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Old 11th December 2003, 06:07 PM   #1
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Default Noise figure vs signal source resistances in JFETS datasheet

Looking at the NF vs Rg plot of the datasheet of 2sk389, 2sk170 and 2sk117, it indicates that to get lowest noise figures, the signal source resistance should be around 10kohms. So doesn't it seem to mean that if you have an output impedance of say 100 ohms from your CD player and you are designing a low-noise preamp using jfets, you should put a resistor of 10kohms at the gate of the jfet to mimic a 10k source resistance ? This is counter-intuitive since that 10kohm will add voltage noise at the input that will be gained up by the preamp.

Let's phrase the question another way. What should I do at the input of the preamp to get the lowest noise performance based on the NF vs Rg plot from the datasheet ?

Thanks,
rlim
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Old 11th December 2003, 06:25 PM   #2
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NO, you have confused the concept. NF is not important with jfets. Never deliberately add gate resistance to a jfet. It just makes it more noisy.
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Old 11th December 2003, 09:12 PM   #3
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General:

> to get lowest noise figures, the signal source resistance should be around 10kohms. So doesn't it seem to mean that ... you should put a resistor of 10kohms at the gate of the jfet to mimic a 10k source resistance?

No.

It means that if you have a perfect transformer, you wind it to step-up or step-down so the amplifier "sees" 10K.

If you had a 100Ω pickup, and that perfect transformer, a 1:10 voltage ratio (1:100 impedance ratio) would give higher source voltage and thus better Signal/Noise ratio.

There are no perfect audio transformers.

Imperfect transformers are widely used for moving-coil phono and microphone inputs. MC phono (and also ribbon mikes) have such low-Z low-Voltage outputs that it is hard to make an amplifier work great directly, and the transformer's flaws may be less than the amplifier's flaws. Transformers are also handy for balancing and floating long lines in professional studios, even when the noise-resistance is not a big issue. However very-high-Gm low-R BJTs and JFETs have been replacing transformers in most applications. Ribbon mikes are still transformered, MC phono may be one or the other, and dynamic mike preamps are transformerless except when high isolation is needed, or last-dB noise is needed, or when transformer coloration is desired.

Specific:

> if you have an output impedance of say 100 ohms from your CD player and you are designing a low-noise preamp using jfets,

Then I bet you will find that you can ignore noise calculations, that a simple design will wind up at far lower noise than the CD noise level or the noise level of the post-DAC amp (if any).

Very roughly: home hi-fi CD players deliver 2VRMS, noise (digital grit) is 96dB lower, or 32 microVolts. Even a TL071 does 2µV. Generic 10/$1 JFETs are similar, if you keep them above 1mA or so. Yes, you may want to keep preamp noise "far" below source self-noise, but I would think that 2 µV is "far" below 32µV.

For "best" noise, use a 100Ω:10KΩ transformer. The CD-format peak signal level is transformed up to 20VRMS, the noise/grit level is at 320µV, and maybe your FET noise stays at 2µV. The 100Ω transformed to 10KΩ adds another 2µV. So you have 20VRMS signal, 320µV digital grit, 4µV source and device noise. This isn't an audible change in noise from the no-transformer situation. And you will probably have overload trouble on 20VRMS (28Vpk) signals.
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Old 11th December 2003, 09:37 PM   #4
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Default No wonder Mr. Curl gets impatient

I have no idea what the last post was about and the only was about and the only bit of sanity in it was the remark about the Jfets noise be small enough to be irelevent at line level signals. Noise figure is really only useful as figure of merit in RF design.

