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The mysteries of RF amplifier stability...

Discussion in 'Electronic Basics' started by billcalley, Feb 10, 2006.

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  1. billcalley

    billcalley Guest

    Hi All,

    I'm having a very hard time understanding the full picture on
    amplifier stability, even if I am an engineer. For instance, if we
    select a transistor that is unconditionally stable at our frequency
    band of interest -- let's say from 900MHz to 1000MHz -- but the
    transistor is only conditionally stable at all other frequencies: So,
    we bias, match, and resistively stabilize the transistor so that we
    finally see, in the linear simulator, that K is greater than 1, and B1
    is greater than 0. This should indicate that we now know that the
    amplifier will not oscillate under any input/output impedance
    conditions. But what happens when you place a filter at the input or
    output port of our newly stabilized amplifier? Since the stopbands of
    the filter are anything *but* 50 ohms when the amplifier is looking
    out-of-band, wouldn't there still be a chance that this
    "unconditionally stable" amplifier could oscillate, since it is only
    based on a conditionally stable transistor?

    Thanks!

    -Bill
     
  2. An unconditionally stable transistor would be of no use other than as a
    door stop, (at least in any definition of unconditionally stable I know
    of). Perhaps you can define what you mean by "unconditionally stable
    transistor".

    You can always take a transistor and turn it into an oscillator if you
    try to.

    An unconditionally stable amplifier is a different beast all together.
    That is a collect of parts. In theory it will not oscillate no matter
    what the source and loads are, even if they are out of band. Even a
    short or open at DC. In practice, with microwave devices it is very hard
    to realise in practice.

    --
    Dave K

    http://www.southminster-branch-line.org.uk/

    Please note my email address changes periodically to avoid spam.
    It is always of the form: [email protected] Hitting reply will work
    for a couple of months only. Later set it manually. The month is
    always written in 3 letters (e.g. Jan, not January etc)
     
  3. A chance for a shameless plug to mention the yahoo rfamplifiers
    group.

    http://groups.yahoo.com/group/rfamplifiers/

    Come on over and ask your questions...

    cheers,
    skipp

    : Hi All,

    : I'm having a very hard time understanding the full picture on
    : amplifier stability, even if I am an engineer. For instance, if we
    : select a transistor that is unconditionally stable at our frequency
    : band of interest -- let's say from 900MHz to 1000MHz -- but the
    : transistor is only conditionally stable at all other frequencies: So,
    : we bias, match, and resistively stabilize the transistor so that we
    : finally see, in the linear simulator, that K is greater than 1, and B1
    : is greater than 0. This should indicate that we now know that the
    : amplifier will not oscillate under any input/output impedance
    : conditions. But what happens when you place a filter at the input or
    : output port of our newly stabilized amplifier? Since the stopbands of
    : the filter are anything *but* 50 ohms when the amplifier is looking
    : out-of-band, wouldn't there still be a chance that this
    : "unconditionally stable" amplifier could oscillate, since it is only
    : based on a conditionally stable transistor?

    : Thanks!

    : -Bill
     
  4. Tim Wescott

    Tim Wescott Guest

    The definition of "unconditionally stable amplifier" is that it will
    stay stable with any passive load attached -- so unless your filter
    presents an impedance with some negative resistance you're OK.

    You can see this when you plot stability boundaries on a Smith chart --
    the chart doesn't really end at the gamma = 1 circle, it continues on
    outward for reflection coefficients higher than one, which imply
    negative resistances. So if the stability boundary is entirely outside
    the usual chart it doesn't mean that _nothing_ will make the amplifier
    unstable, it just means that nothing _passive_ will make the amplifier
    unstable.
     
  5. billcalley

    billcalley Guest

    Thanks Tim and Dave! Great stuff!
    I'm a bit confused by Dave's answer, since I had strongly thought
    that there *was* such a thing as unconditionally stable transistors
    (stability as calculated by the Rollett Stability Factor 'K', and the
    Stability Measure 'B1'), and not just unstable amplifiers -- and that
    in-phase feedback and gain made the amplifiers into oscillators. I'm
    probably missing something here though...

    -Bill
     
  6. Yes, if the K & B1 conditions hold for _all_ frequencies, not only those
    you are interested in. Then it doesn't even matter if the transistor
    initially was unstable on the intended operation frequency.
    It would not oscillate, no matter what the source or load impedance
    were. (Well, unless source or load exhibit negative return loss, but that
    would be unfair.)
    It would oscillate only if there was _additional_ external feedback
    around your amplifier core.
    That feedback might be invisible in your circuit diagram, like
    waveguide modes of your box or long vias thru thick epoxy..

    regards, Gerhard
     
  7. Joerg

    Joerg Guest

    Hello Bill,

    There aren't. This would require ratification of the UST act in congress.

    SCNR, Joerg
     
  8. Tim Wescott

    Tim Wescott Guest

    Transistors have enough capacitance from collector to base (or drain to
    gate) that with the right inductances on the collector (drain) and base
    (gate) they will oscillate. Many overtone crystal oscillators are made
    this way, with the crystal providing inductive reactance to the base and
    an LC tuned circuit providing inductive reactance to the collector.

    There are many, many circuits for which you just don't have to worry
    about a transistor bursting into oscillation -- perhaps this is why you
    thought they were unconditionally stable?

    --

    Tim Wescott
    Wescott Design Services
    http://www.wescottdesign.com

    Posting from Google? See http://cfaj.freeshell.org/google/
     
  9. billcalley

    billcalley Guest

    Thanks Gerhard and Tim for the clarifications. Much appreciated!!

    -Bill
     
  10. Guest

    The capital K refers to the overall (device embedded in the circuit)
    stability, k is the Rollet stability factor. Even if k <1 at some
    frequencies one can make K>1 at all frquencies by lossy embedding and
    or feedback. Having done this the overall circuit will be
    unconditionally stable for all non-feedback terminations with
    reflection coefficients no greater than1.
    Dick
     
  11. billcalley

    billcalley Guest

    Thanks Dick! I hadn't known that there was a K and a k. Makes sense
    though, and the books never seem to fully clarify this either.

    -Bill
     
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