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Why CMOS circuit with variable Vth is considered advantage for low power technology?

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jason_class

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Hello All

Could anyone tell me why cmos circuit with variable Vth is considered an advantage for low power technology?

Kindly advise.
Thank you

best regards
Jason
 

Re: variable Vth

Basically, if you have a leaky circuit and the workload is variable, you can set up a scheme to set Vth to an optimal value based on the workload and trade performance for power. I've seen papers on this somewhere.
 

Re: variable Vth

Variable threshold voltage is one of the many low-power CMOS design techniques used. The aim is to reduce leakage current by increasing Vth, however at the expense of reducing switching frequency or performance in speed.

There is one problem with this technique is that increasing Vth has an adversary or backfire effect. Subthreshold leakage occurs when gate voltage is lower than Vth, increasing Vth increases this leakage.

For more information, there are some renowned experts in this field.

Kaushik Roy (Professor, Purdue USA) - He has written many books.
Yeo Kiat Seng (Professor, NTU Singapore) - Written books with Kaushik Roy
Sharat C. Prasad (CISCO USA) - Written some joint articles and books with Kaushik Roy
Gary Yeap (Motorola Hong Kong and USA) - Written a book
Chandrakasan (Professor MIT USA) - Written numerous articles.
 

Re: variable Vth

Hi Nimue and Sky High

Thanks a lot for the answers and advice. It is nice to read it

By the way, SKy High, kindly advise if variable Vth is applicable for SOI and what is the possible trade off in your opinion compare to bulk cmos.

It will be great if Greg can also join this discussion.

Thanks a lot everyone

best regards
jason
 

Re: variable Vth

Variable Vth is a low-power technique in which the designer has to first consider if the speed has to be compromised for power. In other words, this technique is conditional, not a general technique.

SOI aimed at RF, analog and high-speed digital. Vth will limit the speed if used with SOI, thus defeat the main aim of using SOI for these high frequency circuits.

In fact SOI itself offers reduction in dynamic and static power consumption compared to bulk CMOS, designers might not see the need to use variable Vth at all. Subthreshold leakage introduced in variable Vth is another problem.
 

Re: variable Vth

Thank you Sky High
That's a brilliant comment to hear from you
So there is close relation between speed and power that determines if SOi is suitable.

best regards
Jason
 

Re: variable Vth

No, there is a compromise or tradeoff to make when deciding whether or not to use variable Vth, not SOI.

SOI offers power reduction and high speed, so there is nothing to compromise between these two factors. It's cost and SOI substrate quality.

So there is close relation between speed and power that determines if SOi is suitable.
 

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