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Old 20th September 2012, 07:12 PM   #8031
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Salas, i will publish a graph from Sansen that shows what is going on in general.
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Old 20th September 2012, 07:33 PM   #8032
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So this should be average value for input capacitance of k170 cascoded second stage ?

I am trying to tune my Riaa calculator for this new design.
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Old 20th September 2012, 08:08 PM   #8033
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No, this is only indicative IMO. The K170 Vds under the BF245C or J310 was 2.7-3V in breadboard test. That gets you to 35pF Ciss. Crss goes to 10pF at that Vds. If it was without cascoding it could be around 230pF Miller effect with the gain and Rs used. I hope for 4 to 5 times reduction with JFET cascoding in this non perfect world, and it remains to be measured. With a BJT and higher Vds like in first stage it can go closer to elimination and nearer to just Ciss.
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Old 20th September 2012, 08:20 PM   #8034
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Quote:
Originally Posted by Joachim Gerhard View Post
Salas, i will publish a graph from Sansen that shows what is going on in general.
Nice.
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Old 20th September 2012, 09:15 PM   #8035
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Here we go, sorry the scan was not easy, this book has a HUGe size.
Attached Images
File Type: jpg Miller Effect.jpg (92.8 KB, 156 views)
File Type: jpg Miller Cascode.jpg (101.0 KB, 150 views)
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Old 20th September 2012, 09:18 PM   #8036
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0255 shows that G/BW is not affected so when i trust Bruno Putzeys the speed does not go up when G/BW is the measure of speed. Se " The F Word" in Linear Audio.
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Old 20th September 2012, 09:25 PM   #8037
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The question is " What is a low frequency in this context ? "
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Old 20th September 2012, 09:40 PM   #8038
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Whatever is 10-20times less than the circuit's bandwidth I guess.
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Old 20th September 2012, 10:00 PM   #8039
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The open loop gain falls of with 6dB octave usually so it depends on the feedback factor,
degeneration or GFB come to mind. What is clear is that the cascode gives more gain but also lower open loop bandwidth. In total, given the same resulting gain the cascode is lower in distortion because we win more gain then we loose open loop bandwidth.
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Old 20th September 2012, 10:01 PM   #8040
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0214 last paragraph is actually the B1 and output buffer in this test phono in words.
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