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
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
figure .My question is how exactly a noise figure of a single bip[olar transistor is defined measured .
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 ?
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
"RealInfo"
** Noise figures for transistors are not numbers, but graphs.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
For a bipolar transistor you have both input voltage noise and input curren t noise, so the impedance of the cartridge plays a role. If you increase th e bias of the input devices, the voltage noise goes down by the square root of the current and the input current noise goes up by the square root of c urrent, 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. For all these reasons, a good low-noise transistor will have low bulk base res istance 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 preferred over bipolars because they have 0 input noise current. This makes it easie r to get a good noise figure for a given cartridge impedance.
Whenever I read threads about phono preamps I feel like I've been transport ed in time back to 1970, when these topics were all the rage. Ten years fro m now this topic will be covered by people giving invited talks in retireme nt homes :)
Bob
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
figure .My question is how exactly a noise figure of a single bip[olar transistor is defined measured .
Thanks Elico
IIRC the best noise match impedance of a bipolar transistor amplifier does not change when you go from common-emitter to common base. The power match does, but not the noise match.
If you have a data sheet for a transistor that's designed as a low-noise amplifier it should have various curves showing noise figure vs. various things. IIRC Motorola would plot noise figure vs. collector current and source impedance.
-- Tim Wescott Control system and signal processing consulting www.wescottdesign.com
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
Grab yourself a cold beer from the fridge, and think about how it could be at 40F in a 70F ambient. Same basic answer.
Cheers
Phil Hobbs
-- Dr Philip C D Hobbs Principal Consultant ElectroOptical Innovations LLC Optics, Electro-optics, Photonics, Analog Electronics 160 North State Road #203 Briarcliff Manor NY 10510 hobbs at electrooptical dot net http://electrooptical.net
Well, I suppose _maybe_ I could be a bit less of a smartass about it. ;)
The small-signal emitter resistance of a bipolar transistor is
r_E = kT/(eI_C),
i.e. about 26 mV/I_C at room temperature. If you multiply that by the shot noise of the emitter current, which is
i_N = sqrt(2*e*I_C),
and do two lines of algebra, you get
v_N = sqrt(2*k*T*r_E).
Comparing this with the usual Johnson noise formula, you find that the noise temperature of a forward-biased emitter is T/2, i.e. 150K at room temperature.
(At thermal equilibrium, you can't have a bias voltage or a net emitter current, so the forward and reverse diffusion currents are equal. They each contribute half of the fluctuations, so the factor of 2 is restored. You need the full Ebers-Moll expression to show this, but I'm too lazy to type it out.)
The beta of a BJT is the really low noise thing in electronics. The intrinsic base (i.e. neglecting the actual resistance of the silicon) has an impedance r_B = beta * r_E, but has exactly the same noise as the emitter. (It has to, because there are only two wires involved.)
Thus the noise temperature of the input resistance of an ideal BJT CE amplifier ought to be right around T_J/(2*beta).
It's never quite that good, of course, because the base current has shot noise and there are real physical resistances that have noise of their own.
Cheers
Phil Hobbs
-- Dr Philip C D Hobbs Principal Consultant ElectroOptical Innovations LLC Optics, Electro-optics, Photonics, Analog Electronics 160 North State Road #203 Briarcliff Manor NY 10510 hobbs at electrooptical dot net http://electrooptical.net
figure .My question is how exactly a noise figure of a single bip[olar transistor is defined measured .
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!!!
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.
I've seen suggestions (in, for example, "Radio Frequency Design" by Hayward) that you can get simultaneous power match and noise match in an amplifier using transformer feedback (he called it "advanced feedback methods"). I'm not sure if this gets you below that magic T/2 noise temperature, though -- I haven't even done the math on the things, much less built them and tried them out.
-- My liberal friends think I'm a conservative kook. My conservative friends think I'm a liberal kook. Why am I not happy that they have found common ground? Tim Wescott, Communications, Control, Circuits & Software http://www.wescottdesign.com
That's not what noise figure is. It is defined as the multiple of the syste m resistance noise. The most advanced low noise amplifiers extant are proba bly the RF amps for satellite receiver front ends, typically just a few ten ths of db NF last time I checked.
In an amplifier, it's the output power you care about, not the power delivered to the input stage. Even at low frequency, the "noise match" isn't the same as maximum power transfer on the input.
The output voltage is proportional to the input voltage, which for a constant available input power goes as sqrt(Zs)*Zin/(Zs+Zin).
That peaks at Zs = Zin, as expected, but since the noise temperatures of the two resistances are different, the noise voltage at the input continues to change. That shifts the SNR peak away from maximum power transfer.
Cheers
Phil Hobbs
-- Dr Philip C D Hobbs Principal Consultant ElectroOptical Innovations LLC Optics, Electro-optics, Photonics, Analog Electronics 160 North State Road #203 Briarcliff Manor NY 10510 hobbs at electrooptical dot net http://electrooptical.net
Microwave LNAs have noise temps in the 60K range, fraction of a dB noise figure. The input impedance of an amplifier can look like a very cold 50 ohm resistor.
If you connect a 50 ohm resistor, at room temp, to such an amplifier, it will cool the resistor. A little.
-- John Larkin Highland Technology, Inc jlarkin at highlandtechnology dot com http://www.highlandtechnology.com Precision electronic instrumentation Picosecond-resolution Digital Delay and Pulse generators Custom laser drivers and controllers Photonics and fiberoptic TTL data links VME thermocouple, LVDT, synchro acquisition and simulation
I've tested a lot of InGaAs (? ERA-type stuff) MMICs and almost all seem to have Zin well below 50 ohms, closer to 30 usually. I've assumed that is to optimize NF. Sirenza makes one SiGe part that is really 50 ohms, and can be tuned to exactly 50 by fiddling with the device current.
The RF guys seem to think that an SWR of 2:1 is good enough.
-- John Larkin Highland Technology, Inc jlarkin at highlandtechnology dot com http://www.highlandtechnology.com Precision electronic instrumentation Picosecond-resolution Digital Delay and Pulse generators Custom laser drivers and controllers Photonics and fiberoptic TTL data links VME thermocouple, LVDT, synchro acquisition and simulation
RF amplifiers are always about 2:1 in my experience too. If you don't like it, put in a pad. ;)
Cheers
Phil Hobbs
-- Dr Philip C D Hobbs Principal Consultant ElectroOptical Innovations LLC Optics, Electro-optics, Photonics, Analog Electronics 160 North State Road #203 Briarcliff Manor NY 10510 hobbs at electrooptical dot net http://electrooptical.net
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