RF Transistor Noise Source
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From: John Larkin on 28 Dec 2009 20:58 On Mon, 28 Dec 2009 13:49:05 -0800, RST Engineering <jweir43(a)gmail.com> wrote: >. >. >There was a general discussion in this NG a couple of weeks ago about >using a lightly-biased zener as a noise source. There was no clear >definition about how flat or to what frequency the noise was useful. > >It got me to thinking and I'll do the experiment as soon as I can >clean off my bench, but what do you think I'm going to see for >reasonable noise bandwidth if I use a small signal (like a 2N5770 or >918) and use the emitter-base junction as the zener. Most of them >zener somewhere around 5 volts and that should be reasonable. > >Most of the comments regarding bandwidth using a "regular" zener >centered around the rather large junction area necessary to carry some >decent current; the junction of an RF transistor ought to be at least >an order of magnitude (several??) smaller than that. > >Thoughts? > >Jim That sounds good. A "real" RF transistor might deliver wider bandwidth, from having less junction capacitance. BFS17, BFT25, something like that. Please let us know what you find. John
From: miso on 28 Dec 2009 21:17 On Dec 28, 1:49 pm, RST Engineering <jwei...(a)gmail.com> wrote: > . > . > There was a general discussion in this NG a couple of weeks ago about > using a lightly-biased zener as a noise source. There was no clear > definition about how flat or to what frequency the noise was useful. > > It got me to thinking and I'll do the experiment as soon as I can > clean off my bench, but what do you think I'm going to see for > reasonable noise bandwidth if I use a small signal (like a 2N5770 or > 918) and use the emitter-base junction as the zener. Most of them > zener somewhere around 5 volts and that should be reasonable. > > Most of the comments regarding bandwidth using a "regular" zener > centered around the rather large junction area necessary to carry some > decent current; the junction of an RF transistor ought to be at least > an order of magnitude (several??) smaller than that. > > Thoughts? > > Jim One thing to keep in mind is the transistor is easily damaged when zenered. You should do this with a current limited supply, say 100ua. If you've ever done ESD testing or fuse testing, invariably the reverse biased diode is the thing that is easily fried. When popping metal fuses, you need to insure that the inductive kick of the zapper is such that after popping the fuse, the diode junction gets forward biased. This does dump current into the device, but that can be controlled by the size of the capacitor used in zapping. For ESD structures where there is no diode to the positive rail, the snap back of the "off" nfet saves the parasitic diode junction.
From: John Larkin on 28 Dec 2009 21:27 On Mon, 28 Dec 2009 18:17:39 -0800 (PST), "miso(a)sushi.com" <miso(a)sushi.com> wrote: >On Dec 28, 1:49�pm, RST Engineering <jwei...(a)gmail.com> wrote: >> . >> . >> There was a general discussion in this NG a couple of weeks ago about >> using a lightly-biased zener as a noise source. �There was no clear >> definition about how flat or to what frequency the noise was useful. >> >> It got me to thinking and I'll do the experiment as soon as I can >> clean off my bench, but what do you think I'm going to see for >> reasonable noise bandwidth if I use a small signal (like a 2N5770 or >> 918) and use the emitter-base junction as the zener. �Most of them >> zener somewhere around 5 volts and that should be reasonable. >> >> Most of the comments regarding bandwidth using a "regular" zener >> centered around the rather large junction area necessary to carry some >> decent current; the junction of an RF transistor ought to be at least >> an order of magnitude (several??) smaller than that. >> >> Thoughts? >> >> Jim > >One thing to keep in mind is the transistor is easily damaged when >zenered. You should do this with a current limited supply, say 100ua. It ruins their beta, but if the transistor will never be used as anything but a zener, who cares? I suppose the issue is whether the *zener* properties will change over time. I'm guessing that a transistor, especially an RF transistor, will have a much higher current density than a part designed to be a zener. This suggests some interesting experiments. John
