Im currently again trying to get my head around transistors again.
The Hfe value that I need to calculate the base resistor is dependent on the collector current so I have to have a look on the "Current Gain vs. Collector Current" curve in the data sheet as far as I know.
When I take a look at the datasheet of the 2N2222A for example that just makes no sense for me:
It says its a min value of 100 and a max value of 300. When I take a look at the curve its just not near to that. If I read the value eg. for Ic 400mA at Vce 1V the curve is somewhere at 40 maybe. The value will never be greater than maybe 180.
[quote] When I take a look at the curve its just not near to that. If I read the value eg. for Ic 400mA at Vce 1V the curve is somewhere at 40 maybe.
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And the the spec section, it says that at Ic of 500mA, the current gain is 40 minimum, which is also what the curve shows. Gain apparently deteriorates relatively quickly with Ic about 150mA.
That's pretty consistent with this curve from the Fairchild datasheet, except that Fairchild's graph only goes up to 500mA...
Keep in mind that the values in the graphs are just representative, and the actual values for a given transistor may be different by a factor of two or more.
The circuit design has to take that variability into account, unless it is a one-off custom build, trimmed for a specific application.
I suppose you're not going to use the transistor as a switch,
because Hfe values are given with a considerable constant voltage (10volt) across the transistor.
For switching (which is more used on this forum) you need to look at the saturation graph.
Leo..
BJTs are characterized by their h parameters (lower case), hFE (also called beta) is the DC current gain of the device. There is also an hfe which is the AC current gain. This follows the convention in electronics that upper case variables are used for steady state or DC conditions and lower case variables are used for instantaneous or AC conditions.
