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"0V Vbe" current amplifier?

Discussion in 'Electronic Design' started by [email protected], May 16, 2007.

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  1. Guest

    Is there a clever way to make a current amplifier which acts like a
    NPN transistor except that it has 0 Vbe? That is, with a "base"
    resistor of 10k and an input voltage of 100mV, 10uA gets amplified
    into 1mA at the "collector"? Preferably a method that does not use an
    IC's (opamps, etc..)
     
  2. D from BC

    D from BC Guest

    The OPA861 comes to mind but it's an 8 pin IC..
    Clipped from datasheet...
    "
    The OTA or voltage-controlled current source can be
    viewed as an ideal transistor. Like a transistor, it has
    three terminals—a high impedance input (base), a
    low-impedance input/output (emitter), and the current
    output (collector). The OPA861, however, is self-biased and bipolar.
    "
    D from BC
     
  3. A cascade of an NPN and PNP emitter followers have
    approximately zero offset voltage.

    Integrates amplifiers made of two complementary emitter
    followers in cascade make very fast, fairly low offset
    voltage current amplifiers.
    I know you asked for a non integrated solution, but you
    might be able to get some ideas how this is done from this
    data sheet.
    http://cache.national.com/ds/LM/LMH6321.pdf
     
  4. Eeyore

    Eeyore Guest

    With an op-amp, yes.

    Graham
     
  5. John Larkin

    John Larkin Guest


    Something roughly like...

    +5
    |
    |
    10k |
    | |
    | c
    +----------b npn
    | e
    | |
    e 50r
    ----+-----b pnp |
    | c gnd
    10k |
    | |
    gnd gnd


    John
     
  6. Jim Thompson

    Jim Thompson Guest

    Smoke and mirrors make everything work ;-)

    ...Jim Thompson
     
  7. Phil Hobbs

    Phil Hobbs Guest


    _Magic_ smoke and current mirrors.

    Cheers,

    Phil Hobbs
     
  8. Jim Thompson

    Jim Thompson Guest

    _Magic_ is _MY_ domain ;-)

    ...Jim Thompson
     
  9. Fred Bartoli

    Fred Bartoli Guest

    Jim Thompson a écrit :
    But mirrors is sure not :)
     
  10. Guest

    As opposed to the real world.
     
  11. Winfield

    Winfield Guest

    I like the opa861idbv with its cute SOT23-6 package.

    TI also offers the Burr-Brown SO-8 package opa860, which
    has the same OTA stage, but adds an output buffer.

    These parts replace the opa660, which was discontinued.
    Read the opa660 datasheet to see the schematic.

    While TI calls these operational transconductance
    amplifiers (OTAs), and they characterize them as ideal
    active transistors, they're actually basically current-mode
    opamps with the transconductance node exposed and the
    Gv=1 output stage either omitted or made available with a
    separate connection pin, as in the opa860 (and opa660).
    This type of OTA functionality can be had in several Analog
    Devices opamps, like the ad846 and ad844, which bring
    out the transconductance pin, labeling it "compensation".

    TI's ability to set the quiescent current of the input transistors
    is a nice touch, because you can reduce the +input current
    (what they call the base) and reduce the output-current offset,
    at the expense of speed and a reduced the maximum output
    current capability (note, the maximum output current can
    substantially exceed the quiescent current).
     
  12. _Magic_ is _MY_ domain ;-)[/QUOTE]

    Hell, you wouldn't know real Magic if it jumped up and bit you in the ass.

    Good Luck!
    Rich
     
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