beta 2n3904 bjt change how????

hi need help plz

i use the 2n3904 bjt in fixed bias circuit
and determine beta for it and it was 103

then i replace The 2N3904 With The Other 2N3904 so i get beta= 151

why beta change here ?

safo2t said:

hi need help plz

i use the 2n3904 bjt in fixed bias circuit
and determine beta for it and it was 103

then i replace The 2N3904 With The Other 2N3904 so i get beta= 151

why beta change here ?

Click to expand...

hi,
Look at the spread for HFE from the datasheet.

They cannot make transistors exactly the same unless they are on the same chip like on an IC.

Many things change beta: temperature, voltage and current.

If the bias circuit is designed for the minimum specified beta, then any of the 2N3904 transistors will work in the circuit.

thx for help guys
u helped me so much

You really can't use BJTs in fixed bias circuits because beta goes up with temperature. This causes thermal runaway (the hotter it gets, the more heat it produces), meaning that your transistor will either saturate or explode, depending on the circuit, after a short time.

If you need to use a fixed bias for some reason, and are OK with having to calibrate each individual circuit that you make (it's not for mass production, in other words), then I would recommend using a MOSFET. When they get hot, they conduct less current for the same gate voltage, which gives a form of self-regulation.

A properly designed fixed bias transistor has an emitter resistor as DC negative feedback. Its current barely changes with temperature change.
The emitter resistor also allows a high beta transitor and a low beta transistor to work the same.