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A PNP-NPN output with common-base level shifting

Discussion in 'Electronic Basics' started by Lauri Alanko, Sep 3, 2013.

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  1. Lauri Alanko

    Lauri Alanko Guest

    Hello.

    Here is a circuit I came up with. It is meant to be a general-purpose
    digital output stage: it takes in an inverted digital control signal
    (typically low voltage and low current), and uses that to switch between
    sourcing from a higher voltage and sinking to ground.

    Out
    Q1 | Q2
    V+---v_/-------------+-----------\_^---Gnd
    |Q3 D1 R1 R2 D2 |
    \_^--\<|--\/\--+--\/\--\<|--+
    | |
    Vhi ~In

    Q1 and Q2 do the switching between sourcing and sinking. I'm not sure
    what the proper name for this configuration is. "Push-pull" seems to be
    when NPN is on high side and PNP on low side, and "totem pole" seems to
    refer to either that or to two NPNs on top of each other. But I haven't
    seen this PNP-NPN configuration very much.

    To me it seems it's much better to have PNP high and NPN low, since the
    voltage drop is then much lower. Is this PNP-NPN switch in common use? If
    so, what is it called? If not, what is the problem with it? The fact
    that the transistors get saturated?

    Q3 is an NPN in common-base configuration used to drive Q1 with a lower
    voltage. Vhi is the high level of the input signal, so when input is
    high, Q3 is at cutoff and so is Q1. When input is low, Q3 is active and
    drives Q1 to saturation.

    Do I need a pull-up resistor from the base of Q1 to V+? If so, why?
    Is there some danger in leaving the base of Q1 floating when Q3 is at
    cutoff?

    The zener diodes D1 and D2 are there to prevent strike-through when
    transitioning between high and low. They should drop about Vhi/2 to
    ensure that one transistor won't activate before the other one has gone
    into cutoff.

    To me this seems like the simplest way of, say, driving a mosfet with a
    microcontroller (using only cheap discrete parts). But I haven't seen
    this particular circuit anywhere. Is it okay or is there a problem I have
    overlooked?

    Thanks in advance.


    Lauri
     
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