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Transistor turn ON

  1. Jun 7, 2009 #1
    What is the requirement to turn on a transistor. I always thought Base-Emitter junction should be forward biased.
    But I was looking at the TTL inverter and in that ckt, the collector of one transistor is connected to base of another transistor. How do you find out the voltage on the collector?
    Ic=beta*Ib - is it valid even when B-E junction is not forward biased?
    I am normally used to seeing B-E junction fwd biased and C-B reverse biased. The TTL inverter is a bit confusing.

    Link - fig 10-16, page 159.
    http://books.google.com/books?id=Qq...Ni1Cw&sa=X&oi=book_result&ct=result&resnum=10
     
  2. jcsd
  3. Jun 7, 2009 #2
    You might be confused that the TTL input is normally high "1", and needs to be pulled down to get a "0". For inverters with an open collector output, the output is an open npn collector with the emitter grounded. It likes to pull down about 1k 9depending on TTL series).
     
  4. Jun 7, 2009 #3

    dlgoff

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    I couldn't load the link but I think that when the collector of the first transistor is connected to the base of the second transistor, the first transistors B-E junction will be forward biased until the emitter is brought low.

    Check out http://en.wikipedia.org/wiki/Transistor-transistor_logic#Theory".
     
    Last edited by a moderator: Apr 24, 2017
  5. Jun 7, 2009 #4
    In TTL, one of the transistors operates in the reverse mode of operation, i.e. the base-collector junction gets forward biased, and current flows from emitter to collector. The "current gain, beta", is not "hfe", but "hfc", which is typically 0.01 to 0.02. A good textbook on TTL will elaborate.

    Claude
     
  6. Jun 8, 2009 #5
    dlgoff, so the input transistor's emitter is 0V, the current flows from its base to emitter.
    When its emitter is connected to 5v, the current from base flows to collector and also thru the Base-emitter of transistor V2. correct?
    In the second case, there is a base resistor, CB junction diode, BE junction diode and emitter resistor in the current's path.
     
  7. Jun 8, 2009 #6

    vk6kro

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    The first transistor is initially behaving like a diode. If you test a NPN transistor's collector base junction, it conducts when the base is made positive relative to the collector.

    So, there is a base resistor feeding a diode feeding the base of the second transistor, which will turn on as a result.

    If you ground the input, the first transistor behaves like two diodes, one of which is grounded and takes all the bias current to ground and another diode which is now reverse biassed and carries no current at all. This is because the first diode's anode (the base of the transistor) is now 0.6 volts above ground but the collector has to be two diode voltages above ground before it will conduct.

    So, the second transistor can't turn on when there isn't enough voltage to turn it on.
     
  8. Jun 8, 2009 #7

    dlgoff

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    Yes.
     
  9. Jun 8, 2009 #8
    Thanks guys.
    Going back to the original question, I still don't understand the rationale of connecting pull up resistor between diode cathode and 5v.
     
  10. Jun 8, 2009 #9

    mheslep

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    Well backup, forget the mechanism for a moment. What behaviour do you want to see there? (what voltage under what states). Then start asking yourself what the pull up when connected.
     
  11. Jun 8, 2009 #10

    vk6kro

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    Going back to the original question, I still don't understand the rationale of connecting pull up resistor between diode cathode and 5v.

    There is no diode cathode connected to a pullup resistor.
    The transistor base acts as an anode for two diodes connecting to collector and emitter.

    The diode from the bottom of the resistor to the second transistor is conducting and delivers base current to the second transistor. That is what the resistor is for. It is just a base bias resistor.
     
  12. Jun 8, 2009 #11
    See attached PDF showing open collector NPN with resistor pullup.
     

    Attached Files:

  13. Jun 8, 2009 #12
    VK6kro,

    If you look at this link - http://books.google.com/books?id=Qq0...sult&resnum=10 [Broken]
    The output of the gate is taken at the collector of the bottom transistor(say T4). When T4 is ON, output is LOW. When T4 is OFF, output is HIGH. When T4 is OFF, one way of looking is that the output terminal is connected to diode cathode. Agree?
    Now, a pull up resistor is connected between gate output and +5V. Gate output is nothing but the cathode of diode. So the resistor sees 3.3V on one end and 5V one the other. Why do we see 5V at the output?
     
    Last edited by a moderator: May 4, 2017
  14. Jun 9, 2009 #13

    vk6kro

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    That link doesn't work here.

    With the original circuit you asked about, I have given you an explanation:

    The first transistor is initially behaving like a diode. If you test a NPN transistor's collector base junction, it conducts when the base is made positive relative to the collector or the emitter.

    So, there is a base resistor feeding a diode feeding the base of the second transistor, which will turn on as a result.

    If you ground the input, the first transistor behaves like two diodes, one of which is grounded and takes all the bias current to ground and another diode which is now reverse biassed and carries no current at all. This is because the first diode's anode (the base of the transistor) is now 0.6 volts above ground but the collector has to be two diode voltages above ground before it will conduct (because two diode junctions follow it.)

    So, the second transistor can't turn on when there isn't enough voltage to turn it on.
     
  15. Jun 9, 2009 #14

    vk6kro

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    See attached picture. Is this the one you are talking about?

    Incidentally, the output of TTL is never 5 volts. It is more like 3.5.
     

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  16. Jun 9, 2009 #15
    Only the TTL totem pole output has a 3.5 v output for high state. The open collector output with a pullup has a 5 volt output (@ no current).
     
  17. Jun 9, 2009 #16

    vk6kro

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    Thanks Bob.

    I forgot about the open collector TTLs.
    They could not OUTPUT 5 volts but they could STAND up to 36 volts if you put that on their output transistor.

    One was the 7406.
     
  18. Jun 9, 2009 #17
    Guys, I am asking why the output is 5V when a pull up is connected to the cathode of diode and supply voltage(5v).
    I have no doubt about open collector.
    vk6kro, thats the ckt i am talking about.
     
  19. Jun 9, 2009 #18

    vk6kro

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    I guess I already answered your question.....

    Incidentally, the output of TTL is never 5 volts. It is more like 3.5.

    But this was your original question:
    What is the requirement to turn on a transistor. I always thought Base-Emitter junction should be forward biased.
    But I was looking at the TTL inverter and in that ckt, the collector of one transistor is connected to base of another transistor. How do you find out the voltage on the collector?
    Ic=beta*Ib - is it valid even when B-E junction is not forward biased?
    I am normally used to seeing B-E junction fwd biased and C-B reverse biased. The TTL inverter is a bit confusing.


    How did it become this:

    Guys, I am asking why the output is 5V when a pull up is connected to the cathode of diode and supply voltage(5v).
     
    Last edited: Jun 9, 2009
  20. Jun 10, 2009 #19
    vk6kro, let me apologize first.
    I had posted a question on pull up resistor in another post. I thought this was that.
    Anyway, here's the question - the simplest way to interface TTL to cmos is to connect a pull up resistor to the output of TTL. If the output is open collector type, then I don't have a problem understanding how it works. But if the gate is like the one in your attachment, then how does a pull up resistor actually work. One end is connected to supply and the other, where we take the output is connected to the diode cathode. I say diode cathode bcoz the lower transistor is off.
    Thanks.
     
  21. Jun 10, 2009 #20
    ok. Got it. When a pull up resistor is connected to the diode cathode. The upper transistor base needs 2 diode drops plus 5v to turn it on. So basically, upper transistor never turns on when a pull up resistor is connected.
     
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