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Why this transistor?

Discussion in 'Electronic Design' started by Anthony Fremont, Apr 5, 2007.

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  1. Eeyore

    Eeyore Guest

    It's simply AD = Analog Devices OP07 ! = ADOP07

    Duh !

    Graham
     
  2. Eeyore

    Eeyore Guest

    Toff is only relevant to saturated switching.

    Probably because he's an idiot. There's no way there's any justification for an
    OP07 there. OTOH they're not so expensive as not to stick with one.

    Graham
     
  3. Not any of the ones I have.
    I imagine that Google might prove that incorrect.
    You're from a different part of the world where BC parts are more commonly
    found.
    I'm sure there are. :)
     
  4. Guest

    I wonder if the modern versions of the 2n2369 and similar parts
    are still gold doped. I have about 33 datasheets of various
    versions of '2369 transistors stored in my computer, and only
    the very oldest ones, like Raytheon's datasheet for the bare
    die, mention the gold doping.
     
  5. But we do know what the signal source is. It is from a VFO, so it's quite
    likely that the signal is 1V to 2V peak to peak. With no base resistor,
    would that make it relevant? Seems like an excessive amount of load on the
    circuit supplying the signal, given the purpose. I don't know that for
    sure, I'm just trying to learn.
     
  6. Jim Thompson

    Jim Thompson Guest

    [snip]

    I would presume they are still gold-doped. Otherwise how would they
    meet the storage time spec?

    ...Jim Thompson
     
  7. Tim Williams

    Tim Williams Guest

    How are RF transistors made, anyway? Small-ass dies, dash of spices (like
    gold) to reduce recombination, etc.? Is it then impossible (with silicon)
    to maximize all parameters? Like, say I want a transistor that does like...
    600Vceo, Ic > 1A, t_stg < 10ns, and hFE > 200. (Nevermind lead inductance
    on actually putting that much collector current and bandwidth into
    practice...)

    Tim
     
  8. Joerg

    Joerg Guest

    There will be trade-offs. Such as the hfe penalty that usually comes
    with gold-doping. Don't expect much above 50, typically a lot less.

    Then there are market forces at work. There isn't a lot of market for
    high voltage RF transistors. Some of the power horses I grew up with
    have vanished :-(
     
  9. Tim Williams

    Tim Williams Guest

    Yeah. I mean, HV transistors usually have appalling hFE anyway (typical
    line output transistor circa Vceo ~ 1.5kV, hFE = 4, t_stg = 3us!), but
    that's optimized for something else. What's the ultimate maximum between
    Vceo, hFE, fT, etc. that can be physically produced, you know?
    I certainly wouldn't expect to find something like this for sale! Most RF
    amps are modules at what, 48V or something? Throw together a thousand (plus
    a nutsoid transformer and rectifier for the kiloamp supply rail) and you've
    got a massively-redundant FM transmitter...or something like that...

    Tim
     
  10. Joerg

    Joerg Guest

    No idea, I just take what's on the market (and is cheap...). Guys like
    Jim would most likely know the trade-off details.
    Not quite a thousand but modular combos are popular.
     
  11. Eeyore

    Eeyore Guest

    You appear to have noticed a genuine area of concern.

    Yes, that input stage is total rubbish. What would the source impedance be ? 75
    ohms ?

    Graham
     
  12. Tam/WB2TT

    Tam/WB2TT Guest

    Glad that somebody finally got it right. I have noticed that at 50 MHz, the
    2N2369 will have at least 6 db more gain than the 2222.

    Tam
     
  13. Eeyore

    Eeyore Guest

    In which case they're not 2N2222s. Probably PN2222s.

    One reason I'm not a fan of 2222s is their relatively low current gain by
    today's standards.

    Graham
     
  14. Wimpie

    Wimpie Guest

    Hello Tim,

    It seems that we are focusing on a combination of high collector
    voltage, current and Ft and HFE. You are right, RF transistors have
    extremely small geometry. To get some current capability, many of them
    are in parallel (on the same die). To make them more rugged, they add
    some emitter resistance to get a reasonable current distribution over
    the complete die.

    For an RF amplifier, low HFE means just some more bias current. At HF,
    ft, Ccb and the base spreading resistance (Rbb') is of importance. At
    RF, the input signal is just charging (and recharging) Ccb and the BE
    diffusion capacitance (not confusing with Ceb). This all must go via
    Rbb'. If that one is high, lots of the input power is lost in it,
    hence reducing gain.

    Some datasheets do mention the Ccb*Rbb' product and you can estimate
    Rbb'. When you are using SPICE, the bad modeling of Rbb' gives
    unreliable results with respect to gain and power efficiency.

    With regards to saturated behaviour, the TR (reverse transit time)
    parameter is not or badly extracted. In simulation your circuit
    behaves perfectly, but when you build it, it doesn't function.

    When it comes to amplification (so not saturated behaviour), the
    maximum oscillation frequency would be nice to know also, however it
    is mentioned in rare cases only.

    Silicon bipolair for RF power is becoming less popular. When you look
    to base stations for UHF cellular, mostly metal gate lateral mosfets
    are used (LD MOSFET). Nowadays it is not uncommon to buy a device that
    outputs 100W at 2.5 GHz (continuously). In the lower frequency range,
    you also see more and more (polysilicon gate) MOSFETs. IXIS have nice
    devices (www.ixisrf.com).

    Best Regards,

    Wim
    PA3DJS
     
  15. John

    John Guest

    He had a bunch of them on his shelf.

    John
     
  16. Joerg

    Joerg Guest

    The really big ones (used to) have multiple emitters. Back when I was
    young we bought those that had "partially" failed. Some of them emitters
    must have blown and they'd almost give them to use. They were still good
    even though for a few dB less.

    [...]
     
  17. Gary Tait

    Gary Tait Guest

    You probably have seen 2N2222A, which have a TO-92 plastic case.
    I do have one 2N2222 in a metal can. I don't know why some people would
    actually use one though, unless they don't know about substitution, or
    need to take parts lists literally.

    Here I use 2SA933 and 2SC1815 , as that is what I mostly pull from Asian
    electronics.
     
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