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NOISE FIGURE OF A BIPOLAR TRANSISTOR

Discussion in 'Electronic Design' started by RealInfo, Feb 12, 2013.

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  1. RealInfo

    RealInfo Guest

    Hi all

    In an article about low noise PU preamp for magnetic stylus
    it was written that the bipolar transistors were chosen for their low noise figure .My question is how exactly a noise figure of a single bip[olar transistor is defined measured .

    Thanks
    Elico
     
  2. Buy or borrow a copy of 'Art of Electronics' by Horowitz & Hill
    and read the relevant chapter. That should teach you more than
    any Usenet discussion.

    Jeroen Belleman
     
  3. Guest

    I assume by the context that by PU you are actually referring to some
    audio pick-up preamplifier.

    If your intension is to achieve best power match (source impedance =
    load impedance) then some grounded base amplifier is the best choice
    (a big grounded base 2N3055 or a half doxen grounded base transistors
    in parallel).

    However, typically magnetic pick-ups are designed for much greater
    load impedance to give a flat (after RIAA correction) frequency
    response.

    So what do you exactlly want to do ?
     
  4. You're off on a tangent. He's asking about noise, not about
    power transfer. Moreover, power matching and noise matching
    aren't the same.

    Jeroen Belleman
     
  5. Phil Allison

    Phil Allison Guest

    "RealInfo"
    ** Noise figures for transistors are not numbers, but graphs.

    http://www.cytium.net/hobby/bc107.pdf

    They show the measured results for a typical example of a BC109 for varying
    bandwidths, Ic and input resistance for a fixed Vce of 5 volts.

    The definition of "noise figure" is when the measured noise is so many dB
    *above* the calculated value for the particular source resistance and
    frequency/bandwidth.

    A noise figure of 0dB implies that the device ( under some specified
    condition) adds NO noise to that inherent in the source, a figure of 1dB
    implies that the noise level is 1dB above the theoretical limit.

    FYI:

    All resistive sources have " thermal noise " which follows the formula:

    " Nv = sq.rt. 4.K.T.B.R " where

    Nv = rms noise voltage

    K = Boltzman's constant ( 1.38 exp-23)

    T = absolute temperature in degrees K

    B = effective test bandwidth

    R = resistance value

    Eg:

    For a 200ohm resistor and a 20kHz bandwidth at room temp, the calculated
    value is 0.255uV rms.

    For a 20kohm resistor, the result is 2.55uV rms.


    .... Phil
     
  6. Guest

    For a bipolar transistor you have both input voltage noise and input current noise, so the impedance of the cartridge plays a role. If you increase the bias of the input devices, the voltage noise goes down by the square rootof the current and the input current noise goes up by the square root of current, so there is an optimum bias point for a given impedance. This rule holds until you reach the thermal noise of the bulk base resistance, after which the voltage noise does not continue to fall with increasing bias. Forall these reasons, a good low-noise transistor will have low bulk base resistance and high Beta, which is tricky because these parameters tend to go in opposite directions.
    You can get JFETS with very low voltage noise and these are often preferredover bipolars because they have 0 input noise current. This makes it easier to get a good noise figure for a given cartridge impedance.

    Whenever I read threads about phono preamps I feel like I've been transported in time back to 1970, when these topics were all the rage. Ten years from now this topic will be covered by people giving invited talks in retirement homes :)


    Bob
     
  7. I wonder what to make of the two plots on the 2nd row of page 5.
    Same scales, same measurement conditions, different curves.
    Comparing with the Philips datasheet, I gather the measurement
    frequency was probably different.

    Anyway, say we were to use a BC109 at Ic=10uA and a source
    resistance of 20kOhm, the 1.5dB noise figure works out to
    a tad under 12nV/rtHz, if I got my arithmetic right.

    Almost any JFET can beat that with ease!

    Jeroen Belleman
     
  8. RealInfo

    RealInfo Guest

    Thanks
    Elico
     
  9. Mark

    Mark Guest

    speaking of noise figure...
    something thats troubled me...

    it seems to me the any LNA ***that provides a good input match****
    (talking about RF amplifiers in a 50 Ohm system) and is physically at
    room temperature cannot also have a noise figure better then 3 dB.

    To look at it another way, can you create an active (or otherwise)
    50 Ohm load that creates less noise than a 50 Ohm resistor creates?

