datasheets for 2n4417 2n4418 ?
In a Linear Audio article (in an article about low noise preamplifiers), there is a reference to the use of back-to-back gate/sources of 2n4417/8 as a substitute of "hard to find" G ohm resistors. Strangely, in my search, even the internet seems void of information (datasheets) about such devices or even a manufacture (let alone distributer). My guess several other JFETs serve as a substitute, but what a curious choice. Apparently, they are used "in the ionization chamber of smoke detectors" . ..
In a Linear Audio article (in an article about low noise preamplifiers), there is a reference to the use of back-to-back gate/sources of 2n4417/8 as a substitute of "hard to find" G ohm resistors. Strangely, in my search, even the internet seems void of information (datasheets) about such devices or even a manufacture (let alone distributer). My guess several other JFETs serve as a substitute, but what a curious choice. Apparently, they are used "in the ionization chamber of smoke detectors" . ..
I guess you found a typo? 🙂 It's 2N4117/8 they were quite common and cheaper than the resistors at the time, the trick works best with the lowest possible Is, the real typical reverse leakage on these is 20fA. If the reference mentioned is still on line it shows the equivalent resistance of the junction at 0V is the derivative of the diode equation at 0 which is directly related to Is. Even a 2pA leakage JFET will be much worse. The sensitivity of an ionization chamber or vacuum gauge is limited by the input leakage.
The slope of the diode equation at 0 has units of (big) Ohms and this has the same noise just like as if it was a resistor.
BAV199 dual diodes might work too ~.25$, but I have not tried them.
There were times that small cheap G Ohm resistors were not always available.
Sorry, in one of Barrie's personal history articles he designed a driver for a chart recorder with exceptional fidelity to the input and I thought there might be something applicable to meters there. It might have been another piece.
![DSCF0159[2].jpg](/community/data/attachments/467/467566-b5b340c344cd9cf1a3eb2d02068544c0.jpg?hash=tbNAw0TNnP)
In small quantities, the BAV199 runs $0.25, SMT 1G $1.51, TH 1G $0.64.
Thank you, again, Scott !
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Just caught that the BAV199 is a pair . . .
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Hi AKN,
Hi Scott,
Meters with the proper ballistics, along with peak LEDs would give you a pretty fair idea of music levels. We all got use to using just the meter and they provided enough information to record properly. For amplifiers, the peak LEDs or VFD displays worked extremely well. The Vu meters still gave enough information, but the peak LEDs added would tell you more than enough. I think that the "headroom exhausted" light on Carver products was pretty funny. Bob Carver has a sense of humor for sure.
-Chris
Yes, that's what they were. Very weird ballistics to watch.
Hi Scott,
I'd love to see that. I didn't notice this in the linked article.
Meters with the proper ballistics, along with peak LEDs would give you a pretty fair idea of music levels. We all got use to using just the meter and they provided enough information to record properly. For amplifiers, the peak LEDs or VFD displays worked extremely well. The Vu meters still gave enough information, but the peak LEDs added would tell you more than enough. I think that the "headroom exhausted" light on Carver products was pretty funny. Bob Carver has a sense of humor for sure.
-Chris
Teraohms in chip resistors
Welcome to Eltec Instruments
I had to characterize an Eltec Tohm one for use at 77K. I would have preferred a pulsed-reset charge amp, which was already developed, but was overruled, on the advice to him of a slimeball, by the spineless professor who was terrified of making a mistake and being teased by his peers. That and storm clouds gathering to place me on grant-to-grant funding led to my departure from UCLA in 1985.
Welcome to Eltec Instruments
I had to characterize an Eltec Tohm one for use at 77K. I would have preferred a pulsed-reset charge amp, which was already developed, but was overruled, on the advice to him of a slimeball, by the spineless professor who was terrified of making a mistake and being teased by his peers. That and storm clouds gathering to place me on grant-to-grant funding led to my departure from UCLA in 1985.
The high value SMT resistors seem to be readily available now, since the article I picked up 10 5G ones for only a few $. The average DIY'er needs caution in maintaining these levels of performance, for instance the 10 G SMT resistors I bought as NOS surplus turned out to be non-solderable versions of the termination. The back to back diodes on the MEMS mic were ~1T Ohm.
So they were just meant to be held down mechanically? Weird (although one way to avoid contamination I suppose).
Maybe conductive epoxy or it was just me, the ends came off instantly (very thin gold foil) when touched with a hot iron.
I worked with Ohmcraft and Ohmite 1...10G 0805 - no problems, Ohmite parts are available at digikey
Hi Chris,
Thanks for info.
As a teenager I persuaded my father to by a Alpine deck (just because those meters). If I only had kept it until todays date. I cant help it but I truly admire VU-meters (well designed that is). 🙂
I seem to remember that the equiv. resistance of a junction has only half-thermal noise,
as if the resistor was at 150 Kelvin typ. Must search the text book.
Gerhard
You are right Gerhard. The dynamic resistance (1/Gm) is not directly how the second stage shot noise referred to the input is derived. Gm comes into it, BUT many make this mistake, thinking that the dynamic resistance is the noise contributor from the second stage. It is really 1/2Gm.
thermal noise at zero volts
Scott is correct at zero volts---it is a slope resistance and shows thermal noise. For significant forward bias, i.e., injection, it becomes half-thermal. Oliver, from his great paper Thermal and Quantum Noise: ...If i = 0, we see that P = kTB, which is the thermal noise power available from any resistor. We should be very distressed to find any other result, since, with no applied voltage, the diode is in thermal equilibrium. We note from (57) that under heavy forward bias (i >> Is) the diode is half as noisy as a resistor...
Scott is correct at zero volts---it is a slope resistance and shows thermal noise. For significant forward bias, i.e., injection, it becomes half-thermal. Oliver, from his great paper Thermal and Quantum Noise: ...If i = 0, we see that P = kTB, which is the thermal noise power available from any resistor. We should be very distressed to find any other result, since, with no applied voltage, the diode is in thermal equilibrium. We note from (57) that under heavy forward bias (i >> Is) the diode is half as noisy as a resistor...
No, this is not the same as a transistor gm.
EDIT - Thanks Brad, My Linear Audio article had a reference to the derivation directly from the physics.
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