Different hFE for different Vce,Ic.

[alphacat]

I was taught in university that β (=hFE) is a constant value in the forward active region, that doesnt depend on Vce,Ic.

So how come I see in datasheets that hFE changes when Vce,Ic change?

 

[crutschow]

The university was wrong, the data sheets are right. The hFE can be significantly affected by Ic and somewhat by Vce.

Unfortunately some of the things taught in schools are idealized and turn out to be different in practical applications. The trick is to know which is which.

 

[alphacat]

Thanks, I didnt know this.

 

[Diver300]

hFE varies with temperature and is different for different examples of the same type of transistor as well.

It is good practice to design around a low value of hFE, but also make sure that an infinite value of hFE won't cause problems. The gain of an analogue transistor amplifier will always be far less than the gain of the transistor used to make it, and it is controlled by the value of the other components, that vary less than the transistor gains.

The hFE is still a useful concept because, unless the transistor is saturated, the collector current is controlled by the base current, and the gain varies far less than that base or collector currents.