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How Vbe is CTAT and delta(Vbe) is PTAT for the bipolar?

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chinito

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Could someone explain how Vbe is CTAT and delta(Vbe) is PTAT for the bipolar. Thanks.
 

ptat ctat

Hi,

Any forward voltage of a p-n junction exhibits a negative TC.
Due to a TC of a mobility of minority carries and an intrinsic minority carrier concentration, the TC of Vbe in Silicon is about -1.5 mV/K.

From equation for collector current Ic=Is*exp(Vbe/(kT/q))
you can get equation for Vbe:
Vbe=(kT/q)*ln(Ic/Is).
If two identical transistors are biased at a collector currents of Io and n*Io, then

Delta(Vbe)=(kT/q)*ln(n) ,

]i.e. Tc of delta(Vbe) is positive, proportinal to T.

See in more detail any book about Analog design, for example, Razavi's "Design of Analog CMOS IC", chap. 11.

b/rgds
 

ctat temperature

ptat means proportional to absolutely temperature.
ctat means complimentary to absolutely temperature.
they used in the temperature compensate.
 

ptat ntat

do there have circuit?
 

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