2N3904 hef readings

Discussion in 'General Electronics Chat' started by tk3000, Mar 18, 2013.

  1. tk3000

    Thread Starter New Member

    Sep 11, 2011
    4
    0
    Hello,
    I have some small signal transistor 2N3904 (that shall be deployed amplifying an analog signal which have been converted to analog through a resistor ladder). I tested some of the transistor in two different multimeters and obtained similar results: their hfe (gain) is around 310 to 320. It seems that the min. Is 100 and the max is 300, but one should expect to obtain a hfe readout of about 200. So I was wondering if that is the case and whether or not such transistors are out of specs.



    Thanks,
    Lazarus Wald
     
  2. ScottWang

    Moderator

    Aug 23, 2012
    4,853
    767
    You should change the title of your thread to "2N3904 hfe readings".
    If you using the multimeters to measure the hfe, the results that you got is not the same as spec, unless the conditions is the same as following.

    hFE, DC Current Gain,
    IC = 0.1 mA, VCE = 1.0 V, min = 40, max = 300.
    IC = 1.0 mA, VCE = 1.0 V, min = 70, max = 300.
    IC = 10 mA, VCE = 1.0 V, min = 100, max = 300.
    IC = 50 mA, VCE = 1.0 V, min = 60, max = 300.
    IC = 100 mA, VCE = 1.0 V, min = 30, max = 300.

    It meant that you should set the Ib and measuring the Ic and Vce as above, the multimeters measuring conditions maybe not match the same values as above.
     
  3. Jaguarjoe

    Active Member

    Apr 7, 2010
    770
    90
    I use an arbitrary value of 100 for hfe. It varies all over map from part to part.
    A good design does not depend on hfe to affect its operation. Wikipedia has a good explanation about transistor biasing which shows this..
     
  4. k7elp60

    Senior Member

    Nov 4, 2008
    478
    69
    If you use the minimum hfe of 100,as Juguar suggest in you design the circuit will always work for any 2N3904. The 2n3904 is normally a switching transistor, but it works very well as a linear amplifier.
     
  5. WBahn

    Moderator

    Mar 31, 2012
    17,719
    4,788
    When the spec says "max" it applies to the test conditions stated. It does NOT mean a global maximum that applies to all points withing the recommended (or absolute) operating range.

    If you look at the curves that are provided with most data sheets, such as

    http://www.fairchildsemi.com/ds/2N/2N3904.pdf

    You will notice that at elevated temperatures and, particularly, elevated Vce that the typical current gain can get quite large.
     
  6. Jaguarjoe

    Active Member

    Apr 7, 2010
    770
    90
    The opposite is also true. If the C-E saturation voltage goes below ~0.6v, the B-C junction will become forward biased. This will steal current from the B-E junction which effectively makes the hfe very low, like ~10 or so.
     
  7. WBahn

    Moderator

    Mar 31, 2012
    17,719
    4,788
    Very good point, which is why, if you want to get the kind of very low Vce values that are possible (again, look at the curves) you will typically shoot for base currents that are at least 10% of the collector current.
     
  8. tk3000

    Thread Starter New Member

    Sep 11, 2011
    4
    0
    Sorry about the newbie question. Thanks for all the great responses. I read somewhere that the impirical measurement in normal conditions with any average multimeter for a transistor with such characteristics (min 100, max 300) should produce a reading of 200; clearly that was misleading.
     
  9. WBahn

    Moderator

    Mar 31, 2012
    17,719
    4,788
    No need to apologize about basic questions -- that's one of the primary purposes of this forum. Also, no matter how experienced you will become, there will always be "basic" things that your experience hasn't covered. True, for most of them you will gain the skills needed to answer your own questions, but there will always be times when asking others a basic question is a reasonable thing to do. On top of that, in answering basic questions those of us with "experience" often learn something more about the subtleties involved in material that we already "knew".

    And, yes, I would say that whoever came up with that came about what the multimeter should read was clearly out to lunch.
     
    tk3000 likes this.
Loading...