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Transistor biasing question ?

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tron87

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How do i know which hFE value to use from a bc548 npn transistor data sheet for biasing the base to get the required current on the collector that is controlled by base current ?
For example if its 100 gain i choose resistor input 1 ma into base and get the expected 100ma on the collector-emitter junction?


Does base-emitter voltage change or remain consent at around .6v,.7v?

Does hFE gain change or remain constant in a bipolar npn transistor?
 
Due to processing variance, the DC beta of bipolar transistor will have a range of about 5 to 1 ratio. The bias circuit design should handle this variation.

The Vbe does change slightly for variation in DC beta but it is only a few millivolts variance.

Vbe and DC beta changes over temperature also.
 
The large variation in transistor DC beta is the reason current biasing in not usually used. Typically a resistor is added to the emitter and a resistive divider is used at the base to voltage bias the device (see this).
 
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Adding the resistor to the emitter results in current biasing as well. By setting the emitter current to a known value, the collector current is Ie*alpha. The alpha parameter is beta/(beta+1). Alpha is very stable in value. If beta varies from 50 to 500, alpha will only vary from 0.980 to 0.998.

I don't know of any such thing as "voltage biasing" of a bjt. It would be thermally unstable. Nobody ever controls Vbe directly. We emitter current bias the device. We do not base current bias the device because of beta dependency.
 
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I don't know of any such thing as "voltage biasing" of a bjt..
You took my use of "voltage biasing" too literally. By voltage biasing I mean that by placing a properly valued resistor in the emitter circuit, the emitter current (and thus the collector current) is largely determined by the base voltage, and can be made relatively insensitive to variations in Vbe or beta.
 
"Base voltage to ground" is what you are referring to. That would be correct, since that voltage determines the emitter current. Ic is then "alpha*Ie". "Voltage biasing" was ambiguous, but your clarification makes sense. A good bias stage is beta-proof & Vbe-proof, as well as temp-proof, rail-proof, etc.
 
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