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Simulators >> Circuit Simulators >> Problem with operating region warning control
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Message started by AnalogAroma on Sep 20^th, 2006, 9:03am

Title: Problem with operating region warning control
Post by AnalogAroma on Sep 20^th, 2006, 9:03am

Hi All,

I have simulated a fully differential HBT amplifier with cascode transistors for avoiding Miller effect. The simulator is Spectre 5.0 and the HBT models are VBIC 1.15. In order to find the input swing range , I have done a dc sweep of differential signal using the Kundert's testbench (http://www.designers-guide.org/Analysis/diff.pdf ). And
checked the operating points of the NPN cascose transistors. When the vbc is 99.48 mV and vbe is 912.1 mV , the cascode transistor is in forward active region ( region 1).
But when the vbc is 99.62 mV and vbe is still 912.1 mV , the transistor moves into saturation region ( region 3). This is quite amazing since in the HBT model file ,the B-C forward
voltage ( vbcfwd ) is set to 200mV.

It seems to me that Spectre does not use the "vbcfwd" parameter for giving warning about operating region. Am I right ? For our current project , we wish to allow vbc as high as 400 mV and still the HBT transistor will be physically in the forward active region. How can we pursuade the Spectre to think like us ? I mean, which parameter in the VBIC model should I change ?

With Regards,

Analog Aroma

Title: Re: Problem with operating region warning control
Post by Andrew Beckett on Sep 20^th, 2006, 11:12am

I don't have time to dig into the model equations and answer your question about what the parameters do, but if you wanted to have some additional warnings about ranges of operating region, you can always use the assert capability in spectre. Type spectre -h assert for more details.

Hopefully somebody else may have some bandwidth to answer the detailed model question.

Regards,

Andrew.

Title: Re: Problem with operating region warning control
Post by Geoffrey_Coram on Sep 29^th, 2006, 3:19am

vbcfwd is not a standard VBIC model parameter; it must be something that Cadence added when implementing the model.

Andrew: you'll need to find someone in Cadence who can look at how that parameter is handled to see if it is done properly.