TTL Circuit Help

Discussion in 'Homework Help' started by kou, Oct 15, 2014.

  1. kou

    Thread Starter New Member

    Oct 15, 2014
    3
    0
    So, I'm trying to solve a question about a TTL Gate, but I'm kinda stuck.

    I have this circuit:

    [​IMG]

    And I only have to answer if the diodes and transistors are ON or OFF.
    But I'm not sure what T3's behaviour would be when, let's say, e1 is low and e2 is high, since both T1 and T2 are connected to T3.

    Can somebody help me, please?
    Thanks in advance.
     
  2. MrChips

    Moderator

    Oct 2, 2009
    12,415
    3,354
    T1 and T2 are in parallel. Since T1 is OFF you can ignore it. T2 being ON overrides any effect of T1.

    If T2 is ON, is T3 ON or OFF?
     
  3. kou

    Thread Starter New Member

    Oct 15, 2014
    3
    0
    So T3 will be ON, right?
    That's what I thought too, but then I saw this on the site:
    [​IMG]

    And it said:
    This got me confused.
    Ooh, I think I understand it now. Q5 will be turned on, but since it's a pull-down transistor, the output will be low.
    Is that it?
     
  4. crutschow

    Expert

    Mar 14, 2008
    12,977
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    For reference, that's not a TTL gate, it's a diode-transistor gate (DTL).
     
  5. kou

    Thread Starter New Member

    Oct 15, 2014
    3
    0
    Oh, I forgot to thank you.

    Thank you, @MrChips :)


    @crutschow : I see, thanks for correcting me.
     
  6. MrChips

    Moderator

    Oct 2, 2009
    12,415
    3,354
    In your second example, Q5 is called open-collector output. The output needs a pull-up resistor in order to present a proper LOW/HIGH logic output.
     
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