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#41
February 7th 05, 12:37 AM
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Paul Hovnanian P.E. wrote:
In other words, this data is just a plot of a diode's DC I vs V characteristic, right? What is of more interest is the slope at a given DC operating point. If we pick 0V, for example, the above data (within the limits of its precision) gives a flat line around that point (+5mV 21 Mohms, -5mV 21 Mohms). With a 100 uV signal, you might as well throw a 21 M ohm resistor in there instead. Exactly. At very small currents, the diode is just a resistor (shunted by a capacitance). At slightly small currents, it's a very poor diode, with reverse current almost equal to the forward current, so on each negative half cycle you suck out nearly all the charge you delivered during the positive half cycle. Roy Lewallen, W7EL |
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#42
February 7th 05, 03:58 AM
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Winfield Hill wrote:
No, it means its a better diode at low currents. See my curves again, http://www.picovolt.com/win/elec/com...de-curves.html Note the 1n458 and the JFET diodes, which follow the theoretical 60mV/decade rule down to very low currents. As for Roy Lewallen's "ratio of reverse to forward current" argument, there is no reverse current for these fine fellows, at least for DC and reasonably low frequencies. Sure there is. All diodes have reverse current. It's the very crummy gold-doped 1n4148 that falls over. Awwkk! The gold doping is done to dramatically reduce charge storage time. Without it, the voltage across a diode continues to be in the forward direction for some time after you reverse the current through it. While a non-gold-doped diode might look good in DC tests, it makes a lousy rectifier of RF. In the extreme case, it acts like a PIN diode (which is simply a diode designed intentionally to have a long charge storage, or reverse recovery, time). Alas, life is full of tradeoffs. Roy Lewallen, W7EL |
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#43
February 7th 05, 06:18 AM
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I need to clarify this. My comments apply only to junction diodes, which
virtually all silicon diodes are. Schottky diodes don't exhibit this charge storage effect. That's one reason they're often used in high frequency switching supplies. Their leakage current is, however, much greater than silicon diodes. Roy Lewallen, W7EL Roy Lewallen wrote: The gold doping is done to dramatically reduce charge storage time. Without it, the voltage across a diode continues to be in the forward direction for some time after you reverse the current through it. While a non-gold-doped diode might look good in DC tests, it makes a lousy rectifier of RF. In the extreme case, it acts like a PIN diode (which is simply a diode designed intentionally to have a long charge storage, or reverse recovery, time). Alas, life is full of tradeoffs. Roy Lewallen, W7EL |
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#44
February 7th 05, 02:09 PM
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Mike Monett wrote:
Excellent description - thanks. Only one small problem - as Win pointed out, Bob Pease feels a diode-connected 2N3904 has lower leakage at low voltage than a 1N4148: "What's All This Comparator Stuff, Anyhow?" http://www.elecdesign.com/Articles/A...9517/9517.html Does this mean a 2N3904 has a shallower slope than a 1N4148 through zero, or perhaps one or the other has an offset, such as the Agilent Zero Bias Schottky Detector Diodes shown in AN969? http://www.spelektroniikka.fi/kuvat/schot8.pdf Regards, Mike Monett I'm not sure what you mean by an "offset" -- all diodes cross through the origin of the I-V curve, when excited by DC, anyway -- unless they contain a battery. In the reverse direction, the current pretty much levels off beyond a small reverse voltage. The current of this level part is the saturation current. Again, don't think that good DC characteristics make for a good RF detector. A number of other factors, which have been discussed here, are very important. As I recall, only transistors designed as saturated switches (2N918 comes to mind, but it's been a long time, so don't quote me) are gold doped. Ones which aren't, and I'm quite sure the 2N3904 is in that category, will have long reverse recovery times so will make poor RF rectifiers. Circuits became too fast for saturated switches long, long ago, so I'd be surprised if gold doping is done any more except for replacement transistors in very old equipment. You can learn a lot with a very simple setup consisting of nothing more than a variable amplitude signal generator, a diode, load resistor and capacitor, and a meter or scope. SPICE should also show these effects provided you use good models. Roy Lewallen, W7EL |
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#45
