Band Gap reference BJT

[analog_chip]

Hi,
I m designing a band gap reference circuit and I have some basic doubts.

1. The BJT that I will use is a diode connected with Vbc=0. So, in which region of operation will it work i.e. active or saturation.

2. How this scenerio is handled by SPICE i.e. by eber molls and gummel poon model?

I am using Spectre simulator.

 

[erikl]

1. Depends on Ic : For Vce=Vbe > Vce(sat)=f(Ic) , i.e. for low currents (BandGap application) it works in active region.

2. Depends on your BJT models' level: Usually you get BJT models with level < 3 , these are combinations of the Ebers-Moll and Gummel-Poon models, but there are also more sophisticated BJT models available (e.g. VBIC (level 4), MEXTRAM (level 6), HICUM (level 8) & BSIM (level 49, 53, ..) models), s. this SPICE wiki.

SPECTRE can handle all these models.

 

[analog_chip]

Hi erikl,
Why should we bother about Vce and how for small currents it is in active?
For active, BE is forward and BC is reverse
For saturation, BE is forward and BC is forward
In my case BE is always forward and so question is about BC junction which is theoretically Vbc=0. Now actually in the device (inside) the BC junction is still reverse biased even for Vbc=0.

How does low Ic or Vce affect these?

 

[erikl]

How does low Ic or Vce affect these?

Repeat: For Vce=Vbe > Vce(sat) ... it works in active region.

Vce(sat)=f(Ic) -- s. e.g. this posting -- i.e. for low currents (BandGap application) Vce=Vbe > Vce(sat), that's why it works in active region if operated with low Ic.

For large collector currents Vce(sat) > Vbe=Vce (s. the Ic vs. Vce characteristics in the a.m. posting), in this case a diode connected BJT would work in saturation region. So the Ic value affects in which region the BJT operates.