why does NMOS pass a strong logic 0 and a weak logic 1

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Shanthanayaki

Joined: 26 Jun 2006
Posts: 6

26 Jun 2006 1:39 pmos weak 0

Hello everybody ,
Can anyone tell me, why NMOS pass a strong logic 0 and a weak logic 1 ?.
Vice versa why PMOS pass a strong logic 1 and a weak logic 0 ?.
Thanks,
Shantha

IanP

Joined: 05 Oct 2004
Posts: 6490
Helped: 1542
Location: West Coast

26 Jun 2006 1:55 nmos pass a “strong” 0 but a “weak” 1

Quote:

... in an NMOS process, logic circuitry is often constructed using a weak transistor that is always on and one or more strong transistors that are switched on and off. In NMOS the weak transistor is used to generate a high output voltage level or 1 when the strong transistors are turned off. When a strong transistor turns on it overwhelms the weak transistor and forces the output voltage low, which represents a 0.

Quoted from:
http://www.realworldtech.com/page.cfm?ArticleID=RWT050802020022&p=3

Regards,
IanP

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Shanthanayaki

Joined: 26 Jun 2006
Posts: 6

26 Jun 2006 2:08 nmos weak 1

IanP wrote:

Quote:

Quoted from:
[url]h**p://www.realworldtech.com/page.cfm?ArticleID=RWT050802020022&p=3[/url]

Regards,
IanP

Hi,
Thanks. Appreciate the help
Shantha

gharuda

Joined: 30 May 2006
Posts: 59
Helped: 5

27 Jun 2006 9:45 can both pmos and nmos pass good 1 and good 0

hi

the page just explains about the technology used, intially PMos tech, later Nmos tech was in use and currently Cmos tech is being in use.

but still i didnt get the picture how and why does nmos pass good 0 and bad logic 1
and pmos pass good 1 and bad logic 0.
However still the tranistor pass good 0 and considerable 1 in nmos when used as a pass tranistor - that of threshold voltage.(turn on voltage)

Infact this may due to the majaority carries holes and electrons in p and n type.
like in any circuit two types current flows one is coventional and other is electon drift current. as convential current flows from +ve terminal to -ve terminal and the electron current from -ve to positive.

if we consider a Cmos inveter when the input is logic 0 the pmos tranistor turns on. source of pmos is more positive than the drain terminal, as the majaority carrier is holes whichs consitutes conventional current flows from source which more +ve than drain and flow of conventianal current is from +ve terminal to -ve terminal. thus the output which is drain is pull high faster compared to nmos . (pmos switching action is faster)

when the input is logic 1 the nmos tranistor turns on. source of nmos is more -ve than the drain terminal, as the majaority carrier is electrons in nmos .electon current flows from source to drian .thus pulling down the output drain to low.

switching action of tranistor plays a important role.

kuo

Joined: 27 Jun 2006
Posts: 3

28 Jun 2006 1:05 vgs of nmos

For NMOS to pass 0, as long as Vgs> Vt, NMOS can trun on. It is strong to pass 0.
For NMOS to pass 1, it needs Vgs > Vt to turn on. So it weak to pass 1.
For PMOS to pass 1, the Vg is easy to turn on and pass whole "1"...

So it has PMOS and NMOS to make transmit gate instead of one transistor alone.
It can pass 1 and 0 in good condition.

shyamsundar

Joined: 09 Jun 2007
Posts: 3

24 Jan 2008 9:03 eplanation of working of n-mos

Hello the explanation regarding the Weak logic 1 by NMOS and logic 0 by PMOS, I felt is inadequate. Can anyone elighten on this in detail??
This is one of the major interview questions in physical Design VLSI and MOS Circuit analysis.

Thanks in advance

ninge

Joined: 10 May 2006
Posts: 40
Helped: 2
Location: India

29 Jan 2008 9:27 nmos strong zero and weak one

Hi All,

please go through the attachment page no 7 and 8 which discuss about the topic...
I hope this will help..

Regards,
Ninge

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murthykurapati

Joined: 18 Sep 2009
Posts: 1
Location: singapore

18 Sep 2009 11:31 passing a logic 0

@ ninge,
thanks a lot for the attachment!
It helped me a lot!
I was looking every where, but could not get a satisfying solution!
Thanks once again!

maddy13rocks

Joined: 21 Oct 2009
Posts: 1

21 Oct 2009 3:19 Re: why does NMOS pass a strong logic 0 and a weak logic 1

@ ninge,
thanks a lot for the attachment!
It helped me a lot toooooooo!

carporsche

Joined: 08 Jun 2009
Posts: 8

21 Oct 2009 4:05 why does NMOS pass a strong logic 0 and a weak logic 1

that was a really helpful doc

wangwei420

Joined: 29 Oct 2009
Posts: 1
Location: hk

29 Oct 2009 18:18 Re: why does NMOS pass a strong logic 0 and a weak logic 1

The attachment file is really good!

salma shabayek

Joined: 13 Dec 2008
Posts: 14

02 Nov 2009 13:24 Re: why does NMOS pass a strong logic 0 and a weak logic 1

attached is a lecture explaining it very wel with examples

Sorry, but you need login in to view this attachment

suze

Joined: 17 Sep 2009
Posts: 1
Location: pune

04 Nov 2009 12:45 Re: why does NMOS pass a strong logic 0 and a weak logic 1

thanx really good attachment , i searched a lot for such convincing answer.finally got it in book by (Cmos vlsi design) neil weste page no. 66--- pass transistor char.