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Current Source vs Current Sink?

Discussion in 'General Electronics Chat' started by dknguyen, Sep 10, 2006.

  1. dknguyen

    dknguyen Well-Known Member

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    I've just recently come by something called a current sink...but it looked a lot like a current source to me. What's the difference? Is it just the way they are used in the circuit similar to high-side vs low-side? It was something about NMOS sinking current and PMOS sourcing current, but I can't seem to find the article anymore.
     
  2. mister_e

    mister_e New Member

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    The terms refer to the direction of the current.

    If it's sink, it is flowing into the output. Let's say NPN, emitter to gnd and a load between Vdd and Collector will SINK
    If it's source, it is flowing out of the output. Let's say PNP, emitter to Vdd and a load between Gnd and Collector will Source.
     
  3. ikalogic

    ikalogic Member

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    hello,

    Most IC like op-amps and 555 timers can source and sink current, which means:
    Source: the currents direction goes from the IC to the load
    Sink: the current direction comes from the load to the IC


    Some ICs can only sink current but cannot source current like the TRISTATE i/o ports of an AT89C52 microcontroller

    Usualy the maximum sinking current for an IC is different from the maximum Sourcing current, depending on the internal circuitry of the IC.

    That's the only difference, as far as i know.
     
  4. dave

    Dave New Member

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  5. audioguru

    audioguru Well-Known Member Most Helpful Member

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    A current sink circuit pulls down, it is on the low side.
    A current source circuit pulls up, it is on the high side.
    Here are both as very simple constant current LED drivers:
     

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  6. dknguyen

    dknguyen Well-Known Member

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    Understood. Anyone know anything about the claim "NMOS sinks current better than PMOS, but PMOS sourcescurrent better than PMOS."

    I have always thought this was purely due to the gate voltage requirements to switch the MOSFET on and off, and nothing to do with the source-drain conduction...am I right?
     
  7. Roff

    Roff Well-Known Member

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    NMOS, in order to source, must be configured as a source follower. The output current into a load will the cause source voltage to rise, which will reduce the gate-to-source voltage, which in turn tends to reduce to output current.
    The same is true of a PMOS follower, but the voltages and currents are reversed.
     
  8. Dean Huster

    Dean Huster Well-Known Member

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    The guy above with the earmuffs:

    a) is an airline pilot
    b) is an outdoor audiophile
    c) forgot that Idahoans are supposed to wear cowboy hats
    d) has a chainsaw in one hand, so watch out
    e) is working on his memory
     
  9. dknguyen

    dknguyen Well-Known Member

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    (e) working on his memory. Got to stop stuff from leaking out.
     
  10. Roff

    Roff Well-Known Member

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    Hey, I'm a trend-setter in Idaho. Half the people in the state are now walking around wearing headphones and grungy baseball caps. Below is one of my trainees. Not nearly as cool as me, but then who is...
     

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  11. audioguru

    audioguru Well-Known Member Most Helpful Member

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    Ron wears the headphones from his metal detector. He frequently searches for pennies.

    The fuzz on my chin is beginning to look like his, I better have a shave.
     
  12. Hero999

    Hero999 Banned

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    Source followers are especially bad because the gate threashold is higher than a bjt. A MOSFET configured as a source follower driving a reasonable load can drop as much as 6V.
     
  13. Dean Huster

    Dean Huster Well-Known Member

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    My gosh, Ron. That picture must be old to have that old 12-inch hard drive to the guy's right. It must be at least 128KB, spinning at 33-1/3 rpm, huh?

    I forgot to include answer "f": "Trying to keep the top of his head from sunburning and/or developing a headband stripe."

    Dean
     

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