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Thread: Driving Emitters in a TTL Gate

  1. #1
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    Default Driving Emitters in a TTL Gate

    Hello Folks,

    In a situation when two TTL logic gates, namely TTL-Out and
    TTL-In are connected, and the TTL-Out logic is HIGH....
    how can we have 40 uA of current flowing into the TTL-In gate
    when the inlet on the TTL-In gate is an emitter?

    ziloo

  2. #2
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    In a word--"leakage".

    In more than a word--"don't believe the datasheet schematics".

    For example, consult PDF page 12 on this document and prepare to be disabused.

  3. #3

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    It is a maximum design parameter, most won't draw anywhere near that much.
    Dwight

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    Quote Originally Posted by Chuck(G) View Post
    In a word--"leakage".....

    Alright...it is leaking....but where does it go?
    The emitter is forward-biased!

    ziloo

  5. #5

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    Quote Originally Posted by ziloo View Post


    Alright...it is leaking....but where does it go?
    The emitter is forward-biased!

    ziloo

    Actually, it is reverse biased or at least close to. You need to be more specific as to what condition it is measured at. Are you talking with the input at 5V or at a minimum logic one or what?
    Dwight

  6. #6

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    A transistor has 2 junctions. They are physically close to each other. They are so close that if you inject a current forward bias into the base emitter junction, holes or electrons ( depending on PNP or NPN ) get pulled by the reverse biased base to collector, into the depletion region of the collector to base. Once in that region, they can't go back, against the voltage, and become collector current.
    Surprisingly, if you turned the transistor upside down and wired the emitter to the collector circuit and the collector to the emitter circuit, it would still work the same but with reduced current gain ( as long as the collector circuit voltage was less than about 6.8V ). If the voltage was too high, the emitter to base would zener.
    In order to analyze the typical TTL input you'd have to understand how this happen. When the input voltage is high, of a TTL, the emitter is working like a collector and the collector is biasing the next transistor on, as though it were an emitter. In a regular TTL load there would be about 1.6ma flowing forward bias from the input transistors base, forward biased through the collector junction. As I said, when the biasing of the transistor is reversed, there is some gain making the emitter look like a collector, supplying 40 ua on top of the 1.6ma into the following transistors base lead, turning it on.
    I hope that helps a little.
    To get your terms right, Forward bias is current flowing, reverse bias is less current flowing. When you turn a transistor on, the collector current is flowing through what otherwise would have been a reversed biased junction except for the stray current carriers from the base, cause by the forward biased base to emitter. You can swap emitter and base leads and still have transistor action, just not as much gain.
    I should mention, the emitter base junction is highly doped, making it zener when reverse biased at about 6.8V, for a typical transistor.
    Dwight

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    Quote Originally Posted by Dwight Elvey View Post
    Actually, it is reverse biased or at least close to.....
    The bottom picture (lower case) shows the current flowing towards the emitter
    in the second TTL gate. Where does the flow go.....?

    Attachment 52212


    ziloo

  8. #8

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    Quote Originally Posted by ziloo View Post
    The bottom picture (lower case) shows the current flowing towards the emitter
    in the second TTL gate. Where does the flow go.....?

    Attachment 52212


    ziloo
    You are missing the transistor following the input transistor.
    The current is ( not electrons, in physics current flows from positive to negative. Ben Franklin got it wrong ) flows from the top transistor of the output through the emitter of the input transistor through the collector of the input transistor , along with the forward biased base current of the input transistor through the base emitter of the transistor that you don't show. With the input transistor in the reversed operation, it has a gain of about 0.025. Not a particularly useful gain but it is still some gain. The emitter is working like a collector and the collector is working like an emitter as a poor gain transistor.
    Dwight

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    I'm not following, but evidently Dwight is, thank heavens. Draw a box around everything but the power supply. The sum of the currents going into the PS is the same as the currents flowing the reverse direction. Simplify what you're thinking with Norton equivalents.

    Reverse operation of BJT's isn't common, but it does have its uses. I've seen BJT's operated in collector-to-collector or emitter-to-emitter configurations between the power rails. Germanium BJTs seem to be preferred over silicon.

  10. #10

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    He was right that the emitter is reverse biased. What he doesn't see is that the entire transistor is biased like the emitter is acting as a collector ( poor gain of 0.025 ) but, still, some gain from the current flowing through the base and the forward biased collector. It is still working like a transistor and injecting current carriers into the reverse biased junctions depletion region. It becomes current in the reverse bias emitter-base.
    Dwight

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