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hzfeiyun

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what does vstat mean in hspice log file
Sep 11^th, 2008, 12:31am

one transistor (w/l=80/2) op results:

0:mi100
0:n18
region � �cutoff
id � � 1.99u
vgs � 425m
vds � 859m
vbs � -342m
vth � 507m
vdsat 37m
vod � �-81m
beta � 21m
gm � � 60u
gds � � 160n
gmb � �15u

Q1:from textbook, vdsat (saturation voltage) = vgs -vth, but, obviously, it has different meanings here. What does it mean?

Q2:this transistor is a input tran of an OPAMP, should it be working on saturation region? any problem if it works on cut-off region evan pass all simualtion?

Thx. Wink

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ywguo

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Re: what does vstat means in hspice log file
Reply #1 - Sep 11^th, 2008, 12:58am

Hi hzfeiyun,

Quote:

any problem if it works on cut-off region evan pass all simualtion?

You may call it cut-off region or weak inversion region. There are specific description on MOS transistors in weak inversion region in the textbooks like [Grey and Meyer] .

The vdsat is still saturation voltage although it is not equal to vgs-vth in weak inversion region.

Best regards,
Yawei

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hzfeiyun

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Re: what does vstat mean in hspice log file
Reply #2 - Sep 11^th, 2008, 1:05am

ywguo wrote on Sep 11^th, 2008, 12:58am:

Hi hzfeiyun,

Quote:

any problem if it works on cut-off region evan pass all simualtion?

You may call it cut-off region or weak inversion region. There are specific description on MOS transistors in weak inversion region in the textbooks like [Grey and Meyer] .

The vdsat is still saturation voltage although it is not equal to vgs-vth in weak inversion region.

Best regards,
Yawei

i found some transistor's vdsat is not equal to its vod even in saturation region

and, what does vsat mean in cut-off region?

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ywguo

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Re: what does vstat mean in hspice log file
Reply #3 - Sep 11^th, 2008, 2:54am

Quote:

i found some transistor's vdsat is not equal to its vod even in saturation region

What is vod? Do you mean vgs-vth? Since the popular BSIM3 model is more complicated than level-1 MOS model, vdsat is not equal to vgs-vth strictly. Please refer to BSIM3 manual which is available at Berkeley website bsim.eecs.berkeley.edu.

In weak inversion region, the MOS operates like a bipolar transistor much more. When the vds is greater than vdsat, the Ids is very flat. In other word, the MOS is in saturation region.

In [Grey and Meyer], it states that the drain current is almost constant when Vds>3VT (VT=kT/q). So the minimum vds required to force the transistor to operate as a current source in weak inversion is independent of the overdrive. At 300K, 3VT ≈75 mV. That is a relative coarse but reliable value. It is not uncommon to get vds = 37mV as that in your SPICE log file.

Best regards,
Yawei

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