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Since the Vf vs If plot supplied by Cree is really tailored to the higher end of the current range, I decide to measure it at currents relevant to use in cathode biasing. I set up a simple test rig using a AA battery (1.61V measured) and a series of resistors on the + leg of the circuit, to deliver a range of currents to the diode, then measured the voltage across the diode for each current.The chart above shows the results for the current range of 2.79mA to 65.8mA for the 1A, 600V SiC Schottky Diode, Cree part # CSD01060A. Dynamic resistance looks to be ~1.6 ohms in the range of 10-65mA.
Although the Vf is a bit lower than that of an IR LED at 10mA (.84 vs 1.2V) the dynamic resistance (slope resistance) is more than 3 times less for the SiC Schottky diode, indicating it may be an excellent choice as a cathode bias diode (IR LED data from Morgan Jones' "Valve Amplifiers").
Now if we could just get some comparative noise data on the various diodes... I wonder whether the Schottkys would have an advantage here - they are known to be less noisy in switching applications, but the diodes do not switch on/off in a cathode bias application.
Edits: 07/05/09 07/05/09 07/05/09 07/05/09 07/05/09Follow Ups:
FWIW I've been using standard Schottkies for a some time now and although they offer much lower Vf, and often need to be stacked they are my preference for biasing small signal stages. The first time I tried them I was startled to hear the improvement over a cathode resistor supplying equal bias voltage in terms of detail, clarity and air. After many experiments I came to the (unsubstantiated) conclusion that it must be due to their extremely low dynamic resistance. Be aware though that while this can be an order of magitude lower than other diodes, and down to a small fraction of an ohm it varies dramatically from type to type so check the data sheets, they normally provide graphs. I've used this method of selection since and not been disappointed.
Naz
Thanks for the info, Naz,Any particular Si Schottkys you recommend? At what voltage level are you using them? Do you think the lack of any reverse recovery may be part of the reason (although they technically aren't switching on/off in this application)?
Any info will be greatly appreciated!!
Edits: 07/07/09 07/07/09
Sorry, I'm travelling at the moment. I can only remember the first I tried which were parts we happened to use in our products. These were dual SMT in a D-pak, 12CWQ03FN. The data sheet specifies a forward slope resistance (rt) of just 21.66 m Ohms and the graph bears this out. But there is one proviso and I'm perplexed to say that looking at this again in Spice for the few models I have (which I've never done before), a quick Sim indicates that at low currents (circa 10mA) rd is in the order of 2 - 5 ohms, similar to cree. This figure reduces dramatically with increasing current so I can only conclude that that even the part I've listed might be in the order of half an ohm or so at 10mA because the ultra low figure specified is undoubtedly at max current. Overall I doubt you'd hear the difference between these and cree but yes, Schottkies in general sound great and you are on the right path IMO.
Cheers,
Naz
I think the Crees are probably close in specs to the ones you are using, as their dynamic resistance also drops dramatically as current increases. At the 10mA current range, both are lower than LEDs. The advantage of the SiC is their higher Vf, so less need to be used in a stack. Since I'm moving to a CCS, it should minimize the impact of dynamic resistance anyway.
Doing it graphically I get ~3ohms at 10ma and ~1.17 ohms at 65ma. at 2.7ma I get ~12 ohms. From your numbers below where a current change of +- 2ma nets a voltage change of +-.013 calculates to an impedance of 6.5 ohms.
I don't expect this to make much of a difference in the cathode of your tube, I just wanted to point out that like the Rp of a tube, the "impedance" of a diode varies with current.
dave
I agree completely, Dave. I was doing a quick calculation assuming linearity. Of course, this diode is not linear at low currents. Also, to get to 3.36V I would need 4 diodes in series, raising the resistance by a factor of 4; 1.6*4 = 6.4, close to your 6.5 ohm value. If I use your 3 ohm value at 10mA it turns out to be ~+/- 0.7% variation in Vg.Do you feel that +/- .7% on the grid bias voltage could cause audible issues?
Edits: 07/05/09 07/05/09
Hey,
First guess is no but if you think about it something has to explain why various diodes sound different for a given operation range. Diode bias has been one of those plug an play things for me. I did not like the IR units I tried one bit even though the curve look really nice and linear. The one I ended up preferring has a mugh higher voltage drop and showed less linearity. Who knows maybe I like distortion :-)
dave
The equation I posted below shows that the second harmonic will reduce rather than increase with increased curvature.
No prediction can be made on third and higher harmonics without knowing the higher order factors for the mu of the V/T.
Mark Kelly
this is more useful than you know....these are also the high voltage Schottky's...
so good experimenting...good hunting...and may many benefits come your way for your good work...
i.e. probably lots of good music may come your way...
Peace,
-3db
Looks to be a non linear characteristic, approximately a 3/2 power law.
By eye, V = 0.8 + 0.4(I/10)^2/3.
You could calculate the higher order distortion products that this will produce by reflecting the delta V to the grid and taking derivatives. I doubt that it will be serious as the plate resitance is also a power law.
Mark Kelly
Not sure the non-linearity will be much of an issue for my application. I'll only be swinging at the max +/- 2mA peak to peak where I plan to run the 2C22 tubes in my preamp. For all intents and purposes the curve approaches linearity over such a small range.That current swing translates to a variation of grid bias of +/- 0.013V peak to peak at grid bias voltage of 3.36V, or +/- 0.39%.
Edits: 07/05/09
Thinking about it some more, you have a small negative value of dEg/dIp and a smaller positive value of d^2Eg/dIp^2.
The first will serve to reduce the gain of the stage very slightly (cathode degeneration). The second will result in a small reduction in 2HD as it opposes the sense of the usual d^2Ep/dIp^2 (note Ep not Eg in this )
Mark Kelly
Thanks for taking the time to measure and post your data. What part number and/or current rating was this particular Cree Schottky?
Cree part# CSD01060A, 1A, 600V Silicon Carbide Schottky Diode. Part ID also added to original post.
Edits: 07/05/09
As a relative newbie to DIY stuff, I've never tried anything other than cathode-biasing - but this looks interesting.
If you use a diode (eg Schottky) do you also use a bypass cap ?
Kind of answering my own question, wondering if I'm right ...
No cap needed I think. If I've understood it right, the cap is there to stabilise the voltage where current varies across the bias resistor. With diodes, the voltage drop doesn't vary significantly with small current changes - so no cap needed.
I'd be impressed with myself if the above is right - woohoo !
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