Question about the Rout in NMOS from Razavis book

[patriot]

the Rout in Nmos?

in the Razavi's Analog Cmos Design book,P32
if the device size and the bias are the same,why NMOS has higher Rout than PMOs,
and PMOS is better for ideal currency?
thanks!

 

[cedance]

Re: the Rout in Nmos?

in the Razavi's Analog Cmos Design book,P32
if the device size and the bias are the same,why NMOS has higher Rout than PMOs,
and PMOS is better for ideal currency?
thanks!

are u talking abt the ON resistance of the transistor... then its the otherway i think... pls post page 32 of the book in GIF format... so that others can have a look at it!

/cedance

 

[patriot]

Re: the Rout in Nmos?

on the book described as follow:
In most CMOS technologies,PMOS devices are quite inferior to NMOS transistor.For example,due to the mobility of holes,µpCox≈0.25µnCox in modern process,yielding low current drive and transconductance.
:sm2: Moreover,for given dimensions and bias current,NMOS transistors exhibit a higher output registance,providing more ideal current sources and higher gain in amplifers , :sm2:
for these reasons,it is preferred to incorporate NFETs rather than PFETs wherever possible

 

[cedance]

Re: the Rout in Nmos?

yeh now i got it! ur first question, u mistyped i suppose.. NMOS having higher resistance than PMOS, its the other way! this is called the ON resistance of the transistor.. IN MOS logic, when operated in digital logic, there is theoretically supposed to be a ideal '0' resistance.. but in reality there will be a very small value for resistance say "50 ohms". similarly for PMOS transistors there will be resistance but will be in the order of "150 ohms" or so.. this is because of the basic property of the transistors! NMOS, conduction being electrons have higher mobility and so less resistance.. the conduction in PMOS is due to the holes whose mobility is lesser than the electrons and hence the effective resistance of the channel is higher than that of NMOS...

/cedance

 

[patriot]

Re: the Rout in Nmos?

:sm2: Moreover,for given dimensions and bias current,NMOS transistors exhibit a higher output registance,providing more ideal current sources and higher gain in amplifers , :sm2:

thanks to arunmit168
but the NMOS on the book which works in analog circuits,it has higher output registance than PMOS in amplifiers,i now thought it is due to these reasons:
take a common source amplifier for ex:
gm=(2µ•Cox•W/L•Id•(1+λVds))½
Rout=1/gm||r0
Id stands for the bias current;r0 is the registance related to L Effect
the gm in NMOS is higher than PMOS,hence the Rout of NMOS is higher than PMOS


i don't know if it is this?
thanks to anyone for help!

 

[cedance]

Re: the Rout in Nmos?

you are right i suppose! because,

NMOS transistors have W/L ratio by atleast 2 times lesser than that for PMOS... so, from the gm expression its smaller for NMOS and hence its reciprocal is greater! Hope this adds more to ur understanding!

/cedance

 

[aryajur]

Re: the Rout in Nmos?

I think there is a basic confusion. When a transistor be it NMOS or PMOS is in saturation there are 2 output resistances associated with it, first you should get that distinction clear.
One resistance is tha large signal resistance, which as arunmit tried to explain is a low value of like 50 ohms for NMOS and 150 ohms for PMOS, when the transistor is ON. This resistance can be looked at equivalently as :

Rout (Large Signal) = Vds / Ids,

where Vds is the DC biased voltage across the transistor when it is "ON" and Ids is the biasing current through it. This resistance is quite low.
The second resistance associated with a transistor in saturation, which is of main interest in Analog circuit design is the small signal output resistance. This resistance is defined as:

Rout (small signal) = δVds/δIds

which for example looking at the drain of the transistor in a common source configuration is ro, which is a very high value, which allows the transistor to behave as a good current source (for small signals !!).
This small signal output resistance is approximately equal to:

ro = 1 / (λxIds)

where λ is the channel length modulation factor. Comparing NMOS and PMOS transistors λ for a NMOS transistor is lower than that of a PMOS for the same biasing Ids and Vds, this can be understood by considering this:

λ≡ ΔL/(LxVds)

so this means that if the NMOS and PMOS transistors are identical in dimensions and have same biasing ΔL for an NMOS is lower than that for a PMOS. And this makes sense because for the same Vgs an NMOS has a larger current than a PMOS because of its higher mobility therefore when both are in saturation such that

Ids (NMOS) = Ids (PMOS) this means that
|Vgs| (NMOS) < |Vgs| (PMOS)
thus this means that
|Vgd|(NMOS) < |Vgd| (PMOS) for same |Vds| across NMOS and PMOS

hence ΔL for NMOS will be lesser than that of PMOS for the same biasing, hence λ for NMOS is lesser than that of PMOS for same biasing and therefore the small signal output resistance is larger for the NMOS and it is a better current source at the same biasing and dimensions.
I hope this was helpful.