[SOLVED] Transistor Forward DC gain

First Hi to All the People because it is first time I am writing to any Forum.
I am doing B.Tech in Electronics I have read hundred of Books on Transistor but didnt find much details regarding basic concept.
For example Transistor 2N3904 has a maximum Ic 200mA and generally in all the books its DC gain is given as 100 but when you read data sheet the gain table is very complex. Ic from 0.1mA to 50 mA the gain is 30 to 40 with 100 at Ic 10mA.
My point is we all know there is no calculation possible without DC gain even in voltage divider bias if you want to calculate base current you have to use DC gain but when I see variations in the DC gain how can we use DC gain in calculations. If the gain is maximum at 10 mA then why the manufacturer made current rating to 200mA.
Second why there is no DC gain stated above 50mA in data sheets how come we know what would be the gain above 50mA.
Please do help me....[/B][/B]

well i am no expert...i too am a student...third year now. if i understood ur question u are concerned of the hfe wide variation...
when designing circuits u'll find that u usally design the circuit in a way that the hfe variation has minimal effect on the overall performance of ur circuit.

for example using the BJT for switching devices , hfe is not needed at all....for amplifiers , u will use feedback and amongst others minimize the hfe variation effect on ur amplification...

take a look at some OPamps datasheets(with BJT) u should find the total amplification is in the ranges of 6,7000-9,1000 v/v with big error margin, but when u use the OPamp in ur circuit with negetive feedback the amplification is usually around up to max 10-20 v/v with higher percision (and other good characteristics)

like you i too would appreciate any other insight from any other more experienced engineering/
thanks for posting this issue and i hope i was helpfull

I like to emphasize again the most important statement from pelvisp: negative feedback.
It is known that hfe is not a constant but varies with Ic and - more than that - has a large tolerance (as can be seen in the data sheet).
To cover this large amount of uncertainty during design of amplification stages, negative feedback (emitter degeneration) is applied which drastically reduces the impact of these variations. At the same time, also influences from temperatur variations and/or resistor tolerances are reduced.