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BJT with non constant beta-value burned out?

  1. Aug 2, 2010 #1
    Hello, I have taken some measurments following a proposed experiment in a begginners electronics book to calculate the beta-value of an NPN BJT transistor. The circuit was as follows:

    ____________
    | | |
    | | |
    + R_B R_C
    9V | C_|
    - B
    | E
    |_______ _|

    EDIT: of course it's not clear, at least in my browser. Basically it's a 9V battery with the + that has two branches, one that goes to the Base of the transistor and one to the Collector, each branch has in series a resistor, respectively R_B and R_C. The minus is attached to the Emitter.

    hope it's clear... E B and C are the tranistors emitter, base and collector, R_C a 1kOhm resistor and R_B was a resistor whose value changed during the experiment. Also, not shown, there were two amperometers in series respectively with R_B and R_C (to measure I_B, base current, and I_C, collector current) and a voltmeter between C and E (to measure the collector-emitter voltage).

    So, what *should* have happened is that, starting with R_B=1MOhm and decreasing this value (I didn't use a potentiometer but different resistors), I should have obtained increasing values of I_B and of I_C in such a way that there ratio, beta=I_C/I_B would be more or less constant.

    However, I got the values that can be seen in the image:
    http://i.imgur.com/y9EZZ.jpg

    With R_B=1MOhm beta is 50, and then it constantly decreases till it reaches 1.2!! This is strange...

    So, given that I desoldered the transistor out of a previous made PCB, could I have damaged it by desoldering? Is this non-constant-beta-value a typical symptom of having burned out the transistor?

    thanks for any help,
    renato
     
  2. jcsd
  3. Aug 2, 2010 #2

    uart

    User Avatar
    Science Advisor

    In the last 3 measurements the transistor is saturated. You can't really make a sensible measurement of beta in a saturated transistor so you definitely should ignore those last three lines. (You *really* need to understand that the collector current is limited by the collector resistor and not the base current in this case).

    As for the other measurements they look ok. It’s normal to have some variation in beta with both Ic and Vce. Normally I would have expected beta to increase somewhat with increasing Ic (at least up to several mA) and to decrease somewhat with decreasing Vce. Overall though, the range of variation that you got in beta (25 to 50) is not too large.
     
  4. Aug 3, 2010 #3
    thanks for pointing that out! yeah I think I got it

    Oh, ok. I thought it was large because on the book there was a sample table and beta value for them was allways 100, only one or two times 98; so I expected that kind of variancy (though they warned that they had used high precision equipment and mileage could vary)
     
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