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The bi-polar transistor at RF

Discussion in 'Electronic Design' started by Paul Burridge, Jul 21, 2004.

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  1. Hi all,

    Firstly, does anyone bother designing with Y-parameters *at all* these

    Then... (talking of the common-emitter configuration in this case)
    The only variable according to the Ebers-Moll transistor model apart
    from the device-specific "Is" which has any effect on Ic is the
    potental difference applied across the B/E junction. The signal
    voltage thus applied is loaded by the resistance of this diode. At
    DC., the loading is at a maximum and the entire PD appears across it.
    Right so far? As the applied signal voltage increases in frequency,
    the feedback capacitance (B-C) and the B-E junction capacitance form
    an AC bypass path across the B/E resistance above-mentioned. The two
    capacitances acting in concert shunt more and more of the applied
    signal voltage to ground, bypassing the emitter diode resistance,
    lowering the device input impedance and resulting in less and less
    applied Vbe across this diode and consequently less and less Ic output

    What I'm getting at is that Ebers-Moll is still good at RF, *provided*
    one allows for the bypassing of the emitter diode's resistance by the
    combination of Cb and Ce. Correct?
    And CJC and CJE are the relevant Spice model parameters?


  2. You didn't even mention base spreading resistance, a major
    figure of merit for RF transistors. Newer designers have
    considerably lower r-sub-bb-prime than legacy transistors.
    Look up the hybrid-pi transistor model. SiGe technology's
    claim to fame is low base spreading resistance.

    Rick N6RK
  3. What you say about rbb is true, but SiGe has other 'cool'
    characteristics, some derived from low rbb, and some due
    to other effects of the 'strained' silicon?... Ignoring
    the rbb itself, SiGe tends also to have very low 1/f noise
    and good LF noise in general. Also, it tends to have high
    Beta (at LF.) So, where an RF transistor might tend to have
    a Beta of 20-50, an SiGE part might be 100-300 or higher.

    With the combo of the low rbb (and low 1/f noise), along with
    the high Beta, the total amount of input current noise and
    input voltage noise is damned low.

    SiGe would make good oscillators (for less PM noise) and
    of course, good preamps. This is one case where GaAs FETS
    that are very fast, might be undesirable because of their
    worse 1/F noise characteristics.

    One big disadvantage of the typical SiGe transistors is
    that their breakdown voltage is low. However, the tradeoff
    of breakdown voltage is BETTER for a given frequency response
    and Beta than a normal Si transistor.

    The SiGe transistors are also not very expensive. A part that
    works well with reasonably low distortion and reasonably low
    noise figure at 600MHz would be significantly less than $1.00.
    Unless the transistor is too fast for a given layout, SiGe
    can be used at low frequencies (e.g. VHF) while still avoiding
    the low frequency noise problems that are common from GaAs FETS
    and even other fast BJTs.

  4. Gregg

    Gregg Guest

    Hi John,

    Interesting. As a tubehead myself, I have not heard of these. Who mfrs
    them? What's a typical part #?

    Thanks :)
  5. Robert Baer

    Robert Baer Guest

    At RF, the base spreading resistance can be large when compared with
    the calculated emitter resistance; this makes a serious contribution to
    input noise and the NF of the stage.
    So the particular version of the model one uses can be rather poor in
    determining real-life NF.
    BTW, noise measurements at audio frequencies using different collector
    currents can be used to determine the transistor's base spreading
    Once that is known, and the collector current used in the RF amplifier
    (for determining Re), one can then calculate noise (or NF) and be rather
    close to measured values!
  6. Robert Baer

    Robert Baer Guest

    Beat me to it; see my comment.
  7. John S. Dyson wrote...
    Examples of low-cost high-performance (>30GHz at 10mA) SiGe
    transistors would be Infineon's BFP620 (82 cents at DigiKey)
    and Philips' BFU510 and BFU540

    The Philips transistors look good, but I don't know where to
    get them. Mouser stocks a set of CEL's nice SiGe transistors,

    - Win

    (email: use hill_at_rowland-dot-org for now)
  8. Sorry guys, I did *mean* to include BSR in series with the B/E
    junction resistance, so any reference I made to this junction
    resistance should be taken to mean the total of the two together.
    NF isn't a consideration in this instance; please ignore it.
    And I am well aware of the pi-model. I just want to know if I have it
    right that Ebers-Moll will work accurately into UHF provided one
    allows for the feedback capacitance and emitter junction capacitance
    shunting the input signal around BSR+EBR and thereby reducing the
    signal voltage developed across them. Do I have this right?


