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Low-voltage rail for transistor bases?

Discussion in 'Electronic Basics' started by Lauri Alanko, Sep 21, 2013.

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  1. Lauri Alanko

    Lauri Alanko Guest


    When power transistors are used as switches, the base current can be
    significant, as the saturation beta may be as low as 10. On the other
    hand, the base _voltage_ need only be at Vbe(sat), and any additional
    voltage needs to be dropped by e.g. a resistor. Given the high
    currents, this voltage drop may be a significant loss.

    This suggests to me that ideally the base of a low-side NPN switch
    would be powered by a low-voltage rail, only slightly higher than
    Vbe(sat). This way the conduction loss would be minimized.

    Has such a rail been used with bipolar switches? If not, what's wrong
    with my thinking?

    (Please don't tell me to use MOSFETs because they are better. I know
    that already. I'm looking for understanding, not solutions.)

  2. Phil Allison

    Phil Allison Guest

    "Lauri Alanko" <

    ** No doubt - but it would still have been a 5 or 12 V rail.

    But the DC voltage being switched might be several hundred.

    .... Phil
  3. whit3rd

    whit3rd Guest

    Yes, of course. Old NIM (nuclear instrumentation module) power for switching
    transistors included +24, +12, +6, -12, -24V power supplies, and
    it was common to make use of whatever was most convenient and
    efficient from the plethora.

    Vacuum tube radios typically had A battery for filaments (1 to 6 V), B battery for
    tube bias (20 to 30V) and C battery for plate supplies (as high as you can get).

    Sometimes, you see Vbb for base drive voltage supplies, Vcc for collector.
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