Linear regulator BJT needs to have more distinct VBE turn-ON

[cupoftea]

hi,
We have a linear reg...with a NPN based circuit which senses the VBE of the pass transistor, and pulls down the ref voltage so as to limit overload current...but we want this to start conducting when its vbe is 0.65V and not before...thats impossible....but which is the closest NPN for this behaviour?

 

[KlausST]

Hi,

you may add external circuitry to "modify" Vbe.

For accurate voltage threshold you may add an peltier to keep the BJT at the temperature for exact Vbe = 0.65V.
.. since Vbe drifts with about -2.1mV/°C.

Klaus

 

[crutschow]

when its vbe is 0.65V and not before...thats impossible....but which is the closest NPN for this behaviour?

All BJT's have essentially the same input behavior, as the base-emitter input looks like a forward-biased diode.

You can add a comparator or op amp with a reference voltage to add gain and make the trigger point precise.

 

[Easy peasy]

more electronics required ...

 

[dick_freebird]

Resistor divider on the sensed Vbe, maybe simple revision
or tap of the base shunt network?

Seems to me this is a poor scheme as Vbe varies widely w/
junction temp and is a pretty loose indicator of collector /
emitter current when the main variables are unknowns.