    Mark
     
  10. Robert Baer

    Robert Baer Guest

    Start by biasing the transistor the same way as in final circuit,
    most especially the collector current and base input resistance.
    Use as a common emitter amplifier & pick off signal at collector with
    low noise amplifier.
    Calibrate the gain, use bandwidth filters.
    May calibrate by inserting white noise at input of base (without
    changing the base source impedance) and cranking up to double reading;
    note value, convert to dBm.
    May then use that to calibrate meters at those filters.
    Refine instrumentation and sell it!!!
     
  11. Robert Baer

    Robert Baer Guest

    Check on that last point..good low noise transistors have low base
    spreading resistance which always makes things worse than the theory
    mentioned.
    What is interesting is that noise measured in the audio region
    correlates very well with RF NF, as long as that base spreading
    resistance is low.
    ..and that can be approximated from spot noise measurements at
    nominal currents and very low (collector) currents.
     
  12. Guest

    That's not what noise figure is. It is defined as the multiple of the system resistance noise. The most advanced low noise amplifiers extant are probably the RF amps for satellite receiver front ends, typically just a few tenths of db NF last time I checked.
     
  13. Robert Macy

    Robert Macy Guest

    ok I'll bite.

    The noise at the junction is actually based on sqrt(25ohms) because
    the two 50 ohms are in parallel, the supply Z and 50 ohm load Z are in
    parallel.

    Wait. you say that the 50 ohm resistor makes more noise? Yes, but by
    an additional sqrt(2) then that noise is divided by two to the same
    junction and then is added as the square root of the sum of the
    squares because of the lack of coherence and you're right back to the
    same noise as from a 25 ohm resistor. So that means *if* you compare
    the input noise to that caused by a 50 ohm resistor, anything above
    that becomes the NF.
     
  14. Guest

    In theory it's possible to synthesize a 50 ohm resistor using the Miller effect, and end up with a resistor that has less than 4KTR noise. Assuming the amplifier you use for the Miller effect is ultra-low-noise. This could buy you 3db in the limit, assuming you used this resistor as a termination. Don't know how practical this really is.

    Back to the OP's topic, generally real products use JFETS. You can get themwith less than 2 nv/root-hz, I think. Some of those parts have probably gone obsolete.

    Bob
     
  15. Tim Williams

    Tim Williams Guest

    A pad? As in, resistors?

    /Imagines Phil dragging his fingernails across a chalkboard ;-)

    Tim
     
  16. Phil Allison

    Phil Allison Guest

    "Jeroen Belleman"

    ** So the NF with a JFET might be 1 dB = no audible difference.

    BTW:

    The effective noise Z of a mag PU is about 4000 ohms, when RIAA
    equalised. In real RIAA pre amps, JFETs have disadvantages
    (ie non linearity, low gain & large basic parameter variations)
    that outweigh any tiny noise advantage.




    .... Phil
     
  17. Robert Macy

    Robert Macy Guest

    Linear Technology's LT1011 [I think it is] has around 1 nV/rtHz input
    noise.

    Again, from memory Supertex makes some FETs with less than 1nV/rtHz.
     
  18. On Feb 13, 2:23 am, Phil Hobbs
    Exactly. The trick traces back to the vacuum tube era:
    W.S.Percival, "An Electrically Cold Resistance",
    the Wireless Engineer, May 1939, p. 237. It is necessary
    as a standard practice in room-temperature front ends
    of SQUID readouts, where the generator resistance indeed has
    the 50-ohm Johnson noise *but* it is located in LHe.

    Nowadays eg. the VCA2611 and AD8331 use the technique. I'm
    in impression that most low-noise rf/microwave gain blocks
    utilize that technique to move the noise match and power match
    to roughly the same impedance.

    The technique is effectively the same as the thought
    experiment of damping an indicator needle, discussed in
    many thermodynamics textbooks. I think I got first exposed
    to the idea in the Kittel's book, without realizing how
    widely it is applicable.

    Regards,
    Mikko
     
  19. ´
    That's right, that was Nyquist's original thought experiment.
    I have been wanting to demonstrate it, but it'd be tough to
    get a measurable effect.

    Actually, in Transiton Edge bolometers the Johnson noise
    is modified because of the correlated temperature fluctuation
    in the heat bath - because the electrical power present in
    each upward (downward) voltage swing is drawn from (dumped
    into) the thermal bath.

    Vinante et. al. have used a SQUID and feedback to cool
    a metal bar weighting a ton into microkelvin range of
    temperatures (although the above wording cheats a bit).

    Regards,
    Mikko
     
  20. Indeed. That undocumented peak spoiled our noise-cancelling
    readout back in 1994. There's another peak at 2.5 MHz. We had
    to move to the AD797.

    Regards,
    Mikko
     
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