2N2222... hfe?

Discussion in 'The Projects Forum' started by rougie, Aug 29, 2012.

  1. rougie

    rougie Thread Starter Active Member

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    Hello,

    I am getting into some basic transistor calculations.

    I am using a 2n2222 transistor and I have done the following calculations as shown in the attachment. However, as shown in the 2N2222 spec sheet attachment it says that the HFE should be determined by the corresponding ic calculations.

    If you look at my calculations, my ic is approximately 1.3 ma. According to the spec, shouldn't my HFE be somewhere around 60??

    However, calculations with a HFE of 60 don't make sence with the actual physical readings? But HFE of 100 is much closer to the values I measure.

    So, in my case how is one supposed to determine the HFE for a given transistor such as the 2N2222?

    Thanks for all help!

    Attached Files:

  2. Bill_Marsden

    Bill_Marsden Moderator Staff Member

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    BJTs are wildly variable, even within the same lot. Each transistor has its own gain values, you design to allow for the variation. It has been like this pretty much from day on for BJT parts.
  3. paulktreg

    paulktreg Active Member

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    The HFE of any particular device isn't set in stone, it varies from one transistor to another of the same type. Looking at the datasheet here HFE can be anywhere between 35 and 300 for the 2N2222.
  4. rougie

    rougie Thread Starter Active Member

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    >>>The HFE of any particular device isn't set in stone, it varies from one transistor to another of the same type. Looking at the datasheet here HFE can be anywhere between 35 and 300 for the 2N2222.

    Oh! I see what you are saying.... It is up to me to pick the HFE allowed by the part which would best suite the gain I need!!!

    Thanks all for your help!
    regards
    r
  5. crutschow

    crutschow AAC Fanatic!

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    More specifically, you design the circuit so that it will operate within the range of Hfe that the transistor may have. Typically this is done by using negative feedback, such as from an emitter resistor, so that the minimum gain the stage has is less than minimum Hfe.
  6. rougie

    rougie Thread Starter Active Member

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    now that i think about it ... how can this be?

    if i select 200 as my hfe and use it in my ic calculations....
    ic would equal to 2.6 ma???

    At 2.6 ma my calculations would not match the measured values??

    "you design the circuit so that it will operate within the range of Hfe that the transistor may have. Typically this is done by using negative feedback, such as from an emitter resistor, so that the minimum gain the stage has is less than minimum Hfe."

    using my diagram, can you illustrate an example!

    thanx
    r
  7. ramancini8

    ramancini8 Member

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    You design the circuit such that it will operate from the minimum specified Hfe, 35 in your case. If you do not have feedback high Hfe can cause saturation. You need feedback to stabalize the design.
  8. rougie

    rougie Thread Starter Active Member

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    What do you mean "35" in my case?? I am using "100" in my case!!!

    ic = B * ib
    ic = 100 * (2 - 0.7 / 100000) = 1.3 ma

    if I do it with 35:

    ic = 35 * (2 - 0.7 / 100000) = 455uA??

    455uA is NOT what I measure on the bread board??

    what I measure on the breadboard is approximately 1.3ma which is calculated with the 100 HFE!


    >If you do not have feedback high Hfe can cause saturation. You need feedback to stabalize >the design.

    I am sorry, in reference to my diagram, I am not familiar with the term "Feedback". I guess, "feedback", is used in more advanced transistor circuits than mine... Right now I would like to understand the simple "common emitter" transistor circuit. However, as my calculations are, at gate voltage of *2 volts*, I am not in saturation mode.

    All I am asking is, based on my calculations in my initial post, why did I use 100 as HFE... why not 35, 70, 128, 200... which all would of given me wrong calculations when compared to the actual circuit measurements. I don't get it... discouraged.

    r
  9. rougie

    rougie Thread Starter Active Member

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    I guess what you guys are trying to tell me is that if I would use 200 HFE in my calculations, and I would want a VCE of 3.7 V I would have to lower the value of RC thus increasing ic ... right?

