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Calculating ßdc

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muashr

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Hi,

I have 7 measured values of IC for 7 different values of IB and now I want to calculate βdc and therefore I have to divide IC by IB at the operating point. However, what I don’t understand is how to determine operating point. Or stated otherwise which of the 7 values of IC and IB correspond to the operating point of the transistor?
 
Beta/hfe is going to change some with various operating points, and change radiacally as you approach the non-linear points of saturation and cut-off. Just pick a spot half-way between saturation and cut-off and calculate it there. Don't forget that good circuit design will allow for some pretty radical varations in beta as the circuit parameters will be much larger than affecting transistor parameters. For instance, the gain of a simple common-emitter amplifier should be dependent primarily upon unbypassed emitter resistance and collector resistance while a change in beta from 200 to 300 won't affect the gain. If that weren't true, we'd have to have one or two tweaks in every sub-circuit of a large system that would need adjusted every time a transistor was replaced.
 
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The operating point is whatever the design needs. It can vary from design to design.

The values you measured are for that particular transistor and may be near the typical, minimum, or maximum values for that transistor type, depending upon the particular transistor you measured. Those measured values should not be used to generate a robust design.
 
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