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https://designers-guide.org/forum/YaBB.pl Simulators >> Circuit Simulators >> what does vstat mean in hspice log file https://designers-guide.org/forum/YaBB.pl?num=1221118277 Message started by hzfeiyun on Sep 11th, 2008, 12:31am |
Title: what does vstat mean in hspice log file Post by hzfeiyun on Sep 11th, 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. ;) |
Title: Re: what does vstat means in hspice log file Post by ywguo on Sep 11th, 2008, 12:58am Hi hzfeiyun, Quote:
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 |
Title: Re: what does vstat mean in hspice log file Post by hzfeiyun on Sep 11th, 2008, 1:05am ywguo wrote on Sep 11th, 2008, 12:58am:
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? |
Title: Re: what does vstat mean in hspice log file Post by ywguo on Sep 11th, 2008, 2:54am Quote:
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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