    So if I would of selected a HFE of 200. then:
    ic = 200 * (2 - 0.7 / 100000) = 2.6 ma

    And if I required a 3.7 v at VCE, then
    RC1 = 1.3VRC/2.6ma = 500ohms

    The only diff is that there would be a greater current of ic and my 1K would of reduced to 500 ohms...

    r
    Last edited: Aug 29, 2012
  10. Markd77

    Markd77 Senior Member

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  11. rougie

    rougie Thread Starter Active Member

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  12. Audioguru

    Audioguru New Member

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    You cannot order a transistor with a certain amount of hFE. You get whatever amount they have available. It might be low, medium or high. Temperature also changes the hFE.

    A transistor might have a low hFE or it might have a high hFE which is shown as a range of numbers on its datasheet. You do not "pick" the hFE.
    Transistor amplifier circuits are designed using negative feedback so that the circuit works properly with any transistor whether its hFE is low or is high.

    If the transistor is used as a simple on-off switch then its hFE is assumed to be only 10 because the saturated hFE is very low.

    You mentioned "gain". hFE is current gain, not voltage gain.
  13. rougie

    rougie Thread Starter Active Member

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    <
    A transistor might have a low hFE or it might have a high hFE which is shown as a range of numbers on its datasheet. You do not "pick" the hFE.
    >

    Don't mean to be stubborn here and I do apollogize if I am
    but the 2n2222 that I used in my example comes with a range of 35 to 300 I
    believe... And I somehow found a tutorial that exclusively picked
    100????

    Why 100?

    If one needs to completely understand all the finest details
    about transistors to understand why this is so, then I will
    continue learning....

    <
    If the transistor is used as a simple on-off switch then its hFE is assumed to be only 10 because the saturated hFE is very low.
    >

    How 10?.... The spec says 35 to 300???

    I know that transistors come with a predetermined HFE range.
    But all I want to do is use a transistor as an on/off device.

    I did my calculations with a HFE of 100 (and not 10) and using 6 volts as opposed to 2volts at gate I am still able to use it as an on/off switch.

    The only problem is I need different voltages and currents!

    still confused... But ready to put it to rest LOL
    Regards
    R
  14. Papabravo

    Papabravo AAC Fanatic!

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    The only reason a tutorial would pick a value is to walk you through the calculations you would make when and if you know the value. "Now your challenge, Jim should you decide to accept it, is to design a circuit that DOES NOT care what value of Hfe a transistor has and still meets the design requirements."

    "This message will self-destruct in 5 seconds and the Secretary will disavow all knowledge of your actions"
    --loosely adapted from a popular TV show "Mission Impossible"
  15. rougie

    rougie Thread Starter Active Member

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    i watched an interesting video at:

    http://www.allaboutcircuits.com/videos/61.html

    it seems that a transistor comes with the possible range of hfe values. so the manufacturer guarantees that the gain can vary somewhere between min hfe and max hfe based on construction, temperature and so forth.

    i am new to this transistor stuff, but if i were to design a circuit that doesn't care about what the hfe is, i would have to read up further about transistor theory... but hey an insight would be very welcome :)

    thanks for your reply.... its appreciated

    r
  16. Audioguru

    Audioguru New Member

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    Negative feedback in a transistor circuit reduces the range of hFE, reduces the amount and range of voltage gain, reduces distortion and widens the bandwidth.
    Frequently the negative feedback is produced by a resistor to ground in series with the emitter.

    Attached Files:

    absf likes this.
  17. rougie

    rougie Thread Starter Active Member

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    <
    Negative feedback in a transistor circuit reduces the range of hFE, reduces the amount and range of voltage gain, reduces distortion and widens the bandwidth.
    Frequently the negative feedback is produced by a resistor to ground in series with the emitter.
    >

    Okay so it's an emitter bias type circuit!

    Thanks for your help!
    r
  18. ramancini8

    ramancini8 Member

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    The data sheet shows mostly minimum values. The beta range at 150mA is 100 to 300, and that shows the variability of beta. To measure beta, measure Vce and use this formula Ic = (Vcc-Vce)/Rc. What is your measured Vce? Your calculation for Ic should be Ic=Hfe(.013mA). Get Ic from the measured value of Vce and calculate Hfe. If you have a mA meter you can measure Ic and Ib.

    With a guaranteed Hfe range of 3 to 1 you can see how a large value of Rc can limit the range of Vce causing saturation, and that is why feedback is used. If you want to persue this subject more contact me offline.
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