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Old 23rd December 2009, 05:15 AM   #21
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Okay, I guess I got phase margin wrong too. I was quoting the margin where the open loop gain equaled 1 (0dB) and the phase margin is the difference between the phase and 180 degrees phase shift at the frequency where the closed loop gain and open loop gain are the same.

This preamp has a closed loop gain of 18dB and at the open loop gain is 18dB at 3MHz. The phase margin is 360 deg - 330 deg, that is 30 degrees.

Phase margin - Wikipedia, the free encyclopedia

And wiki says it should be 35 deg. But since this isn't a power amp driving a capacitative load, 30 deg should be okay I think.
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Old 23rd December 2009, 07:56 AM   #22
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Thanks for the Wiki info on feedback and phase margin, Johnloudb. I usually rely on degeneration for feedback--think I'll stick with it as it seems to work fine for me.
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Old 23rd December 2009, 06:08 PM   #23
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Here's the open loop response when using the LSK170C JFETs and it has almost 80 degrees phase margin. I get slightly better simulated performance with the LSK170C also, but the difference is very slight and probably doesn't mean anything in the real world. But it looks like the more stable of the two, in this circuit anyway. I've used the LSK170C in my last preamp and it sounds really good, so I may stick with those if I can get my hands on some more.

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I usually rely on degeneration for feedback--think I'll stick with it as it seems to work fine for me.
Is that like the feedback Pass used in his F5?
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File Type: png C_BOZ_openloop_LSK170C.png (3.8 KB, 286 views)
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Last edited by Johnloudb; 23rd December 2009 at 06:10 PM.
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Old 24th December 2009, 02:23 AM   #24
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More like this (R11 and R20 are the source degeneration resistors):
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File Type: png Current amp first stage.PNG (13.4 KB, 229 views)
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Old 28th December 2009, 12:55 PM   #25
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Looks nice Carpenter!

I made some modifications to my circuit to increase the phase margin using the bf862, now it is just above 50 degrees. So, I hope this works as good as the sim, since I'd like use that transistor. Any opinions about transistor matching in a circuit like this? I don't think it's really necessary, as long as they are reasonably close, but if there is a benefit I'll buy enough to match them.

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Open loop response.

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10 KHz square wave.

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File Type: png C_BOZ3_openloop.png (3.7 KB, 250 views)
File Type: png Cascode_BOZ3_bf862.png (5.1 KB, 276 views)
File Type: png 10KHz_SquareWave.png (2.8 KB, 271 views)
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Old 28th December 2009, 01:02 PM   #26
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R2 is actually 90k for a closed loop gain of 18dB, not 70k like shown in that schematic.
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Old 29th December 2009, 11:37 PM   #27
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Well, I found the real problem with my circuit. I had all the gates on one channel wired to the sources and the gates on the other channel wired to the drains. Could that cause a problem???

I just had the 220 ohm resistors lined up wrong on the generic PCB, and I just never thought about that possibility, and never checked that out. But while checking for continuity of the JFET mounting pads, I found the shorts.

Got to check everything!!! Oh, well, at least the preamp will be more stable now.
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Old 30th December 2009, 01:58 AM   #28
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Ooooooh.... that does add an interesting wrinkle to the mix. Good luck with the next attempt.
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Old 30th December 2009, 02:51 AM   #29
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Quote:
Originally Posted by carpenter View Post
Good luck with the next attempt.
I'll need it.
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Old 30th December 2009, 05:27 AM   #30
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Triple check; check at different times in the day.

Make us proud!
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