Simplistic NJFET RIAA - Page 804

Joachim Gerhard · 20th September 2012, 07:12 PM

Salas, i will publish a graph from Sansen that shows what is going on in general.

RCruz · 20th September 2012, 07:33 PM

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.

Salas · 20th September 2012, 08:08 PM

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.

Salas · 20th September 2012, 08:20 PM

Quote:

Originally Posted by Joachim Gerhard

Salas, i will publish a graph from Sansen that shows what is going on in general.

Nice.

Joachim Gerhard · 20th September 2012, 09:15 PM

Here we go, sorry the scan was not easy, this book has a HUGe size.

Joachim Gerhard · 20th September 2012, 09:18 PM

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.

Joachim Gerhard · 20th September 2012, 09:25 PM

The question is " What is a low frequency in this context ? "

Salas · 20th September 2012, 09:40 PM

Whatever is 10-20times less than the circuit's bandwidth I guess.

Joachim Gerhard · 20th September 2012, 10:00 PM

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.

Salas · 20th September 2012, 10:01 PM

0214 last paragraph is actually the B1 and output buffer in this test phono in words.

20th September 2012, 07:33 PM	#8032
RCruz diyAudio Member Join Date: Sep 2006 Location: Saxon Blog Entries: 1	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. __________________ RC

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20th September 2012, 07:12 PM	#8031
Joachim Gerhard diyAudio Member Join Date: Mar 2009	Salas, i will publish a graph from Sansen that shows what is going on in general.

20th September 2012, 08:08 PM	#8033
Salas diyAudio Chief Moderator Join Date: Oct 2002 Location: Athens-Greece	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.

20th September 2012, 09:18 PM	#8036
Joachim Gerhard diyAudio Member Join Date: Mar 2009	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.

20th September 2012, 09:25 PM	#8037
Joachim Gerhard diyAudio Member Join Date: Mar 2009	The question is " What is a low frequency in this context ? "

20th September 2012, 09:40 PM	#8038
Salas diyAudio Chief Moderator Join Date: Oct 2002 Location: Athens-Greece	Whatever is 10-20times less than the circuit's bandwidth I guess.

20th September 2012, 10:00 PM	#8039
Joachim Gerhard diyAudio Member Join Date: Mar 2009	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.

20th September 2012, 10:01 PM	#8040
Salas diyAudio Chief Moderator Join Date: Oct 2002 Location: Athens-Greece	0214 last paragraph is actually the B1 and output buffer in this test phono in words.