NOISE FIGURE OF A BIPOLAR TRANSISTOR

[RealInfo]

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

 

[Jeroen Belleman]

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 .

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

 

[[email protected]]

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 .

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 ?

 

[Jeroen Belleman]

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 .

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

 

[Phil Allison]

"RealInfo"

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 bipolar transistor is defined measured .

** 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

 

[[email protected]]

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

 

[Jeroen Belleman]

"RealInfo"

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 bipolar transistor is defined measured .

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

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

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

 

[RealInfo]

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

Thanks
Elico

 

[Mark]

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.

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

 

[Robert Baer]

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

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!!!

 

[Robert Baer]

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

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.

 

[[email protected]]

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

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.

 

[Robert Macy]

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

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.

 

[[email protected]]

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

 

[Tim Williams]

RF amplifiers are always about 2:1 in my experience too. If you don't
like it, put in a pad.

A pad? As in, resistors?

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

Tim

 

[Phil Allison]

"Jeroen Belleman"

"RealInfo"

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 bipolar transistor is defined measured .

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

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

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!

** 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

 

[Robert Macy]

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 them with less than 2 nv/root-hz, I think. Some of those parts have probably gone obsolete.

Bob

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.

 

[Mr Stonebeach]

On Feb 13, 2:23 am, Phil Hobbs

Not Miller, which is capacitive, but a similar idea--you use a quiet
inverting amplifier to jiggle the opposite end of a resistor to make it
look smaller.

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

 

[Mr Stonebeach]

If you connect a 50 ohm resistor, at room temp, to such an amplifier,
it will cool the resistor. A little.

´
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

 

[Mr Stonebeach]

LT1028A has a nasty noise peak around 300 kHz that they don't tell you
about.  The datasheet noise plot conveniently ends well below that, even
though it's a 100 MHz op amp.  (Marketing again.)

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