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Why in theory transistors have fixed β but in reali

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wolfheart_2001

wolfheart_2001

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BJT hfe

hi,
i used to use transistor's in theory with fixed β(hfe) but in practice i found for that the 2n3904 transistor (ex..)has many values for β, each for specific Ic value ,so this mean any analog input will not be amplified linearly. can anyone explain this to me!!
 

Re: BJT hfe

wolfheart_2001 the thing is that when you design a discrete transistor amplifier you have to ensure that your output is not beta dependent.

the reason is as you have found out that different transistors of the same part number have different values of beta and infact transistors on the same silicon will have different values of beta. and to make things worse beta changes with temperature and that change is certainly not linear.

there is a solution to this problem. in discrete transistor amplifiers we use current feedback by including an emitter resistor so that the design doesnt depend on beta. you can find such circuits in textbooks or on the internet.

so whenever you design an amplifier make it beta proof!!!!
 

Re: BJT hfe

dear samcheetah,
can you give me a name of a good book or a webpage that discuss trans's in practice, since all the books i read suggest to use single β for transistors
and give little attention for practical usage.
 

Re: BJT hfe

well yes of course

i would recommend the following books

The Art of Electronics by Horowitz and Hill
Microelectronic Circuits by Sedra Smith
Electronic Devices and Circuits by Bogart

and in addition to the above books i would recommend you to go to the webpage

http://amasci.com/amateur/transis.html
 

Re: BJT hfe

wolfheart_2001
the reason can be explianed through the Gummel plot ....
there are three regions of β -

1) @low-level injection β increases as Ic increases, because the base current increases slowly compared to the Ic due to generation and recombination in the base.
β « Ic^(1-1/n) ,n -> recombination factor

2) @high-level injection β « Ic^(-1) because Ib increases faster than Ic. This region is useful in power applications.

3) @mid-level injection β is constant. It is this region in which most amplifiers operate.
....u can refer the book by 'streetman' for additional info

regards
 

Re: BJT hfe

actually β depends on depends on doping concentration thats y it varies from IC to IC but for paricular IC it will be constant
 

Re: BJT hfe

haseebcu said:
actually β depends on depends on doping concentration thats y it varies from IC to IC but for paricular IC it will be constant
Click to expand...

it also varies for a particular transistor with changes in the ambient temperature. so, this β is a really nasty character :D
 

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