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Old 11th March 2009, 08:21 PM   #201
jwb is offline jwb  United States
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I've been trying to work that out, too. There aren't a lot of 220-packaged BJTs with similar specs to the good old 2907. I only need 100mA for this application. If I needed higher power, I might parallel them instead. you can parallel as many CCS as you want in this application. It's harmless.
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Old 11th March 2009, 08:25 PM   #202
Salas is offline Salas  Greece
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What is its main characteristic that you value? Bandwidth?
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Old 11th March 2009, 08:30 PM   #203
Salas is offline Salas  Greece
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P.S. Isn't BCW68G the pass element?
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Old 11th March 2009, 08:38 PM   #204
jwb is offline jwb  United States
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Yes, it was, but then I noticed that BCW68G, while a nice device, is only available in the SOT-23 package and the thermal resistance is too high.

I'm leaning toward PZT2907A in the SOT-223 package, which has better thermal properties. How much current do you need in your application? My power needs are somewhat low because I have a 12V input and a 9V output, only a 3V Vce drop.

The valuable characteristic in the CCS pass device is low capacitance.
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Old 11th March 2009, 08:46 PM   #205
Salas is offline Salas  Greece
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7V drop, up to 200mA. My transformers are allowing for a double Mosfet VGS drop. Vin-Out
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Old 11th March 2009, 08:52 PM   #206
jwb is offline jwb  United States
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You could use a D45H11. Performance is not as good, but it's still better than the FET CCS.
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Old 11th March 2009, 09:17 PM   #207
Salas is offline Salas  Greece
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Will judge subjectively. Regulation is more than adequate in all those improved shunts. A simple CRC pre filter can help the most wanting one. Did you get a chance to listen at all?
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Old 11th March 2009, 10:10 PM   #208
iko is offline iko  Canada
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Quote:
Originally posted by jwb
The ripple rejection on this one is about 1000x better.
Nice variation, I tried something like this one too, but didn't get such good results. Your circuit does not show 1000x better results. I can post the plot. It has better results in the higher frequencies 1kHz up.

If you replace the top 2k resistor with a 2sk170 you'll get even better results.
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Old 12th March 2009, 01:12 AM   #209
iko is offline iko  Canada
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In real life it oscillates very easily. Modified it a bit and it stopped oscillating but it doesn't seem to perform as well as the alternative I have. jwb, it looks like a promising circuit, perhaps you can make it work. One of the problems is that for a 24V output version the pass bjt needs to dissipate about 750mW. Too much for a single 2N2905A. I did parallel it with another, with emitter degeneration, then it was ok. However, the circuit needs a bit more work.
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Old 12th March 2009, 02:44 AM   #210
jwb is offline jwb  United States
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You didn't leave out the base stopper on my Q3 did you? Without that it does certainly oscillate easily. I have it breadboarded here and it's not oscillating, or it's oscillating at a frequency well above 400MHz.
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