What the curve means is that the noise contribution of the jfet refered to the input is less than the thermal noise of the source resistance for points on the curve below 0 dB. Look at the E sub N figures if youwant to know what the noise voltages are. The impedances in line level circuit are chiosen for bandwith and distotion. Low impedances for better bandwidth but not low enoungh to increase the distortion of the circuits driving them.


http://www.eutelsat.com/glossary/8_1_2.html
Noise Figure A method for quantifying the electrical noise generated by a practical device. The noise figure is the ratio of the noise power at the output of a device to the noise power at the input to the device, where the input noise temperature is equal to the reference temperature (290 K). The noise figure is usually expressed in decibels.

http://www.rf-amplifiers.com/index.p...c=noise_figure
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Old 11th December 2003, 10:50 PM   #5
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Both of the above threads are correct, but NF is almost useless with audio jfets. I know, I know, they still put it on the spec sheet, BUT it is a hangover from low noise bipolar transistors, where it really makes a difference. All of the fets that you mentioned are pretty quiet. This means that they have low voltage noise. If you are transformer coupled, then adding gain to the transformer will give you a better noise figure, because you are then operating at a higher input level. The tradeoff is the output impedance of the transformer, which means virtually NOTHING to both tubes and fets, BUT is VERY important with bipolar transistors. This is because bipolar transistors have an extra noise source in the base emitter junction that will go bonkers when you have too much input source impedance. I hope that this helps.
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Old 11th December 2003, 11:10 PM   #6
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Default I agree with John, actually....

As my background is RF design, NF is an important concept there, as it also realizes that components further down stream in the RF chain can also contribute noise. In those situations, you need to know the gain, and NF, of every part in the chain.

I have never worried about NF at audio, period.

But.....having said that.......

"Never" is an awful long time, John. I suppose that if you put a 10 ohm gate damper into one those Toshiba JFETs that we both use, the noise contribution of that individual stage would go up 2 dB or so, compared to a dead short on the input.

However, in the real world, the average guy here will not have a perfect, zero ohm source. The use of a gate damper, when judisciously chosen, will not have a staggering effect on noise contribution. In fact, in a lot of "real world" applications, its effect may almost be totally swamped out. Not completely, but almost.

Mind you, I am not talking about doing something obviously ridiculous, like putting a 1K gate damper in a MC phono preamp.

Jocko
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Old 12th December 2003, 12:11 AM   #7
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For the last 30 years, my total input noise has been equivalent to a 10 ohm resistor. Try that, with added resistance on the input.
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Old 12th December 2003, 12:33 AM   #8
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Hi,

Quote:
For the last 30 years, my total input noise has been equivalent to a 10 ohm resistor. Try that, with added resistance on the input.
Good, I know pretty well what Jocko means and what John means.

The net result of these gatestopper/gridstopper craze is that I see an awful lot of people in both transistor based and tube based designs using values that are quite likely to not not only damp gates but also severely reduce bandwidth for no reason.

My advice is, if you don't have a scope and you don't experience oscillation, keep gatestopper/gridstopper values to a minimum if not you pay twice: you lose bandwidth and information and instead of gaining S/N you'll lose it even more.

With CDP replay, curtailing frequency response is one way to mask a problem but when it hits way below the 20Kc mark it also becomes a problem when countermeasures are overdone.

Concluding I'd recommend tackling RF right at the source trying to prevent it from entering the audiocircuit in the first place.

Cheers,
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Frank
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Old 12th December 2003, 12:39 AM   #9
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Default Horses for courses

30 years ago we didn't have cell phones and 1Ghz PCs putting RFI into everything.
I think I will take a hit of a few db in noise for the RF suppresion and lack of ringing from high Q resonate circuits at RF frequencies. It is hardly a secret that good RF design techniques pay pretty good dividends even in audio frequency circuits.
Throw digital audio into the equation and you had better be designing for what goes on at RF frequencies with equal importance to how the circuit performs at audio frequencies. RFI will not go away just because you ignor thinking about it. It gets worse the more RF spectrum devices go into use every day.
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Old 12th December 2003, 12:42 AM   #10
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Thumbs up Good luck

"Concluding I'd recommend tackling RF right at the source trying to prevent it from entering the audiocircuit in the first place."

You had better take away everyones cell phones and PCs then...........
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