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MMIC tolerances

Discussion in 'Electronic Design' started by Joerg, Dec 5, 2006.

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  1. Joerg

    Joerg Guest

    Still thinking about using the old BGA2001:

    I know that the stability circles predict hail and thunder from the
    north-east but I'll try my best not to let anything become inductive. It
    just fits the bills nicely. Anyhow, none of the myriad MMICs that I
    looked at had many entries in the min/max columns. The only entry is
    usually for supply current and that varies a whole lot, like by a factor
    of two between min and max. What are the typical tolerances for the
    other parameters such as gain and compression point?

    Side question: Is the SOT343 package likely going to remain popular? So
    far I have seen it used by NXP, Avago, and Infineon.
  2. John  Larkin

    John Larkin Guest

    For the classic InGaAs darlington mmic, you force bias current into
    the output pin, so it doesn't have any choice in the matter. I've
    found the gains (Mini-Circuits, Sirenza, W-J) to be very consistant,
    nothing like 2:1 span.

    You *can* also poke current into the input pin to shift the bias
    point. Sometimes that's handy.

    Any self-respecting mmic is unconditionally stable.

  3. maxfoo

    maxfoo Guest

    Interesting, do you have an test data showing how much improvement biasing the
    input pin makes? It seems like this method would only be valid if the input was
    not matched to 50 ohms with the previous stage, no?
  4. Joerg

    Joerg Guest

    Do you remember how much difference you saw in gain?

    In my case more than 0.5dB would throw this application off the rocker.
    Then I'd have to use an opamp and in my case I'd have to use it in
    non-TIA configuration. THS4021 or something like that in standard
    non-inverting fashion with the photodiode into a resistor at the
    non-inverting input. Reason is that I was just informed that the PD
    version can change and I don't want things to become unstable because
    the new one has a different capacitance.

    Yes, I agree. But the very low NF varieties seem to all have that danger
    zone up in the inductive part of the Smith chart. Actually, so do many
    "roll your own" low noise amps.
  5. John  Larkin

    John Larkin Guest

    As long as the part is still in its linear range, pushing the bias
    won't change the impedances much. For example, if the expected signal
    swing range is small, you could pull up the input a bit, pulling down
    the output, and run the part at lower voltage and power. I use these
    parts in time-domain apps, and if I know that the signal is
    predominantly in one direction (say, pulses from a microchannel plate
    or a photodiode) I can bias the output in the opposite direction and
    get both lower power dissipation and more peak signal swing.

    Most of the ERA-type mmics are simple inside: just a darlington with a
    feedback resistor, easy to model at DC.

  6. John  Larkin

    John Larkin Guest

    If you need extreme gain accuracy, I wouldn't use any mmic. They
    wouldn't be temperature stable to a fraction of a dB... they're just
    darlingtons with crappy feedback resistors. So an opamp, either as a
    tia or just using a load resistor followed by a positive-gain stage
    would be a lot more stable.

    Have you looked at the THS4302 series? Vicious little beasts. I was
    thinking of dumping a photodiode into a grounded resistor-inductor
    series pair, and amping that with a THS4303 maybe. Tweak the L to
    cancel some of the pd capacitance and extend the bandwidth a bit. I'd
    consider a t-coil, if I understood them better, which I don't.

    +30--------|<---------+------------to amp

  7. Joerg

    Joerg Guest

    After looking at some other MMICs I have just deleted that part of the
    schematic. Even the good stuff from Mini Circuits is slightly above
    tolerance spec for this app. At least it's not like in the olden days
    when I'd have lots of eraser turds on the floor now :)

    So it's the load resistor opamp combo. Somehow that doesn't look
    high-tech and cutting edge in terms of cost but, oh well, it'll work.

    I don't need to go that high, just to 100MHz. Also, I'd like to get away
    without more regulators and the THS4021 can live nicely with +/-12V rails.

    A peaker coil? That would be cool but this amp is a true hotrod. 12GHz
    GBW, wow. Possibly the only way to create a reliable inductance here
    would be to use one embedded in the layout. Just make sure its magnetic
    field doesn't see any part of the opamp feedback.

    Probably this is the frequency range where you might benefit from a
    discrete design with one of those 45GHz RF transistors. Problem is that
    many of these are from EU manufacturers and I have experienced
    procurement nightmares there. Great products but very poor marketing.

    T-coil? I've only heard that in connection with hearing aid coupler
    circuits in churches and public auditoriums.
  8. John  Larkin

    John Larkin Guest

    I wish people would look up existing terminology before they make up
    names. A real, classical t-coil has nothing to do with deaf people.

    If you couple the plate of a tube to the grid of the next one, the
    plate load resistor and Cp+Cg have a time constant tau, and the
    bandwidth is 1/2.2tau. Adding an inductor in series with the plate
    load resistor is "shunt peaking" and improves things roughly 40%.
    Adding another between plate and grid is "series peaking" and helps
    more. An ideal t-coil improves bandwidth 2.8 times the basic circuit.

  9. Joerg

    Joerg Guest

    Yeah, nowadays marketeers create such terminology. Some of the major
    hearing aid companies use the word T-coil. But the term "deaf people"
    isn't correct either since it's only useful for folks with some
    remaining hearing ;-)

    I have never really understood why magnetic loops are needed in the
    first place. All the industry would have had to agree upon is a common
    method of close range LF transmission.

    Interesting. I've seen coils in line with the plate but when they were
    used to muffle a load capacitance they were often called peaker coil.
    But most of the time it was there to mute unwanted oscillation way above
    the operating frequency (coil wound around a resistor).
  10. Robert

    Robert Guest

    Thomas Lee's got a nice write-up of the different ways to do Inductive
    Peaking including the Bridged T-Coil in:
    The Design of CMOS Radio-Frequency Integrated Circuits, Second Edition

    I also liked his first chapters on the early history of Radio Circuits as
    well but a lot of people don't like what they call his "disdain of rigor".

  11. Joerg

    Joerg Guest

    Amazon now want my email address before allowing to preview. What are
    they thinking? Or, are they thinking?

    Yeah, not scientific enough probably. I like those books. Have to look
    next time at Borders. Although I am not sure I'll go there anymore
    because ours has shrunk the EE section to almost zero :-(
  12. John  Larkin

    John Larkin Guest

    Other good books that disdain rigor:


    Phil Hobbs' "Building Electro-Optical Systems". Full of quirky
    quotations, cartoons, lore. Also dense with good stuff.

    Thomas Lee's "Planar Microwave Engineering." But I don't know why he
    calls it "Planar", since it covers most everything.

    All three are worth reading cover to cover.

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