cmos in linear or saturation

cmos saturation

hi! all,

this may sound like a stupid question. when i was in college, i was taught that when designing a FET amplifier, i should keep the FET in the linear region.

however, i was told now, whenever i design an CMOS analog circuit, i should keep the transistor in saturation region.

so, what is going on?

thanx!

speedracer

cmos saturation region

well your confusion is due to various classification existing for the MOS device.
Well when they tell that the device should we be in linear region for the faitful amplification they use to talk about the bipolar device..
Where as for the CMOS device to have the linear charecterstic and high gain they need to operate in saturation region.. i.e Vgs>Vth and Vds> Vgs-Vth (am talking about the nmos device for pmos polarity will reverce.) ..
And because of this bipolar convention they use to say the mos should be in linear region. but they mean it should be in saturation region.

Amit

cmos linear region

It is a little bit funny with that linear-saturation thing. For bipolar transistors saturation means when the Vce is 0.2v and below and linear (or active) when it is above 0.2 that is when the operation is in the flat part of the Ic-Vce characteristic. For MOS transistors, terminology is exactly the oposite. The MOST is in linear (or ohmic) region when the Vds is somewhere below 0.2v or where the transistor behaves more like a linear resistor. MOST is in satulation or active region when there is enough Vds, that is Vds>Vgs-Vt>0.2v. There you're in the flat part of the Id-Vds curve and the transistor behaves more like a current source. To design analog circuits/amplifiers you'd want the transistors in that latter region where they behave like current sources. Usually these kind of circuits are called linear.
I know it is a bit confusing, but once you get used to it, everything comes into place.

linear and saturation

Hi if you re taslking about simple two transister amplifier then

----|------vdd
|= m1 (p)
|
|-
|
|= m2
|
------gnd

then as m1 working as resister(active load) so be in linear or triode region and accoringly gate is tied with drain(i.e. output for this case).

and m2 remain in saturation region notice that gm is higher in saturation region compare to triode or linear region.

so gain of stage = gm2*(ro1||r02|| any output load )

now if you see characteristics of this whole configuration
Vo|
|****** <--------------------------------m2 in linear region
| A *
| * <---------------------------m2,m1 in saturation
| B *
| * C
| ********* <-------------m1 in linear region
|-----------------------------------Vin

if you see A and C represents nonlinear characteristics of whole amplifier and so to get amplification we need to make proper bias (in this case gate of m2) so that amplifier work in region B (which is linear region) but transisters are in saturation.!!

hope i have answered your query
regards...sim

cmos saturation current

Hi if you re taslking about simple two transister amplifier then

please look to figure atached here with----

----|------vdd
|= m1 (p)
|
|-
|
|= m2
|
------gnd

then as m1 working as resister(active load) so be in linear or triode region and accoringly gate is tied with drain(i.e. output for this case).

and m2 remain in saturation region notice that gm is higher in saturation region compare to triode or linear region.

so gain of stage = gm2*(ro1||r02|| any output load )

now if you see characteristics of this whole configuration
Vo|
|****** <--------------------------------m2 in linear region
| A *
| * <---------------------------m2,m1 in saturation
| B *
| * C
| ********* <-------------m1 in linear region
|-----------------------------------Vin

if you see A and C represents nonlinear characteristics of whole amplifier and so to get amplification we need to make proper bias (in this case gate of m2) so that amplifier work in region B (which is linear region) but transisters are in saturation.!!

hope i have answered your query

regards...sim

saturation cmos

well CMOS and Bipolar transistors are totally different devices but the operational regions of these have the same name. Also what you learnt in college does not happen always, you might design something in the weak inversion region when all your college designs where in the strong inversion region.

saturation linear region

in order to remember again the region of bipolar and CMOS i suggest to you to read Sedra & Smith book. It will help you to see the differences betwenn these 2 devices

triode region, saturation , linearity better

simphibie,

I think that in your circuit above M1 can never be in triode, because its gate is tied to the drain, so it is always in saturation. In region C M2 can be in triode or in saturation depending on the current the gate voltage and the sizes of the transistors - most probably it is in triode.

cmos in saturation

All of this confusion of conflicting nomenclature has a historic background. In the beginning with bipolar transistors all circuits were analog and the transistor states were named after the circuit performance.

When MOS came in all of the circuits it was used in were digital which did not have a linear state. These people used the same words as before to describe the transistor characteristics.
There was a disconnect between the two groups which causes this confusion.


