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Old 5th September 2012, 11:40 PM   #761
coluke is offline coluke  Italy
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Quote:
Originally Posted by elektroj View Post
Noise floor in the pictue looks strange, though. Are you using weighting filter?
Nope - just a quite conservative HF * input * filter: low level signals from my EMU are quite noisy, and at 60+ dB gain I just didn't want to turn the amp under test in a small LW tx . Output noise floor follows filter envelope, but level of the harmonics should be unaffected. I'm going to repeat the test during the weekend using the old faithful (HP 239A) as signal source - it's by far more clean in the HF range than my soundcard - I'll let you know. Have to understand why OL THD is lower (or CL THD higher) than expected.

L.

Last edited by coluke; 5th September 2012 at 11:54 PM.
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Old 5th September 2012, 11:56 PM   #762
gpapag is offline gpapag  Greece
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Originally Posted by scott wurcer View Post
Obscure, here you go germanium JFET's at 4.2K.

http://ntrs.nasa.gov/archive/nasa/ca...2000033912.pdf
Thank you Scott.

Fig.3 frequency scale looks a bit strange to me. Logarithmic, 3 decades, 8-188Hz.
Fig. 4 I think has been shown and discussed again.


Quote:
Originally Posted by benb View Post
It may lower noise, but lower temperatures definitely lowers conductivity of semiconductors, and at lower temperatures they just won't turn on.
Benb
I am really out of my waters here, but how lower “conductivity” matches with an increase in Drain current (fig. 2) and increase of transconductance (fig 4) of the above pdf?


George
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Old 6th September 2012, 01:44 AM   #763
bcarso is offline bcarso  United States
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Originally Posted by benb View Post
At the time (when I was in college, late 1970's) I thought it was pretty useless information unless one was going to work in a transistor manufacturing plant, but now my opinion is one can't have too much knowledge.
So VERY true.
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Old 6th September 2012, 01:51 AM   #764
RNMarsh is offline RNMarsh  United States
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Default thermal couples

Quote:
Originally Posted by gpapag View Post
(Just before the beer)

I am surprised.
First by the answer from the experts, as I think that this problem had to be addressed in microcircuits and micromachines.

Second by the “research” on copper/solder/copper. What research? This is the most common/dominating case of electrical connection in electronic circuits for how many decades?

It seems that a lot of work went/goes into finding similar coeffecient of thermal expansion for semiconductor processes, materials and packaging.
As a practical note - the closer the two metals/materials are on the Periodic table, the lower the thermal electric effects.
But there are diffusion issues with solder alloys and metals... like gold easily diffuses into solder and makes the joint brittle. Et al. Tonnes written on such things. -RNM

Last edited by RNMarsh; 6th September 2012 at 02:17 AM.
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Old 6th September 2012, 01:52 AM   #765
bcarso is offline bcarso  United States
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Quote:
Originally Posted by gpapag View Post
Thank you Scott.

Fig.3 frequency scale looks a bit strange to me. Logarithmic, 3 decades, 8-188Hz.
Fig. 4 I think has been shown and discussed again.




Benb
I am really out of my waters here, but how lower “conductivity” matches with an increase in Drain current (fig. 2) and increase of transconductance (fig 4) of the above pdf?


George
Lower temperature helps for a while with Silicon, but just not much colder than 100 K. The improvement is both as an increase in gm and a reduction in the temperature and thus thermal noise of the material. And if you are concerned about measuring tiny amounts of charge, the reduction in gate leakage is profound, provided the drain-gate voltage is low enough.

Germanium is a lot different than Silicon. GaAs also works pretty well at Helium temps.

One anecdote: low-temperature physicists did some circuitry that had to be very cold with bipolars. In order to have mobile carriers for transistor action, they included "grain-of-wheat" incandescents in the mix and had a little near-IR light shine on the chips. This I heard from Edward L. "Ned" Wright, a brilliant guy at UCLA. Eventually they got some devices made that worked without the light, by "doping the pee out of them" Of course they didn't work very well at room temperature
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Old 6th September 2012, 03:46 AM   #766
bcarso is offline bcarso  United States
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Default and now for something rather different

Stranded on a desert island needing a wideband unity-gain low-distortion inverter, and with almost nothing but a supply of 2SK2145s

