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Bandgap reference circuit (bjt area ratio)

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shrikantbp

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Hi,

In band-gap reference circuit.Why there is 1:n ration of BJT?

Regards
Shrikant
 

Hi,

What circuit are you referring to?
Where do you find the 1:n ratio?
Please give pictures, links...

Klaus
 

Because you are looking to run two equal currents
through devices at two unequal current densities.
So you need unequal areas.

1:N ratioing is the IC designer's best way to get
a maximally consistent ratio. Drawing two different
device geometries (like a single 1X emitter, and
a single 16X-area emitter) puts you at the mercy
of various lithographic influences that at best are
inconsistent, and probably modeled with a "high
degree of fiction", meaning your design may be
way off and run sloppy in production. The integer
instance ratio removes this influence (aside from
field loading litho issues, and this is where you add
dummies around the reference diode core).
 

shrikantbp said:
Hi,

In band-gap reference circuit.Why there is 1:n ration of BJT?

Regards
Shrikant
Click to expand...
In order to create PTAT current source.This current will be used to compensate VBE vs. Temp value.( remember that VBE drops down approx. -2mV/deg )
 

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