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BJT saturation

Discussion in 'Electronic Design' started by [email protected], Mar 25, 2013.

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  1. Guest

    Regarding BJT saturation point, the books say that the transistor is said to be saturated if increasing Ib no longer increases Ic. For example, if I have a circuit where the the collector of NPN is at +5V through a 1K resistor, maximum Ic is 5ma. Assuming hfe is 100, above 50uA of Ib doesn't increase Ic due to 1K resistor. Is this the saturation point for the transistor? If this is the case
    then it largely depends on the current limiting resistor of Ic. Am I missing something here?

    -Mark JS
     
  2. mike

    mike Guest

    You're missing an objective.
    What are you trying to accomplish?
    Discussing a definition does little to make an undefined
    circuit work.
     
  3. Bill Sloman

    Bill Sloman Guest

    If you are seriously interested in getting a low saturation voltage,
    you use an inverted high-current-gain transistor, driving your base
    current through the base collector junction and treating the emitter
    as the collector.

    Old-fashioned textbooks still cover this.Trevor. H. Wilmshurst's
    "Analog Circuit Techniques" seems to be one of them.
     
  4. Jon Kirwan

    Jon Kirwan Guest

    When looking for a definition I can easily explain and others
    can precisely understand without equivocation, that's the
    view I take.

    Jon
     
  5. Tim Williams

    Tim Williams Guest

    Power transistors don't saturate?

    Silly Jimmy ;-)

    Tim
     
  6. Jamie

    Jamie Guest

    But shit for gain..

    Jamie
     
  7. Bill Sloman

    Bill Sloman Guest

    We'll forgive your ignorance.
     
  8. Guest

    That's a nice technique.
     
  9. mike

    mike Guest

    Can you explain how that works? Where you tie the collector can't
    affect the saturation characteristics of the transistor???

    Stated another way, can't you do the same thing with two diodes.
    Saturation not involved?????
     
  10. Bill Sloman

    Bill Sloman Guest

    <snipped John Larkin getting personal>

    But you didn't know that an inverted transistor does it better, which
    was common knowledge back in the 1970s.

    Obviously, you can bodge the saturation voltage down to zero, but the
    crucial point is that the offset voltage of a saturated inverted
    transistor is a lot less than that of the same transistor biased
    conventionally. I've no idea whether the bodge current for exactly
    zero offset would be more or less, but you start off with less offset
    to cancel out.

    One more case where you weren't paying enough attention as an
    undergraduate.

    I got an extended lesson in the technique back in 1974, when my boss
    at Kent Instruments in Luton had me put together a slow but cheap and
    accurate two-quadrant PWM-based multiplier, where we used two
    saturating switches in series to get a very low offset voltage when
    the swtiches were on. The first transistor was run conventionally, and
    sank the bulk of the current, while the second was run inverted to
    minimise the offset voltage. Slow, but 0.1% accurate.
     
  11. Robert Baer

    Robert Baer Guest

    At (the original) Fairchild, the C-B forward bias had to equal the
    E-B forward bias before we considered the transistor to not be active.
     
  12. Robert Baer

    Robert Baer Guest

    Check, but why limit to the NPN?
     
  13. Robert Baer

    Robert Baer Guest

    I picked a randumb NPN and that "reversal" is seen in the region of a
    forced beta of 0.1 and Ic in the region of <1mA.
     
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