MOS as switch question

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suhas_shiv

Joined: 30 Nov 2005
Posts: 139
Helped: 7

08 Sep 2006 7:25 MOS as switch question

I am trying to simulate the attached circuit. The problem is with transistors acting as switches. The thing that is confusing me is, does the transistor, say the first one, have to be in saturation? When I simulate it is not in saturation as seen by the voltages in the circuit. Also, if it has to be in saturation how do I bias it? Is my baising wrong? The source voltage is same as drain and hence Vds=0V.
Can someone please explain how to analyze this problem? Thanks.

Suhas

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lics

Joined: 22 Aug 2006
Posts: 18
Helped: 1

08 Sep 2006 8:28 MOS as switch question

I think it's no need for the thransistor to be in saturation because your transistors act as swiches. They operate in deep linear region. You just let Vgs>Vth.

Davood Amerion

Joined: 01 Mar 2005
Posts: 589
Helped: 90
Location: Persia

08 Sep 2006 9:16 MOS as switch question

in your circuit M1 configured as source follower!
if you want to use M1 as switch:
1- use PMOS for M1
or
2- use higher drive voltage for V1 (1.5+output max voltage)

kwkam

Joined: 25 Feb 2002
Posts: 299
Helped: 15
Location: Somewhere on earth

08 Sep 2006 15:00 MOS as switch question

your circuit can't show what you want. Anyway, tansistor as a switch will don't care about saturation or not. The problem is can the transistor be fully turn-on

pkalavakuru

Joined: 17 Feb 2006
Posts: 44
Helped: 1

08 Sep 2006 15:44 Re: MOS as switch question

ur using 1.5 V is enough to turn on ur NMOS TRANSISTOR..take ur PULSE 0 to 5V
and check it

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suhas_shiv

Joined: 30 Nov 2005
Posts: 139
Helped: 7

11 Sep 2006 6:52 MOS as switch question

Hi again,

Just wanted to know one thing, is it ok for the MOS to be in triode while acting as a switch or is it a must that it should be in saturation? I have heard people supporting both. Can onyone please throw some light? Thanks.

Davood Amerion

Joined: 01 Mar 2005
Posts: 589
Helped: 90
Location: Persia

11 Sep 2006 8:06 Re: MOS as switch question

suhas_shiv wrote:

first you must consider real switch.
In real switch, when closed, we have a near zero resistance. when we pass current through it voltage drop acros it is negligible.
In MOSFET, this is located in triod region.

and you can consider switch in open circuit (infinite resistance). In MOSFET, this region located in off condition.

Regards,
Davood Amerion.

laglead

Joined: 21 Feb 2006
Posts: 273
Helped: 17

14 Sep 2006 14:55 Re: MOS as switch question

no need in saturation.

aborigini

Joined: 01 Aug 2006
Posts: 23

14 Sep 2006 15:59 Re: MOS as switch question

The NMOS must be fully turn ON. When it is working as a switch, it will act in triode with low resistance path.

You can check the digital ic notes.

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shaikhsarfraz

Joined: 08 Aug 2006
Posts: 309
Helped: 19

15 Sep 2006 6:06 Re: MOS as switch question

Hi,
For a MOS transistor to work as swtich it is not necessary that it should work in saturation region.

kwkam wrote:

your circuit can't show what you want. Anyway, tansistor as a switch will don't care about saturation or not. The problem is can the transistor be fully turn-on

The only thing is that when MOS is cutoff the resistance is so high that it works as open circuit.
When it is in linear or saturation region it works as short circuit although it has on resistance associated with it.

Thanks
Shaikh sarfraz