  9. Sorry! Corrected above. IOW: whilst the emitter diode resistance is
    bypassed at RF by these two capacitances, the base spreading
    resistance *isn't* - apart from that, the rest of the post is now
    correct, yes? IOW, as the signal frequency increases, the BSR becomes
    the dominant component of the device's input impedance... Phew!
    Unless of course, someone knows otherwise...
  10. I read in that Paul Burridge
    >) about 'The bi-polar transistor at RF', on Thu, 22 Jul
    Emitter lead inductance?
  11. Er, yes, but I'm only interested in the *internal* characteristics of
    the device here, so even the bonding wires' inductance isn't an issue.
    Thanks for giving me the chance to clarify, though.
  12. I read in that Paul Burridge
    >) about 'The bi-polar transistor at RF', on Thu, 22 Jul
    A bit of the emitter lead is inside the encapsulation, and a bit is on
    the die.
  13. While the IP3 and compression figures may good to comparable devices
    working in the GHz bands, the huge gain with the low Vce (typically
    less than 2.3 V) will damage the input IP3 values quite quickly at
    VHF. This can be a problem in the VHF and lower UHF bands, in which
    signal levels can be quite high and multiple strong signals may pass
    the front end selectivity.

    For VHF applications, a high current SiGe device operating at
    impedance levels well below 50 ohms would give good IP3 figures, but
    apparently the noise figure increases quite rapidly with high
    collector currents.

    Paul OH3LWR
  14. Thank you, John, that gives me another chance to re-state the question
    more succinctly. I'm not concerned with inductances here at all,
    Does the Ebers-Moll equation hold good at UHF+, provided the value one
    inserts for Vbe is adjusted to account for the loss of signal voltage
    the B/E junction will suffer as much of it (the applied signal
    voltage) is shunted around it via the device's internal capacitances?

    There! I think I've nailed it this time!
  15. "Noiseless feedback' is very helpful to mitigate the excess amounts
    of gain, while pushing the return loss match (the impedance match)
    closer to the noise match (the input impedance where the noise is lowest.)
    A simple emitter (source) inductor and a little bit of parallel, noisy
    feedback can be used to tame some of the interesting UHF+ components.
    (The emitter (source) inductor is a case where a small amount of
    inductance is much better than too much, because instability can ensue
    with too much series feedback (too large an emitter (source) inductor).)

    I do agree with your implication that a high current device can be helpful
    at low frequencies, but some SiGe components do seem to maintain
    reasonable noise performance at high currents.

    In any case, the SiGe components do give the GaAs type components a
    run for their money. In some cases, the SiGe components are actually
    better. The PHEMTs (HP 54143) are also an interesting variation on
    the 'fet' theme, which helps to mitigate some of the problems WRT
    GaAs. For example, a PHEMT can provide a good noise match at 50ohms,
    a transconductance of almost 1MHO at 60ma, and reasonable IP3 (38dBm isn't
    impossible.) Feedback can be especially helpful with the high
    transconductance FETs.

  16. Has anybody actually used noiseless feedback with these devices at
    VHF? After all, the fT of these SiGe transistors are in the 30 GHz+
    range, so I would guess that the parasitics would mess the situation
    quite badly, especially when using feedback components.

    How critical is the layout compared to for instance MAR-x series MMICs
    (that are essentially darlingtons) ? Can these SiGe transistors used
    with dual sided boards and microstrips or do they require multilayer
    boards and full striplines in order to use feedback ?

    Paul OH3LWR
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