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low power design guide

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evilguy

evilguy

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i really need advice in low power design guide. in analog design, what minimum current should i choose to drive the circuit, is it essential to bias the transistors in subthreshold region.
 

yes it is essential to bias the transistor in subthreshold region.
searcf for the Tsividis paper for low power design in IEEE JSSC.
 

    evilguy

    evilguy

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thanks. correct me if i'm wrong. in order to bias it in subtreshold design, vgs must be less than vt? how about the current? is 50µA can be consider small enough to drive the transistors?
 

evilguy said:
thanks. correct me if i'm wrong. in order to bias it in subtreshold design, vgs must be less than vt? how about the current? is 50µA can be consider small enough to drive the transistors?
Click to expand...
In my opinion 50µA isn't small enough, even several µA isn's small enough for subthreshold operation, you know in most of the circuits several µA is very common, so if you want your circuit to operate in subthreshold region I think the current should be even smaller.
 

cherryic said:
evilguy said:
thanks. correct me if i'm wrong. in order to bias it in subtreshold design, vgs must be less than vt? how about the current? is 50µA can be consider small enough to drive the transistors?
Click to expand...
In my opinion 50µA isn't small enough, even several µA isn's small enough for subthreshold operation, you know in most of the circuits several µA is very common, so if you want your circuit to operate in subthreshold region I think the current should be even smaller.
Click to expand...
hi,
i don't think so.
a transistor working in its weak inversion is not depedent on its current but mainly on the overdrive voltage(vgs<=vt).

jeff
 

jfyan said:
cherryic said:
evilguy said:
thanks. correct me if i'm wrong. in order to bias it in subtreshold design, vgs must be less than vt? how about the current? is 50µA can be consider small enough to drive the transistors?
Click to expand...
In my opinion 50µA isn't small enough, even several µA isn's small enough for subthreshold operation, you know in most of the circuits several µA is very common, so if you want your circuit to operate in subthreshold region I think the current should be even smaller.
Click to expand...
hi,
i don't think so.
a transistor working in its weak inversion is not depedent on its current but mainly on the overdrive voltage(vgs<=vt).

jeff
Click to expand...

are you saying that in subtreshold region, drain current can be hundred of µA? let say if my drain current is 100µA, i also can bias the transistor into subtreshold region by assure that my vgs<=vt?
 

yes,that's my own opition.
you can raise the size ratio to increase the current while keep vgs is close to vth.

good luck.
jeff
 

    evilguy

    evilguy

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i would agree with jeff also. but this is purely academic.

in low-power design, u would like to bias your circuit in nA range with vgs < vth.

my question:

with such low current, slew rate becomes a problem. what techniques do you use to tackle it? i.e., limiting voltage swing? example?
 

szekit said:
i would agree with jeff also. but this is purely academic.

in low-power design, u would like to bias your circuit in nA range with vgs < vth.

my question:

with such low current, slew rate becomes a problem. what techniques do you use to tackle it? i.e., limiting voltage swing? example?
Click to expand...

no... i dont go that far yet. i mean i dont search for a specific technique to tackle the issue. i'm thinking on limiting voltage swing. however i'm confius. if we bias our transistor in nA, is it enough? let say if we want high gain Opamp, is nA current enough to drive the Opamp? beside, what value is suitable for voltage swing to assure drain current in nA?

are there any circuit example for me to refer?
 

u can use i = C dv / dt to calculate the current u need.
 

szekit said:
u can use i = C dv / dt to calculate the current u need.
Click to expand...

can you explain more how to use this equation?
 

jfyan said:
cherryic said:
evilguy said:
thanks. correct me if i'm wrong. in order to bias it in subtreshold design, vgs must be less than vt? how about the current? is 50µA can be consider small enough to drive the transistors?
Click to expand...
In my opinion 50µA isn't small enough, even several µA isn's small enough for subthreshold operation, you know in most of the circuits several µA is very common, so if you want your circuit to operate in subthreshold region I think the current should be even smaller.
Click to expand...
hi,
i don't think so.
a transistor working in its weak inversion is not depedent on its current but mainly on the overdrive voltage(vgs<=vt).

jeff
Click to expand...
I know that, but we are talking about practical applications not theories right?
 

jeff
I know that, but we are talking about practical applications not theories right?
Click to expand...

i'm more confius now. what do you mean by that?
 

evilguy said:
jeff
I know that, but we are talking about practical applications not theories right?
Click to expand...

i'm more confius now. what do you mean by that?
Click to expand...

Yes, subthreshold operation does mean Vgs<Vth, and ID=ISo(W/L)exp(Vgs/nVt)[1-exp(-VDS/Vt)], and you can get different orders of values by this equation, but you should consider practical appilications, because in practical applications you have some experiences about the range of subthreshold currents, I mean what I have encountered in some projects.
 

    evilguy

    evilguy

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thanks... the main problem for me is i dont really know how to decide the value of drain current. some experience analog engineer should find this easy and can simply decide what is the value.
 

evilguy said:
thanks... the main problem for me is i dont really know how to decide the value of drain current. some experience analog engineer should find this easy and can simply decide what is the value.
Click to expand...
I think maybe you can refer to some IEEE papers to gain some useful insights about this issue.
 

ua is the subtherhold current order
 

beckchm said:
ua is the subtherhold current order
Click to expand...

i dont understand that, can you eloborate it further
 

evilguy said,
if we bias our transistor in nA, is it enough? let say if we want high gain Opamp, is nA current enough to drive the Opamp?
in the weak inversion region you can get higher gain but lower speed, so your problem is speed rather than gain.
 

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