Varying Beta (hfe) definitions?.?

[STEEDA]

Hello, I am trying to make sense of some data I just read in a text-book regarding transistor gain (hfe).

From what I've always understood, hfe = ΔIc / ΔIb . Correct?

In this text-book, it states that hfe can be measured as a ratio of input current (Ib) to output current (Ie). As I thought about it, it made sense but surely BOTH can't be correct? So, is hfe the measured ratio of collector current to base current (Ic / Ib) or Emitter current to base current (Ie / Ib)?

 

[KeepItSimpleStupid]

Think about it a bit. Ie=Ib+Ic right. You wouldn't want hfe to be defined as Hfe = Ib/(ib+Ic) would you? That's way to messy. If Hfe = current gain, then Hfe = Ic/Ib makes sense. hfe = ΔIc / ΔIb makes sense too. Current is multiplied. Hfe isn't a well controlled parameter.

 

[Claude Abraham]

There's ac current gain, particularly high frequency value "hfe". Then there's low frequency/dc value "hFE".
hFE = Ic/Ib.
hfe = ic/ib.
"Ic, Ib" are the dc current values, & "ic, ib" are the sc current values.
As frequency increases, more base current, ac part, is needed for a specific value of ac collector current. Eventually, at some particular value of frequency, ic/ib = hfe takes a value of unity. This frequency is called the "transition frequency", aka "ft".

 

[STEEDA]

Thank you BOTH for your replies! I think understand the difference between ac gain and dc gain but what's a bit confusing is HOW these are written.

I've seen dc gain written as hfe, HFE, hFE, Hfe
AND ac gain written as hfe, HFE, Hfe, hFE

I was taught that upper case is used to denote large signal dc (eg: VIN) and lower case for small signal dc (eg: vin)

I THOUGHT that h (h-parameter) was always written in lower case. The subscript written in lower case (fe - for ac / large signal) and upper case (FE - for dc/small signal)!

So which is the correct way to write gain for large signal / small signal, dc and ac???

I don't mean to nit-pick.... I just want to "get it" right.


thanks.

 

[bloki]

Yes h parameters are always written in lower case and DC in upper case. When you are in doubt consult datasheet and look at other specifications for which hfe is given.