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reliablity of hFE


shekhar_dandya
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I know that a transistor is biased a it's mid current value. From there the power must be decided on and that will determine the voltage. I suppose half power is reasonable. The real test is to determine the beta and the changes in VCE that will keep the beta constant. I would like to tell of an easy way to find that beta and and voltage. I think there does exist a Q point.

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Shekhar,
If you are talking about biasing the base of a common-emitter transistor, then you won't know what the value of hFE will be.
See the data sheet for the 2N3904 transistor here:
http://www.fairchildsemi.com/ds/2N/2N3904.pdf

1) Page 2 shows that at room temperature, the hFE can be from 30 to more than 300, depending on the transistor's tolerance and amount of collector current.
2) Page 3 shows that the hFE varies with temperature and amount of collector current.
3) Page 3 shows that the Vbe also changes with the temperature and amount of collector current.
So if you bias the base of a common-emitter transistor with simply a single resistor to its base from the supply voltage, then its Q-point could be anywhere.
To overcome the obstacles above, it is recommended to use negative-feedback, in the form of an emitter resistor, and a constant-voltage voltage divider to the base from the supply voltage (with the current through the voltage divider about 10 times the highest specified base current), and/or a feedback voltage divider to the base from the collector of the transistor.
Then the Q-point of the transistor will be very close to your plan.

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