2n3904 - what resistor to base?

Discussion in 'General Electronics Chat' started by Razor Concepts, Apr 11, 2009.

  1. Razor Concepts

    Thread Starter Active Member

    Oct 7, 2008
    212
    1
    I'm having trouble figuring out what resistor I should use on base when wiring up a NPN transistor.

    So, everything runs at 5 volts. The maximum current a 3904 can handle is 100ma, so is it okay too choose a resistor for a 100ma current so anything I hook up will work as long as it takes less than 100ma?

    I was poking around and found that there is a voltage drop of about .6v. So say if the transistor has a gain of 50 (not sure what the 3904 has), and I want 100ma. So I would need a 2ma current to base. So using ohms law with 4.4 volts and 2ma, I get 2200 ohms. So a 2200 ohm resistor should work with any load up to 100ma- I think.

    Now how do I find the gain of the transistor in the datasheet? I am looking at this one:
    http://www.fairchildsemi.com/ds/2N/2N3904.pdf

    Thanks!
     
  2. sendniranjan

    New Member

    Apr 11, 2009
    1
    0
    hi i am an engineering student in ECE branch
    i think amplification in transistor depends upon resistance of collector which is forward biased.
    more is collector resistor more is the amplification.
    we connect resistor to the base only to prevent it from damege as base is lightly doped.
     
  3. electronictech

    Active Member

    Apr 1, 2009
    35
    0
    The gain of the transistor will vary slightly with the collector current. For most applications up to the 'Absolute Maximum Ratings', the typical values will suffice. You have the correct understanding of the current gain, or as I like to call it 'Beta DC'.....so you should be able to accurately measure the base current versus the collector current and derive the gain. Most inexpensive digital multimeters have a transistor checker that will give you the 'hFE' or 'Beta DC' of a PNP or NPN transistor, and that measured value will always be accurate.

    Anyways, have fun!
     
  4. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    Actually, for the 2N3904 you'll need a bit more base current when you're running it that hard (saturated).

    When Ic is 10mA, the hFE is optimal; a minimum of 100 and a max of 300.
    When Ic is 100mA, the hFE drops to around 30, and Vbe increases; the latter could be nearly a volt. You'll need to examine a datasheet.

    Since hFE drops to around 30, you'd need at least 3.33mA base current to get 100mA Ic. Let's just go with 3.5mA. Since Vbe increases to nearly a volt, you'll need to figure that in as well.

    If you will be supplying the base from a 5v source, you'll need a base resistor of (5v-Vbe)/3.5mA = (5-1)/0.0035 = 1143 Ohms. 1.1k Ohms is the closest standard E24 resistance that is less than 1143 Ohms. Chart: http://www.logwell.com/tech/components/resistor_values.html
     
  5. Audioguru

    New Member

    Dec 20, 2007
    9,411
    896
    The datasheet shows that its current gain is a minimum of 30 when its collector current is 100mA and its collector voltage is 1V which is not saturation.
    But it shows it saturating when its base current is 1/10th its collector current. So the base current should be 10mA, not just 3.5mA.
     
  6. eblc1388

    Senior Member

    Nov 28, 2008
    1,542
    102
    I agree with Audioguru about the increase in base current to get the 2N3904 to saturate.

    In the image, if we use Beta=10 to calculate the base current we can get Vsat down to only 0.13V at 100mA.

    [​IMG]
     
  7. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    If you want less current thru the base, use a more capable transistor like a 2N2222/PN2222.
     
  8. italo

    New Member

    Nov 20, 2005
    205
    1
    Hfe is just an empirical number it realy means nothing unless it is specificaly related to the Ibe and IC. For a 100 ma ic the 2n3904 is basicaly into the saturation region. So now it is a switch rule of thumbe a forcew beta of 10 or 10ma Ib. I think HIS question maybe how to bias a transitor as class Amplifier and if so then he need to re-introduce the question. just guessing.
     
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