February 7th 05, 05:55 PM
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In article ,
Roy Lewallen wrote: As I recall, only transistors designed as saturated switches (2N918 comes to mind, but it's been a long time, so don't quote me) are gold doped. Ones which aren't, and I'm quite sure the 2N3904 is in that category, will have long reverse recovery times so will make poor RF rectifiers. Circuits became too fast for saturated switches long, long ago, so I'd be surprised if gold doping is done any more except for replacement transistors in very old equipment. Don't people still use 2N2369As, or at least the plastic version? If not, what do they use instead? (Does gold doping work for PNP transistors? I don't see why it wouldn't, but I've never seen a specific reference to a gold-doped PNP.) |
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#47
February 7th 05, 06:26 PM
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I read in sci.electronics.design that Jim Thompson
wrote (in k58f015dte50otru2ams8qavq858c3sl8b@ 4ax.com) about 'gold doping (was Diode and very small amplitude high frequencies signals)', on Mon, 7 Feb 2005: No. Doping "sex" (of gold) is wrong for the N-type base, which is where you're trying to induce recombination. What to use, then? Ion-implanted krypton? -- Regards, John Woodgate, OOO - Own Opinions Only. The good news is that nothing is compulsory. The bad news is that everything is prohibited. http://www.jmwa.demon.co.uk Also see http://www.isce.org.uk |
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#48
February 7th 05, 08:21 PM
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On Mon, 7 Feb 2005 17:26:30 +0000, John Woodgate
wrote: I read in sci.electronics.design that Jim Thompson wrote (in k58f015dte50otru2ams8qavq858c3sl8b@ 4ax.com) about 'gold doping (was Diode and very small amplitude high frequencies signals)', on Mon, 7 Feb 2005: No. Doping "sex" (of gold) is wrong for the N-type base, which is where you're trying to induce recombination. What to use, then? Ion-implanted krypton? Sounds good to me, and it'll glow in the dark ;-) ...Jim Thompson -- | James E.Thompson, P.E. | mens | | Analog Innovations, Inc. | et | | Analog/Mixed-Signal ASIC's and Discrete Systems | manus | | Phoenix, Arizona Voice  480)460-2350 | || E-mail Address at Website Fax 480)460-2142 | Brass Rat || http://www.analog-innovations.com | 1962 | I love to cook with wine. Sometimes I even put it in the food. |
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#49
February 8th 05, 04:25 AM
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When I needed diode clamps that would work with +/-100mV with less
than 10pA of leakage, I always used selected Jfets with some back bias. The DPAD10, etc. series of diodes that were designed for this were too expensive. But that was when I was at Fluke and I could specify exactly what I wanted and the vendors would come knocking. Most of the parts were specially selected from National process 50, which was used to make 2N4416 RF transistors. The diode equation works over all ranges of the diode. We also used 2n2484 over 9 decades of current (pA to mA) of DC and tested them to be very log-linear. At AC, as already stated, you have to include the capacitance in the model. A finger print on the transistor body would ruin the response. Steve. -- Steven D. Swift, , http://www.novatech-instr.com NOVATECH INSTRUMENTS, INC. P.O. Box 55997 206.301.8986, fax 206.363.4367 Seattle, Washington 98155 USA |
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#50
February 8th 05, 03:25 PM
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Steven Swift wrote...
When I needed diode clamps that would work with +/-100mV with less than 10pA of leakage, I always used selected Jfets with some back bias. The DPAD10, etc. series of diodes that were designed for this were too expensive. But that was when I was at Fluke and I could specify exactly what I wanted and the vendors would come knocking. Most of the parts were specially selected from National process 50, which was used to make 2N4416 RF transistors. Yes, the PAD-1 and pn4117 JFET curves on my plot show your point, http://www.picovolt.com/win/elec/com...de-curves.html with about 150 to 200mV forward voltage at 1pA. In that setup, the current for under 100mV was well below what I could detect. The diode equation works over all ranges of the diode. We also used 2n2484 over 9 decades of current (pA to mA) of DC and tested them to be very log-linear. At AC, as already stated, you have to include the capacitance in the model. A finger print on the transistor body would ruin the response. Not to mention the long reverse-recovery time of those parts. I put up my graph to counter Dave's claim of a 600mV "barrier potential", not to argue that these diodes make good low-level RF rectifiers. :) -- Thanks, - Win |
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