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Biasing MMIC amplifiers (GALI-5)

Discussion in 'Electronic Design' started by [email protected], Oct 13, 2007.

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  1. Guest

    Can someone recommend a simple current regulator circuit that I can
    bias devices like the Mini-Circuits GALI-5 with? These devices like to
    run at a constant current (60 mA), and a series resistor is usually
    used to approximate a constant current source. However, power
    consumption is a concern in this design, so I don't want to dissipate
    power in the series resistor. I was considering an LDO regulator until
    I realized I need constant current, not constant voltage. Is there an
    integrated current regulator that I could use that would address my
    power consumption concern? Who makes it?

  2. Phil Allison

    Phil Allison Guest

    ** No.

    An electronic current source dissipates heat just like a resistor - the
    advantage is the current level is more stable with varying supply voltage
    than a simple resistor and the effective impedance much higher.

    Your best bet IS to use the recommended 40 ohm resistor with a 7 volt
    regulated supply & use a switching regulator to drop the voltage from your
    main supply.

    .......... Phil
  3. Steve

    Steve Guest

    The bias inductor is often unable to provide a very high impedance, so that
    the bias network shunts RF power (gain loss) and degrades output VSWR. In
    those cases, the resistor also serves to raise the RF load impedance of the
    bias network. Any current source you design will need to present a high
    impedance beyond the frequency band of interest, unless your series inductor
    can do the entire job below its self resonant point.

    There are constant current designs using certain adjustable linear
    regulators. The sense/set resistor is the "upper resistor" and the "lower
    resistor" is your load. You'd need a regulator that has the fixed voltage
    drop across the upper resistor, not the lower one. Assuming the typical
    1.25V drop, at 60 mA, the resistor is only about 20 ohms. If you check your
    design with a network analyzer, you might find that it maintains a high
    impedance at your RF frequencies (I doubt it). Otherwise, the series
    inductor has to provide all of the isolation.

    Good luck
  4. Guest

    I think I'll try a current mirror with either PNP bipolar transistors
    or P-channel FETs. I've done this before when I wasn't as concerned
    about excess power disspipation, and I could use a fairly high bias
    voltage. The problem in the current case is that excess bias voltage
    leads to a proportional amount of excess power consumption that I want
    to minimize. So I will have to determine how low I can go with the
    supply voltage while maintaining a reasonably constant current.
  5. Joerg

    Joerg Guest

    Check the LM317 datasheet. Such a regulator and one lone resistor can
    make a nice current source. Personally I'd stay away from LDOs because
    they are expensive and most have documented as well as undocumented
    pathologies. Some of them of the kablouie category, pieces flying around
    and stuff.

    As Steve wrote you need to verify what load resistor the MMIC wants for
    RF purposes. You can use an inductor plus load resistor but watch out
    for resonances. Provide good bypassing at the other end of that.

    The ultimate Uncle Scrooge method in power economy would be a switcher
    that gets the current as feedback instead of a voltage. Then you could
    keep the collector voltage at the far end of the inductor precisely
    where it needs to be for your 60mA.
  6. John Larkin

    John Larkin Guest

    The extreme power-saving case would be to pull up the output through
    an inductor, from a stiff supply, and sense/servo the operating
    current by applying a bit of DC to the input. An opamp and a few
    passives can do that.

    Otherwise, any classic current source will work.

    | |
    | |
    | Re
    LED a |
    k |
    | |
    | e
    +---Rb----b PNP
    | c
    | |
    Rled |
    | L
    | |
    | |
    gnd to mmic

    I think Zetex and maybe TI make integrated current mirrors that might
    do this with less parts. Just be careful they don't oscillate, given
    the inductor load.

    Incidentally, most of these parts have a below-50 ohm input impedance
    at recommended operating current, and go up at lower currents.

  7. John Larkin

    John Larkin Guest

    A fet would probably have more output capacitance than a bipolar. One
    good combination is a small PNP, a ferrite bead, then an inductor. The
    whole mess will be a fairly good broadband high Z.

    Be careful with Minicircuits. They outsource these parts and
    occasionally change vendors, and the new parts only vaguely behave
    like the old ones. And they tend to tell us to drop dead when we have
    problems. I'd go with W-J or Sirenza or somebody dependable.

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