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rail-to-rail output stage secrets?

Discussion in 'Electronic Components' started by [email protected], Dec 17, 2003.

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  1. Guest

    Hi,

    the other day I was looking at rail-to-rail OpAmps and was pleased to
    see that an internal circuit schematic is revealed in the MC33201
    datasheet <http://www.onsemi.com/pub/Collateral/MC33201-D.PDF>, a
    practice that seems to have become rare nowadays.

    The MC33201 is a purely bipolar device operating from 1.8 to 12V, with
    an open-loop gain of 300000, a gain-bandwidth product of 2.2MHz, and a
    slew rate of 1V/us - sort of a low-voltage rail-to-rail version of the
    uA741.

    I soon realized that I failed to comprehend how the rail-to-rail output
    stage is supposed to work: while the current sink (or is it called the
    source?) toward VEE poses no problem (it's just two cascaded NPN
    transistors), the current source (the sink?) toward VCC consists of an
    PNP current mirror driven by two cascaded NPNs (again) which are driven
    by another PNP current mirror which is driven by a *constant-current*
    source! (There are no current limiting resistors at all, by the way.)

    The device is said to have an output current capability of 50..80 mA on
    the one hand, and a quiescent supply current of only 0.9mA on the other.
    Is this a case of deliberate hiding of information? Or just an honest
    omission? Can somebody supply the missing piece of information?

    I had hoped to learn how a smooth crossover from sinking to sourcing of
    output current is managed in a rail-to-rail output stage - without a
    dead zone (like the uA709 had), which would show up as steplike signal
    distortions, epecially for fast transitions - and without shorting Vcc
    to Vee, of course.

    TIA,

    Martin.
     
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