What is the relationship between base resistance and beta in

[ljy4468]

Hi, all

What is the relationship between base resistance and beta in BJT??

I want to know the reason, also.

Thanks in advance.

 

[Audioguru]

The transistor has an internal base-to-emitter resistance that depends on the current gain of the transistor. The input impedance of a transistor is its base biasing resistors in parallel with its input impedance of its preamp transistor.

The transistor has an input impedance that is its hfe (AC current gain) times its internal plus external emitter resistance. If the beta is low then its input impedance will also be low.

A darlington transistor has a very high beta and has a very high input impedance.

 

[ljy4468]

thanks for your answer, audioguru,

but my question is a little bit different.

In epi-process, there is BL(buried layer) N+ region which is low resistance path for collector to emitter current. (example of NPN)

And is also provide low resistance path for base of parasitic PNP. And it lowers beta of parasitic PNP.

So there is little chance that parasitic PNP operate due to low beta.

My question is why beta is low if base resistance is low physically?

Thanks.

 

[Audioguru]

An ordinary transistor does not have a buried layer nor a parasitic PNP. it is simply an N layer then a p layer then another N layer.

Maybe you are talking about a Mosfet.

The input impedance of an ordinary transistor is its AC beta times its internal plus external emitter rresistance.
If the external emitter resistance is unbypassed then the input impedance of the transistor is high.