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I am trying to figure out "beta" value of an NPN transistor, but I am not allowed to measure current directly. (Although I do have access to lab equipment and other components such as multimeters, resistors, etc). I figure I must use either

beta = (I_c/I_b)

or

beta = (I_e/I_b)-1

but I'm not sure how to find and of the branch currents without knowing beta to begin with. As you can see I'm in a bit of a catch 22. Any guidance would be much appreciated.

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  • \$\begingroup\$ You are not able to look it up on the datasheet? \$\endgroup\$
    – user92289
    Commented Dec 2, 2015 at 3:06
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    \$\begingroup\$ This sounds like a homework question, given that you are "not allowed" to measure current. Anyway, V=IR may be of use here if you put a resistor on each leg. \$\endgroup\$
    – Tustique
    Commented Dec 2, 2015 at 3:47
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    \$\begingroup\$ Yeah, if you are allowed to measure voltages, and you have resistors of known value, you can just build a circuit that reveals beta when voltage is measured. Make sure you are not in saturation. You will get the "forced beta" not the true beta. \$\endgroup\$
    – user57037
    Commented Dec 2, 2015 at 5:29

1 Answer 1

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The beta of a transistor is a set of Ic and Ib combinations.This means that it varies(hfe=x-y).You can make use of the fact that you know what the resistance will be,using resistors.Now we'll think about the 3 transistors states :saturation,active mode and cut off.Take a beta of 10 and see if your transistor is saturated.Now if you change the value of the base resistor,you shouldn't notice any change in collector current(try connecting some motor to the collector).The beta you will get will be the forced beta.It's not yet what you need.Keep decreasing the base resistance until you notice a change in Ic.Now you will find the actual beta of the transistor(you'll be in the linear region-active mode).Collector current is now affected by Ib.Write this one down and keep decreasing base resistance until it shuts down and enters the cut-off state.The beta before your BJT turned off is the last.Write this one down,too.Now you have your answer(example:200-500).Note that for this,you should use beta= Ic/Ib.

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