Technical Question - How to determine transistor gain using a data sheet

[Baktown]

Greetings all,

Can someone explain to me in 50 words or less how to determine transistor gain from a data sheet?  I know that the symbol for gain is Hfe, but on most data sheets there are several lines of values for Hfe, based on different current values (as far as I can tell).

Which value do I use as a reference point for building effects?  Also, on my DMM I can test Hfe, how much current do DMM's typically test with?  I'm sure it varies from brand to brand, or is there a standard followed by all manufacturers?

Sorry for the newbie question, but I'm confused....

Rock on,

Rick J

[zachomega]

Unless your meter tells you in the manual how much current it uses, your guess is as good as mine. 

The data sheets are pretty much worthless.  Gains vary dramatically from the sheets.  Your best bet is either to make a circuit that isn't too dependent on the actual gain of the transistor or buy a bunch and start testing them. 

-Zach

[R.G.]

Ah. Finally - a parishioner asking for The Sermon. 

Data sheets tell you the absolute truth - which varies. Those lines with hfe or Hfe at different currents are the maker's way of telling you that current gain varies with collector current, which it does. The "Min Typical Max" columns are the maker's way of saying "I'll sell you anything I produce with this part number as long as it's gain is more than min and less than max. I think a lot of them will be typical, but counting on that is our way of tripping up junior engineers."

zachomega has the right idea. And it's the way I was taught to use transistors: design circuits that make the specific gain of the transistor not matter.

[Baktown]

Well,

I'm not really designing anything (yet), I'm just trying to sort out all the transistors I have according to gain.  I'm starting to think this isn't such a good idea....

Rick

[zachomega]

If they are Silicon transistors, don't bother...Modern Si transistors have large gains usually. 

Older ones supposedly have lower gains and might make it worth sorting, but don't get hung up too much on the details, just stick them in the meter and sort. 

Germanium transistors should probably be gone through for leakage and while you are there, sorted for gain.  It's a pain but if you are making something like a fuzz face/tonebender it is nice to just grab presorted parts. 

That's my two cents anyway. 

-Zach 

[brett]

Hi
to get an idea, you can use the hFE vs Ic (collector current) chart for a "typical device".  That'll get you close enough for most purposes.

You can calculate the Ic in many stompbox applications by dividing the collector voltage (if known) by the collector resistor. e.g. 3V/4.7k is about 0.6 mA.  If you don't know the collector voltage, a good guess is half of the supply voltage.
cheers

[R.G.]

I'm not really designing anything (yet), I'm just trying to sort out all the transistors I have according to gain.  I'm starting to think this isn't such a good idea....

It's a fine idea. You're looking for an indication, not precision. Datasheets are fine for that. For small plastic-cased devices to use in effects, pick the datasheet typical gain at 1mA. (and for all devices, typical for the current it will be used at)