A similar thing is done with two port networks. Real experts in this area realize that the two input terminals of two networks can be placed in series or parallel. The terminology for combining two networks says inputs series/parallel and outputs series/parallel. The term tandem is used for connecting the output of one network to the input of the next network. People who are not experts in this area draw signal flow diagrams with boxes with one arrow connecting them and call the whole lot a series connection of stages. This is a good way to determine if a person has a real solid education in electronics.

transistor linear, saturation

In digital circuit, MOS usually work in saturation region, in analog circuit MOS usually work in linear region.

saturation current cmos

Hi, leftsea
I can not agree with you. Actually , In digital IC,
MOS will not work in saturation region.

transistor characteristics linear saturation

hi,
In digital IC, MOS transistor usually works in linear region (Vds<Vgs-Vth).
In analog IC, MOS transistor mostly works in saturation region.

triode linear saturation

I alway got confusion when I first started doing design. This is worthwhile to recover the memories. Also, when I got started, always got confusion between PMOS and NMOS

Re: triode region, saturation , linearity better

sutapanaki said:

simphibie,

I think that in your circuit above M1 can never be in triode, because its gate is tied to the drain, so it is always in saturation. In region C M2 can be in triode or in saturation depending on the current the gate voltage and the sizes of the transistors - most probably it is in triode.

Click to expand...

HI , sutapanaki,

I cannot agree with u either. In region C, M2 can only be in triode, because at a certain point Vod=Vgs-Vth must equals Vout and then Vod>Vout since the ouput decrease as the input increase. this point is the turning point from saturation to triode. am i right?

---------- Post added at 16:23 ---------- Previous post was at 16:18 ----------

sutapanaki said:

simphibie,

I think that in your circuit above M1 can never be in triode, because its gate is tied to the drain, so it is always in saturation. In region C M2 can be in triode or in saturation depending on the current the gate voltage and the sizes of the transistors - most probably it is in triode.

Click to expand...

Oh, I got it. Do u mean the turning point may not be reached so M2 still stays in saturation in Region C?

I am a newbie in analog ic design, and i always consider the operation region of cmos transistors depend on the purpose of them, for example: if i want that mos to achieve high gain, i will control it to operate in saturation, and also, if i need the controlable current, mos has to operate in saturation, but some time mos transistor just only need to be on triode to permit the current through.
Maybe i am not correct in this problem, hope youguy help me more clearly!
Regard.

Re: triode region, saturation , linearity better

Yes, that's what I meant. Imagine M2 being a long device such that even when its gate is at Vdd, the current that it draws ia still small. A small current means Vgs~Vth for the pmos M1. So Vgs-Vth=Vdd-Vth for M1. On the other hand output is at ~Vdd-Vth for the pmos M1. If Vth for both nmos and pmos are kind of similar in value, you'll have for M2 that Vdd-Vth=Vgs-Vth=Vdd-Vth=Vds which is in saturation. Of course, if M2 was a strong device, pulling down a lot, then its Vds will be small and the transistor will be in linear (ohmic) region.

rogeret said:

HI , sutapanaki,

I cannot agree with u either. In region C, M2 can only be in triode, because at a certain point Vod=Vgs-Vth must equals Vout and then Vod>Vout since the ouput decrease as the input increase. this point is the turning point from saturation to triode. am i right?

---------- Post added at 16:23 ---------- Previous post was at 16:18 ----------



Oh, I got it. Do u mean the turning point may not be reached so M2 still stays in saturation in Region C?

Click to expand...

Re: triode region, saturation , linearity better

sutapanaki said:

Yes, that's what I meant. Imagine M2 being a long device such that even when its gate is at Vdd, the current that it draws ia still small. A small current means Vgs~Vth for the pmos M1. So Vgs-Vth=Vdd-Vth for M1. On the other hand output is at ~Vdd-Vth for the pmos M1. If Vth for both nmos and pmos are kind of similar in value, you'll have for M2 that Vdd-Vth=Vgs-Vth=Vdd-Vth=Vds which is in saturation. Of course, if M2 was a strong device, pulling down a lot, then its Vds will be small and the transistor will be in linear (ohmic) region.

Click to expand...

Thanks for replying.
Supposing the M2 is a weak device, Vout may near Vdd-Vth and Vin cannot exceed Vdd, so M2 may stay in saturation all the time. But may Vin exceed Vdd by some method?

Re: triode region, saturation , linearity better

These are already specifics. Yes, Vin can exceed Vdd, for example if you use some sort of voltage boosting. Or if you have dual supply, etc. But this opens the door to a whole new topic of overvoltage limits the device can survive.