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MMIC amps: Input impedance

Discussion in 'Electronic Design' started by Joerg, Nov 30, 2006.

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  1. Joerg

    Joerg Guest

    Hello Folks,

    The BGA2001 from NXP looks like a good deal at around $0.30/1k. Hard to
    justify rolling your own amp at that price, plus it's got a really nice
    noise figure of under 1.5dB.

    I am planning to use it well under 100MHz, actually down to maybe
    5-10MHz. Looking at figure 12 the input impedance seems to exceed
    500ohms down there (they don't show anything below 100MHz). Except for
    500MHz its input is pretty far off 50ohms on most bands. Is this normal?

    I wonder why they spec input intercept and other things in dBm then. Not
    complaining here since a higher Rin actually helps my photodiode app
    because I won't need a RF transformer. Just wondering. I have always
    rolled my own, first time I am considering a MMIC.
  2. Joop

    Joop Guest

    Well, this particular MMIC is not specified as "a generic 50 ohm
    building block" as some others are. I guess the dBm simply refers to
    power level in general, not usual RF dBm(50) or telco dBm(600).
    That would also mean that you might need more input voltage level to
    reach the same input power in dBm than you would if the preceeding
    stage is designed for 50 ohm load.

    But even a BGA616 which IS specified as "Cascadable 50 ohm gain block"
    does not always behave as close to 50 ohm in/out in my simulations
    (SPICE is available). Almost a factor of two off can take place
    depending on biasing and input impedance.

  3. Joop

    Joop Guest

    That came out wrong. I mean the input/output impedance varies with the
    impedance of the stage on the other side of the device. Supply biasing
    through resistor only or resistor/inductor combi has influence as
  4. Joop

    Joop Guest

  5. Joerg,
    they want you to AC couple anyway. So from the reverse
    biased photodiode an inductor to GND and a cap to the

  6. Joerg

    Joerg Guest

    Thanks, Joop. I'll use a resistor/inductor series combo since the needed
    frequency range is wide and there is that dreaded inductor series resonance.

    The output also looks a bit high-Z-ish so I'll have to spring for a
    BFS17 follower there. The BGA616 looks better but not perfect either and
    its noise figure isn't so hot.
  7. Joerg

    Joerg Guest

    Yes, Rene, that is about how I plan to do it. Looking at the output
    chart S22 it seems that this MMIC (and lots of others) doesn't really
    like to drive a 50ohm load. More like several hundred ohms. Guess I'll
    spring for the usual BFS17 follower. If it was a really high volume
    product I'd also do the MMIC part with a couple transistors but not in
    this case.
  8. John  Larkin

    John Larkin Guest

    Looks nasty. It's not unconditionally stable, and has weird impedances
    vs frequency. Whenever you see a mmic with a nf much below 3 dB, you
    can be certain that it's for a narrowband, tuned application. It looks
    sort of like a transistor with a bias generator.

    The datasheet says that the nf is measured at optimum match, which I
    interpret as "tuned."

    The Sirenza SGA-3586 SiGe part is about the lowest-noise, wideband,
    "true 50 ohm mmic" around.

  9. maxfoo

    maxfoo Guest

    Check this one out cross-ref to sga-3586, specs look a little better too.
  10. Joerg

    Joerg Guest

    I think that's what it is, a transistor with a bias circuit. I hope it's
    stable if I provide a nice 200ohm load. The plan was to follow it with a
    buffer so whatever else is connected (not under my control) won't upset
    the apple cart.

    But how could they call it MMIC then? Marketeers at work?

    It looks nice but the S11 and S22 also take a nose dive around 500MHz.
    I'll be staying under 100MHz. What amazes me is how cheap SiGe parts are
    now. There goes the art of designing our own amps...
  11. Joerg

    Joerg Guest

  12. John  Larkin

    John Larkin Guest

    Eventually nasty people (like a certain JT who I won't identify)
    integrate anything that's fun, so it's not worth designing ourselves
    any more. So we have to move up the abstraction stack.

  13. John  Larkin

    John Larkin Guest

    The Sirenza is a lower power part and the NF is a bit better. But
    Hittite is making some kickass parts lately, like their '465 20 GHz
    distributed amps and things.

    I like the Sirenza because it's really 50 ohms in, and in fact you can
    tune the input impedance by varying the bias current, so it's nice for
    time-domain signals at the end of a coax. It's a very nice
    medium-speed mmic, although is is a bit fragile as regards input zaps.

  14. Joerg

    Joerg Guest

    A bit less power would be great, I don't like stuff in my designs that
    get too hot to the touch. Except, of course, tubes :)

    Here the only way to create a zap would be a really bright flash. And
    when that one comes then I guess the performance of this amp will be of
    secondary concern...
  15. Joerg

    Joerg Guest

    There seem to be some more good analog chip guys out there. But even 30c
    a pop is too much at times. Not in this app but when it's a mass product
    I'll always be designing around a 5c RF transistor. Especially in cases
    when, as you had mentioned, the MMIC isn't much more than a glorified
    transistor with a bias. You can get a real noise figure of under 1.5dB
    for less. Just like with burgers, if you make your own from ground beef
    they cost less and taste better (we don't do them any other way).
  16. maxfoo

    maxfoo Guest

    I like a cheap Mini-circuit's ERA-33SM when I need to boost an rf signal.
  17. Joerg

    Joerg Guest

    I'll check that out, Max. Haven't looked at Mini-Circuits much because
    they are often single-sourced. But then again they have never
    disappointed me in my whole career and always delivered (mixers etc.).
    Unlike another company where the name also starts with "M" ;-)
  18. John  Larkin

    John Larkin Guest

    The ERA-series parts are fine. They are absolutely stable and fairly
    rugged for an exotic part. I use them as fast pulse amps and as
    sub-100 ps photodiode amps, although I do have a fiber-coupled optical
    trigger pulse that's measured in watts! Their input impedances tend to
    run low, in the 30's maybe, when the output is matched. And they do
    run hot, although InGaAs melts at a higher temp than silicon!

    All the classic darlington mmics have a Zin that depends strongly on
    output loading. They are very handy parts.

    Mini-Circuits now has some faster, 8-GHz range, parts, and some nice
    higher-power SOT-89's. W-J makes nice SOT-89 mmics, too, probably the
    highest power ones around.

  19. Joerg

    Joerg Guest

    Sub-100psec? Wow, that is performance. I just don't like semiconductor
    stuff that gets really hot.

    A little higher Z-in would be good in my case. Although I am afraid I am
    going to be flooded with DFB noise but that's what I can't know until
    the circuit runneth.

    Have you seen pathologies or instabilities with the darlingtons? The old
    BGA2001 still looks enticing with it's high Z-in. Intercept is a bit
    marginal because it's a low current device.

    Some of the SOT-89 parts I looked at appeared to be able to unsolder
  20. John  Larkin

    John Larkin Guest

    My observation is that if you want a NF below 3 dB, from a 50 ohm
    signal, you've gotta tune the input. Some of the NEC phemts get below
    1 dB, since the Cin is very low and the matching network can give a
    bunch of free voltage gain.
    I just had a burger with Mo at the Zuni Cafe, and I'd be astonished if
    any mere mortal can make their own burger that good. They serve it on
    fresh focaccia with homemade pickles and pickled onions and homemade
    aioli. $12.

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