Confusion with basic transistor.Please help.

Discussion in 'General Electronics Chat' started by anhnha, Jul 22, 2012.

  1. anhnha

    Thread Starter Active Member

    Apr 19, 2012
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    Hi,
    Could anyone please help me point out whether there is current flow through the Rc resistor or not?
    In the circuit Vin is 0.8V is connected directly between base and emitter of transistor without any resistor is used.Assume that the wire has zero resistor and internal resistor of source also is zero ohm, so the current Ib will be zero amp and Ic is also zero amp? Is it right? I am not sure about this?
    Thanks.
     
  2. #12

    Expert

    Nov 30, 2010
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    .8 volts is enough to get a bipolar transistor to allow current to flow. Just look up any small transistor like 2n3904, 2N2222 and see what happens when the Volts base to emitter is .8
     
  3. anhnha

    Thread Starter Active Member

    Apr 19, 2012
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    Thanks,
    But assume that BE diode is conducted at Vbe is 0.7V, and than in the circuit
    without any resistance, is current will increse to infinite?
    And the same what will happen when BE diode is conducted at Vbe is 0.8V?
    What is the Ic?
     
  4. #12

    Expert

    Nov 30, 2010
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    Can you answer this question now or should we move this to the Homework Forum?
     
  5. WBahn

    Moderator

    Mar 31, 2012
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    If you apply a voltage directly across the BE junction, then you need to use the exponential diode equation in order to determine the base current. For most BJT near room temperature, the base (and collector) current change currents change by a factor of 10 for each 60mV change in base-emitter voltage.
     
  6. ramancini8

    Member

    Jul 18, 2012
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    If the source is applied positive to the base and negative to the emitter there is current flow at least for a while. With no curent limiting resistor, the B-E junction may burn out. If Vce is close to zero volts collector, and emitter, current is flowing, but if Vce is Vcc no current is flowing.
     
  7. WBahn

    Moderator

    Mar 31, 2012
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    As long at the applied Vbe results in currents the device can handle there is no problem. It will not be very stable and is susceptible to thermal runaway, but as long as a sufficiently low Vbe is used, that won't happen (not to say that this is a good idea, just that the lack of a current limiting resistor does not automatically equate to unrestrained currents).

    Also, if Vce is ANYTHING less than Vcc (not just close to zero, but anything that is less than Vcc), current is flowing (because there is a voltage drop across the collector resistor).

    But even this description relies on the emitter voltage being tied to 0V so that Vce is equal to the collector voltage, and it is the collector voltage relative to Vcc, not the collector-emitter voltage, that matters.
     
  8. takao21203

    Distinguished Member

    Apr 28, 2012
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    Base resistors are required , at least when the available voltage has higher potential than 0.7 ,, 0.8V

    Otherwise the transistor will draw large currents into the base, and operation will become disturbed. Or as for PNP, the base would emit large currents.

    I know because I tried. You need a small base resistor in most cases, differently to digital MOSFET, which are perfectly fine without one.

    Interestingly I also often observed voltage after the base resistor will become clamped automatically to a level around 0.7V (if we assume for instance NPN, 2k base resistor, and 5V = Vb = Vcc, let say, having a green LED in the path from Vcc to collector).
     
  9. WBahn

    Moderator

    Mar 31, 2012
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    If you can control the base-emitter voltage so that it matches the base current you are trying to achieve, then you do not need a base resistor. There are a number of ways of designing the circuit so that it includes a feedback mechanism to make this happen. One is to simply use an emitter resistor. But you can also take the voltage at some other point in the circuit that is directly related to the base current (or base-emitter voltage) and servo the base voltage using that.

    And also assuming the emitter is tied to 0V. Nothing interesting about it. When active, the base-emitter voltage drop is going to be that of a forward-biased diode.
     
  10. takao21203

    Distinguished Member

    Apr 28, 2012
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    Well I made some transistor testers but from what I understood myself.

    The very basics of BJT are clear to me. Maybe not all details.

    OP does have very basics questions so I believe for instance never built transistor tester.
     
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