What would you do?
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File Type: jpg All 2SK2145 inverter 09-05-12 001.jpg (221.2 KB, 351 views)
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Old 6th September 2012, 10:30 AM   #767
bcarso is offline bcarso  United States
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To clarify what is up in #766: Consider everything above R12 and R16. It's a fairly straightforward common-source amplifier with a matched current source load, with a floating level shift to a common-drain voltage follower with a matched current sink load. Closing the loop around it produces an inverting gain of about unity, with the noise dominated by resistor thermal noise. The performance by itself is not bad at all considering the minimal component count. The level shift could be done in a number of ways, perhaps the best except for physical size being a battery.

The stuff added to it enhances performance markedly, by effectively increasing loop gain and eliminating loading on the upper section's output. And yet the auxiliary section adds almost no noise --- the upper section remains by and large "in control".

The performance results will be spoiled to some extent by thermals in the input devices, since the full voltage swing appears there, although the BL parts should be close to a zero tempco operating point which will help a lot. There are ways around this with more parts. If the floating bias voltage in the upper section cannot be fairly low impedance, the helper resistor R16 can be capacitively coupled to the drain of Q1B.

Power supply rejection, distortion, and bandwidth predictions are all very good indeed. Nonfloating bias sources can be made but it adds a lot of parts. Variants include multiple output devices for higher drive current, higher-than-unity gain, etc.
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Old 6th September 2012, 02:02 PM   #768
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Originally Posted by Kindhornman View Post
jneutron,
Very off topic, but if you take a super-conductor to very low temperatures it will stop the flow of electrons then? Just made me wonder your comment to George.
No. Electron transport in a superconductor is significantly different than it is within semiconductors. The gap goop stuff was touched upon by benb.

Quote:
Originally Posted by gpapag View Post
(Just before the beer)



I am surprised.
First by the answer from the experts, as I think that this problem had to be addressed in microcircuits and micromachines.

Second by the “research” on copper/solder/copper. What research? This is the most common/dominating case of electrical connection in electronic circuits for how many decades?
George
I did attempt to post a link, but it became very messy.
I'll try again.
Here's the home page..

Lake Shore Cryotronics, Inc. is a Leader in developing innovative measurement and control solutions.


Yesterday I found reference information which included copper and solder joints. They didn't say much though, did provide techniques for making sure the joints didn't affect the measurement. I'll try again later.


Here's a helpful guide:
http://www.lakeshore.com/Documents/LSTC_appendixC_l.pdf
Here's calibration stuff.

http://www.lakeshore.com/Documents/LSTC_appendixD_l.pdf

jn
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Old 6th September 2012, 04:01 PM   #769
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Quote:
Originally Posted by gpapag View Post
(Just before the beer)

I am surprised.
First by the answer from the experts, as I think that this problem had to be addressed in microcircuits and micromachines.


George
Sorry if I wasn't clear, I was looking into typical packages, a few microns of gold over kovar, and bond wires which are often alloys. I consulted an expert on the very thin metal film issues, in the end I concluded it didn't matter much by experiment.
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Old 6th September 2012, 05:09 PM   #770
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Quote:
Originally Posted by RNMarsh View Post
Patents can be tricky things -- a current source where before there was a resistor and a jFET where there was a bipolar. Small things sometimes but in a competitive market it can help. For me it is the concept that counts - but knowing that the results are in the details.

I published the same bias topology with an all bipolar compl-push-pull line stage in the quarterly, TAA (3/80).
[The work, of course was done long before it got around to publishing.]

Pioneer never called me when I published my circuit maybe because it would put their patent in question as obvious appl of existing design. Note: Pioneer applied for thier patent same year (1980). And, granted in 1982. Certainly the circuit details are different (allowing a patent) but the bias topology had been done before their patent.

PS -- many of you know that when you work for a university or government - all inventions belong to them. One invention was a circuit topology for a MC pre-preamp appl. I got release from the Lab's patent lawyers to patent it myself... it had no use to their work so didnt want to pay for the patent -- I published it instead. What do I need a patent for that i should spend 10 grand to get it? And, I liked my day job. - Dick Marsh
One can say the same for the circuit you published in TAA, JLH published the original idea back in 1968 although he did use resistors and not ccs.
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