Newbie question about transistor Hfe

[jrcfg]

Hi all...if a transistor is biased with a +5V, 0.2mA and is switching a +14V source to a load drawing 20mA, then my Hfe is 100. Correct?

What happens if the load draws more current...say 40 mA...does the bias current stay the same and only the ratio (Hfe) change to 200?

I guess what I'm asking is does changing the load current (or voltage)effect the bias? Thanks. John

 

[audioguru2]

Hi JRCFG,
Hfe is for AC gain and hFE is for DC, I think.
You can't calculate or measure hFE when the transistor is switched and saturated. What if the base current was equal to the collector current? Is its hFE only 1? Of course not! You can measure hFE when the transistor is linear, not saturated nor cutoff.
A transistor's hFE changes only a little when you change its collector current, therefore its base current remains almost the same. If you increase the collector current then you must also increase the base bias current, or it might not saturate very well.
The hFE decreases a lot when the voltage at the collector is low, when it is switching. That is why datasheets spec the collector saturation voltage with a base current that is 1/10th or 1/20th of the collector current. A 2N3055 power transistor has its IC=10A saturation voltage spec'd with a base current that is 1/3rd of the collector current. That is an awful lot of base current and the transistor is not near its maximum rated collector current, but it still saturates poorly because it needs even more base current.