From: Tim Wescott on 29 Dec 2009 00:07 On Mon, 28 Dec 2009 19:08:29 -0500, Phil Hobbs wrote: > On 12/28/2009 6:59 PM, Tim Wescott wrote: >> On Mon, 28 Dec 2009 14:49:06 -0800, RST Engineering wrote: >> >>> On Mon, 28 Dec 2009 16:07:24 -0600, Tim Wescott<tim(a)seemywebsite.com> >>> wrote: >>> >>>> On Mon, 28 Dec 2009 13:49:05 -0800, RST Engineering wrote: >>> >>> >>>> It'll be good to know what your results are. >>>> >>>> Twenty years ago you could buy a noise diode from MA-COM (IIRC; it >>>> may have been some other company), home-brew your own circuit to hold >>>> it, then send it back to MA-COM for calibration. I don't know if you >>>> still can. >>>> >>>> A noise diode was, of course, 'just a zener', optimized for use at >>>> microwave frequencies. >>> >>> Noisecom and Micronetics are the only two I know of. Noisecom used to >>> sell "factory seconds" to hams for pennies on the dollar but that >>> practice seems to have gone by the wayside. They, as you noted, would >>> also do a calibration of your design for a few bucks. Gone also. >>> >>> It will be fun to get back to experimenting with something where I >>> don't have a real good idea what the answer is going to be. >>> >>> Jim >> >> I think it's Noisecom that I was thinking of. Dang; I should have >> taken advantage while I could. >> >> I have thought that if you were building something low-noise enough you >> could measure the noise figure with a pair of transmission lines >> terminated in resistors: drop one into ice water (or dry-ice/acetone, >> or LN2), and heat the other one up (boiling water, or a >> not-quite-melted- solder heat furnace). Then switch between them. >> With no current flowing through the resistors, you'd certainly know >> their noise temperatures! >> >> > A common approach in physics labs is to terminate the input with a 300 > kelvin resistor, measure the noise, dunk the resistor in liquid > nitrogen, and measure it again. Works great. > Goodness you keep the heat turned up -- or is that in the summer? It's about 293K in here now, because I can get comfort cheaper with a sweater than by turning up the heat. -- www.wescottdesign.com
From: miso on 29 Dec 2009 00:28
On Dec 28, 6:27 pm, John Larkin <jjlar...(a)highNOTlandTHIStechnologyPART.com> wrote: > On Mon, 28 Dec 2009 18:17:39 -0800 (PST), "m...(a)sushi.com" > > > > <m...(a)sushi.com> wrote: > >On Dec 28, 1:49 pm, RST Engineering <jwei...(a)gmail.com> wrote: > >> . > >> . > >> There was a general discussion in this NG a couple of weeks ago about > >> using a lightly-biased zener as a noise source. There was no clear > >> definition about how flat or to what frequency the noise was useful. > > >> It got me to thinking and I'll do the experiment as soon as I can > >> clean off my bench, but what do you think I'm going to see for > >> reasonable noise bandwidth if I use a small signal (like a 2N5770 or > >> 918) and use the emitter-base junction as the zener. Most of them > >> zener somewhere around 5 volts and that should be reasonable. > > >> Most of the comments regarding bandwidth using a "regular" zener > >> centered around the rather large junction area necessary to carry some > >> decent current; the junction of an RF transistor ought to be at least > >> an order of magnitude (several??) smaller than that. > > >> Thoughts? > > >> Jim > > >One thing to keep in mind is the transistor is easily damaged when > >zenered. You should do this with a current limited supply, say 100ua. > > It ruins their beta, but if the transistor will never be used as > anything but a zener, who cares? I suppose the issue is whether the > *zener* properties will change over time. I'm guessing that a > transistor, especially an RF transistor, will have a much higher > current density than a part designed to be a zener. > > This suggests some interesting experiments. > > John The garden variety transistors (2n3904 for instance) has a very low current when it zeners. It makes a micropower reference. I'm not sure about RF transistors. The device is easily damaged as in hard failure when zenered. I can't speak for off the shelf transistors, but for IC diode junctions I've tested, what happens is the current flows at a hot spot on the junction, probably at a wafer defect. It gets zapped and goes ohmic. I've never done zener zap trimming, but it might work in that manner. If you had a bare die and an emission microscope or maybe even a sensitive video camera, you would see the hot spot as it zeners. Once it goes ohmic, you would put liquid crystals on the die and see a boiling effect at the point of failure with applied power. I'm not a reliability engineer, but have done the tests a few times. [I'm guessing reliability people don't hang out on design newgroups. ;-)] |