats a "got to have" to get generic CMOS logic to plug and play.
What you are stuck with is closer to 50% which makes life difficult at best.

Can you request a secondary power supply at a higher voltage? (without violating Vgs restrictions?)
Can you create your own power supply by charge pumping?

There are ways of doing circuits with only three elements stacked power to rail, but things like cascodes and similar are not going to be in your tool bag.

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ACWWong

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Re: Design of Rail-Rail OTA in 45nm Process
Reply #6 - Apr 12^th, 2008, 12:58pm

neoflash wrote on Apr 4^th, 2008, 7:04pm:

ACWWong wrote on Apr 4^th, 2008, 4:42am:

You could also try to exploit RSCE and back gate biasing to lower threshold voltage.

Hi, Wong:

Thanks for the head up. I was always trying not use that technique because they look risky.

Is this inevitable choice?

I don't see it as risky, if the device models in your pdk are good.

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kiran123

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Re: Design of Rail-Rail OTA in 45nm Process
Reply #7 - Apr 14^th, 2008, 9:02pm

ACWWong wrote on Apr 12^th, 2008, 12:58pm:

neoflash wrote on Apr 4^th, 2008, 7:04pm:

ACWWong wrote on Apr 4^th, 2008, 4:42am:

You could also try to exploit RSCE and back gate biasing to lower threshold voltage.

Hi, Wong:

Thanks for the head up. I was always trying not use that technique because they look risky.

Is this inevitable choice? �

I don't see it as risky, if the device models in your pdk are good.

Hi Experts,
� � Can anyone provide me Input Rail-to Rail opamap design steps for the figure attached herewith along with proper o/p summing ckt �( is from Ieee paper [i]"A Compact Power Efficient 3V CMOS Rail-to- Rail Input/Output Operational Amplifiers for VLSI Cell Libraries" by Ron Hogervorst , J.P.Tero, J.H.Hijsing [/i] )
� or �any suggested papers or documents to help me in this regard

for designing steps even i referred books written by authors (opamp theory and design, low Voltage analog desin considerations ...etc) i have request to work on 0.18u technology

Regards
Kiran

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Rail-Rail_Opamp.jpg

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neoflash

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Re: Design of Rail-Rail OTA in 45nm Process
Reply #8 - Apr 15^th, 2008, 5:14am

kiran123 wrote on Apr 14^th, 2008, 9:02pm:

ACWWong wrote on Apr 12^th, 2008, 12:58pm:

neoflash wrote on Apr 4^th, 2008, 7:04pm:

ACWWong wrote on Apr 4^th, 2008, 4:42am:

You could also try to exploit RSCE and back gate biasing to lower threshold voltage.

Hi, Wong:

Thanks for the head up. I was always trying not use that technique because they look risky.

Is this inevitable choice?

I don't see it as risky, if the device models in your pdk are good.

Hi Experts,
Can anyone provide me Input Rail-to Rail opamap design steps for the figure attached herewith along with proper o/p summing ckt ( is from Ieee paper [i]"A Compact Power Efficient 3V CMOS Rail-to- Rail Input/Output Operational Amplifiers for VLSI Cell Libraries" by Ron Hogervorst , J.P.Tero, J.H.Hijsing [/i] )
or any suggested papers or documents to help me in this regard

for designing steps even i referred books written by authors (opamp theory and design, low Voltage analog desin considerations ...etc) i have request to work on 0.18u technology

Regards
Kiran

This is not rail-rail since output requires 2xVdsat to keep gain.

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Berti

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Re: Design of Rail-Rail OTA in 45nm Process
Reply #9 - Apr 15^th, 2008, 6:36am

Neoflash is right: Kiran, The figure you attached doesn't show the rail-to-rail output stage, but only the first stage of the amplifier.
For design steps: I think the amplifier is basically a two-stage amplifier with a folded-cascode first stage (with PMOS and NMOS diff. pair in parallel) and
a class A push-pull second-stage. All those techniques are well covered by text books (e.g. P. Allen, CMOS Analog Circuit Design).

Cheers

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kiran123

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Re: Design of Rail-Rail OTA in 45nm Process
Reply #10 - Apr 16^th, 2008, 10:37pm

Hi thanks Berti,
I agree with you that input stage is one of the many techniques of rail to rail constant gm stage and whereas o/p is not o/p rail to rail one , at present i am looking the design steps of this input rail to rail constant gm stage + suitable and simple o/p stage as adder

in this regard i don't think book allen is explained about ( input rail to rail constant gm techniques) if so do tell me chapter no( chapter 6 discusses with Class A and Class AB kind of opamap o/p stages not rail to rail input or o/p stage design rite?)

if you can forward any good documents or referencess exactly to aid this design will be really helpful

thanks
Kiran

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loose-electron

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Re: Design of Rail-Rail OTA in 45nm Process
Reply #11 - Apr 17^th, 2008, 2:16pm

Input rail to rail design is typically down with a PMOS differential pair and an NMOS differential pair structure -

Each has its own current source, and each has its own "current mirror diode" active load.

The second gain stage of each device mirrors out as a current, and then the signals from the two structures are summed together, and used to drive the output driver.

The concept is roughly illustrated in the attached picture. Although this implementation uses diode connected loads, but the general idea is shown.

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Picture1_001.gif

Jerry Twomey
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kiran123

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Re: Design of Rail-Rail OTA in 45nm Process
Reply #12 - Apr 17^th, 2008, 8:12pm

loose-electron wrote on Apr 17^th, 2008, 2:16pm:

Input rail to rail design is typically down with a PMOS differential pair and an NMOS differential pair structure -

Each has its own current source, and each has its own "current mirror diode" active load.

The second gain stage of each device mirrors out as a current, and then the signals from the two structures are summed together, and used to drive the output driver.

The concept is roughly illustrated in the attached picture. Although this implementation uses diode connected loads, but the general idea is shown.

Hi loose-electron,
thanks for your post , this looks very different i have not seen this structure anywhere in the textbooks, so no idea on how to design to get constant gm over entire input common-mode range

i have doubt regarding this can you explain the need of feeding the o/p (n1,n2) o/p of PMOS stage to NMOS stage loads @ p3,n4 through transistors P8 and n4 ?

do let me know regarding this rail-rail design which is the best book? (i have refered many of them listed in above) but none of them gives design strategy as the one Allen holberg gives

thanks
kiran

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neoflash

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Re: Design of Rail-Rail OTA in 45nm Process
Reply #13 - Apr 18^th, 2008, 5:54am

loose-electron wrote on Apr 17^th, 2008, 2:16pm:

Input rail to rail design is typically down with a PMOS differential pair and an NMOS differential pair structure -

Each has its own current source, and each has its own "current mirror diode" active load.

The second gain stage of each device mirrors out as a current, and then the signals from the two structures are summed together, and used to drive the output driver.

The concept is roughly illustrated in the attached picture. Although this implementation uses diode connected loads, but the general idea is shown.

Doing this design in 45nm becomes extremely difficult...

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Berti Community Fellow Offline Posts: 356	Re: Design of Rail-Rail OTA in 45nm Process Reply #14 - Apr 20^th, 2008, 9:18am Hi Neoflash, Would you please comment why you think that this structure might be difficult to realize in 45nm. Regards
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