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Low on-resistance bipolar transistor - does it exist?

Discussion in 'Electronic Basics' started by John Popelish, Apr 6, 2007.

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  1. Saturated BJTs are not as simply ohmic as mosfets are, but
    they can have pretty low forward drops. I don't know what
    range of voltage and current you are talking about
    switching, but Zetex makes some small, very low drop
    transistors. E.g. ZTX1049A
    has a collector to emitter drop of no more than 220 mV while
    carrying 4 amps of collector current and driven with only 50
    mA of base current. The resistance with a similar drop at 4
    amps would be 0.055 ohms. And this is a little TO-92 sized
    device. Unfortunate, the resistance is not linear with
    variable current. In other words, at zero collector
    current, the collector to emitter voltage does not go to zero.
  2. This doesn't sound like a problem with the transistor, but
    with the circuit design. Could you send a schematic?
  3. I can't say whether that is the method, or an incremental
    method is used (small change in collector current causes a
    small change in collector voltage drop, Ron= delta V / delta
    I ).
    Yes. The base current leakage into the collector produces a
    minimum collector voltage drop for a given base current,
    even if the external collector current goes to zero. I
    think that for any given collector current, there is an
    optimum base drive, for lowest collector to emitter drop.
    More is not necessarily better.
  4. Some of the mythology about MOSFETs is based on not thinking things through.
    They have very high input impedance, which means that unloaded, high
    voltages can remain high voltages when applied to the gate. There's nothing
    to dissipate the high voltage. A bipolar sees the same voltage on its
    base from static discharge, but the relative low impedance of the input
    means the "high voltage source" gets loaded down significantly, and isn't
    high enough to do damage. The voltage doesn't change, the "voltage
    sensitivity of the input" doesn't change, the impedance of the input of
    the device changes.

    Hence, MOSFETs are more prone to static damage when lying loose. It
    was a real problem in the early days (or at least every time a MOSFET
    was mentioned, they mentioned being very very careful about protecting
    the device), though it was relatively soon after that the manufacturers
    started putting protection diodes on the gates to help protect against
    this (the protective diodes would not conduct normally, but would conduct
    when the voltage on the gate went above a certain level, providing a path
    to discharge the high static voltage on the gate).

    But the worry about static damage was with the MOSFET lying loose. Once
    it was installed in a circuit, you could remove the jumper around the
    leads (some devices in the early days came with such a jumper, and
    if it didn't all the articles warned about putting your own jumper
    on the device before you started soldering). The circuit itself
    would generally protect the device, because there was a very real
    path to ground from the gate in the form of a resistor or something
    else that would provide a path to ground (such as a coil in the case
    of radio circuitry). The circuits could still be relatively high
    impedance, but even with relatively high value resistors they loaded
    things down enough to dissipate the static, if it ever got to that
    point in the circuit.

    The real exceptions would be circuitry where a MOSFET gate is somehow
    open to the world, and there's nothing to protect that input. So
    something like an electrometer that depends on a really high input
    impedance to do it's work and needs the gate exposed to the world
    might suffer. That MOSFET as an "active antenna" in that shortwave
    receiver (where the device is more to transform a really high impedance
    point of the whip antenna to a lower impedance that the rest of
    the receiver can use) might be vulnerable, if the manufacturer didn't
    put protective diodes in the circuit. Plug in boards that have a MOSFET
    input connected to the connector might be vulnerable, if there is no
    pullup or pulldown resistor on the gate that provides a discharge
    path for static electricity on the pin.

  5. tempus fugit

    tempus fugit Guest

    Hey all;

    A transistor thread in this group got me wondering about transistor
    characteristics. I have seen low (and very low) on resistance MOSFETS (2
    ohms and less) but haven't seen a similar spec in a standard BJT. In fact,
    sifting through the Fairchild site, one can sort the MOSFETS based on Ron,
    but the same isn't true for the BJTs. Are some BJTs lower Ron than others,
    or is low Ron something that BJTs simply are not capable of? It's my
    understanding that a MOSFET is more susceptible to damage from static
    discharge than a BJT, and so it would seem to limit it's applications as a
    switch if you weren't supposed to have any voltage at the drain before
    things were powered up.

  6. tempus fugit

    tempus fugit Guest

    Thanks for the reply John. I'm switching 12v at the collector with 5v at the
    base. The 12v is coming through a 100K resistor from the previous device, so
    the current is pretty small. I've found that using a standard (2N2222 type)
    transistor doesn't quite switch things all the way off ( there is an LED
    indicator that still glows faintly).
  7. tempus fugit

    tempus fugit Guest

    Oh, one other thing - is this how the Ron is calculated, i.e., Collector to
    emitter drop divided by collector current?
    If this were the case, my low current would increase Ron.
  8. jasen

    jasen Guest

    you probably want to look at Vces with BJTs it serves a somewhat
    equivalent purpose

  9. tempus fugit

    tempus fugit Guest

    Thanks for the input Michael. I actually have a MAX4662 analog switch
    ( that I had hoped
    to use for this purpose, but once I got learning more about MOSFETS, I
    decided against using it because it was possible that the 12v I'm trying to
    switch may be applied to the com terminal before the power was actually on.
    But you're saying that this shouldn't be a problem? If not I'd wire it in
    and give it a try.

    John, I have a schematic but unfortunately I don't have a scanner, so I'll
    try to take a picture and post it on ABSE. The schematic is from my guitar
    amp, and it is a control signal for channel switching and volume boost that
    I'm trying to switch remotely.
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