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transistors, and capacitors

Discussion in 'General Electronics Discussion' started by tedstruk, Apr 6, 2017.

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  1. tedstruk

    tedstruk

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    Jan 7, 2012
    A transistor is basically a capacitor with an attitude. Rather than open after it heats up a little, It refuses to open unless power is supplied to the base. Same basic deal, different application conditions. I have a cheap Harbor freight multimeter, with NPN and PNP testing capabilities. It tells me the HFe of the transistor. Where in my basic electronics did I miss what HFe is?
     
  2. Audioguru

    Audioguru

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    Sep 24, 2016
    The hFE is shown in all transistor datasheets to be the current gain. A transistor with an hFE of 100 when its collector current is 50mA will have a base current of only 50mA/100= 0.5mA when it is active and is not saturated.
    The hFE varies with the collector current as shown in a graph in the datasheet, it drops at higher currents.
    I have used a multimeter hFE test to match transistors.
     
  3. Arouse1973

    Arouse1973 Adam

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    Dec 18, 2013
    Just curious, why is a transistor like a capacitor, am I missing something?
    Adam
     
  4. Ratch

    Ratch

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    Mar 10, 2013
    A transistor is transconductance device (voltage controls current). It cannot be compared to a capacitor because a transistor is an active device, whereas a capacitor is not. The base current of a transistor is an indicator of the collector current (CC), but it does not control the CC. Vbe is what controls the CC in the active region of operation. The beta or Hfe is a measure of the unavoidable base waste current produced during device operation from carrier diffusion. A high Hfe transistor has less waste current for the same CC than a low Hfe transistor. If you drive the base with a current source, you have a current amplifying circuit which includes a transistor, not a lone transistor current amplifier.

    Ratch
     
  5. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Jan 21, 2010
    It is nothing like it.

    I recommend you learn some basics about electronics.

    Please don't make baseless statements like this. The is a small but real chance that someone might read what you say and think it has some basis in fact.

    At least this post is not in response to someone else. I won't have to delete it.
     
  6. OBW0549

    OBW0549

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    Jul 5, 2016
    That explanation was probably in the same section you skipped over, the one where they explained that transistors are absolutely NOT anything like capacitors.
     
    Arouse1973 and LectricCircuit like this.
  7. Audioguru

    Audioguru

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    Sep 24, 2016
    I wonder why all transistor datasheets do not mention transconductance but show current gain instead?
     
  8. Ratch

    Ratch

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    Mar 10, 2013
    Correct, caps store energy, transistors don't.

    Ratch
     
  9. Ratch

    Ratch

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    Mar 10, 2013
    Although a transistor is a transconductance device, it it is not a linear parameter because of its diffusion operation. Driving the transistor with a current source shows a linear relationship with the collector current. This is because the high resistance of the current source swamps out the variation of the base-emitter resistance. Therefore, the base current vs. the collector current is of use. The base current is an indicator the the collector current.

    Ratch
     
  10. davenn

    davenn Moderator

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    Sep 5, 2009
    the OP's errors and Q's have been pointed out by several

    as point out by others, this statement is just so wrong

    thread closed
     
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