John Curl's Blowtorch preamplifier part II - Page 1306 - diyAudio
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Old 23rd June 2011, 06:22 PM   #13051
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Any comments on the topology above?

An obvious limitation is the difficulty in degenerating transconductance enough for a no-long-loop-feedback line stage. Much more suited for a phono equalizer or a conventional loop-fedback line stage.

The cascoding is important both for linearity and for power supply immunity because inputs are referenced to supply rails.

I've never seen this in print, but probably *somebody's* thunk it up before. Any previous mentions would be appreciated. Thanks,
Chris
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Old 23rd June 2011, 08:11 PM   #13052
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If it works, it looks good.
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Old 25th June 2011, 08:02 PM   #13053
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Quote:
Originally Posted by Chris Hornbeck View Post
The resistor with ?? marks just came to me
Without the resistor, very little happens.

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Old 25th June 2011, 08:42 PM   #13054
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Quote:
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Without the resistor, very little happens.

The resistor allows adjustment of transconductance independent of quiescient current. If the resistor is not there there is no gain at all.
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Old 26th June 2011, 12:58 AM   #13055
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My earlier pictures were terrible, so maybe these will help show signal polarities that are causing my heroes some consternation.

Chris page1.jpg

Chris page2.jpg

Chris page3.jpg

Figure 1 is a simplified John Curl input stage, drawn to emphasize its four quadrants. Figure 2 shows it divided in half and is redrawn in figure 3, diff-in single-ended-out. Figure 4 is the top half of figure 3 doubled for diff-in and diff-out, including the (bad idea, sorry) ?resistor. Redrawn for clarity in figure 5. Figure 6 is a simplified high gain topology drawn with the cascodes that keep it linear. The source degeneration resistors in the second ("VAS") stage must be chosen for proper DC conditions leaving transconductance to you'll-get-what-you-get-and-like-it. Hoping someone might see a way around that foible. Thanks very much for any comments.
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Old 26th June 2011, 01:34 AM   #13056
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Sorry, should of course be " *without* the cascodes...".
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Old 26th June 2011, 01:54 AM   #13057
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I like the "series differential" concept. I saw it somewhere eons ago but this is a good application. Getting this to work will be a real balancing act given the number of matches that need to be sorted out.
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Old 26th June 2011, 06:20 AM   #13058
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Hi,

Quote:
Originally Posted by Chris Hornbeck View Post
Attachment 228820

Figure 6 is a simplified high gain topology drawn with the cascodes that keep it linear. The source degeneration resistors in the second ("VAS") stage must be chosen for proper DC conditions leaving transconductance to you'll-get-what-you-get-and-like-it. Hoping someone might see a way around that foible. Thanks very much for any comments.
I like the use of the Rush Cascode. In order to gain more control about the transconductanceone might deliberatly add some DC between the two gates of the Rush Cascode (tapping the resistors) to either minimise or maximise transconductance, however, I not necessarily see this as being appreciably superior to John Curls original topology shown below.

Ciao T
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Old 26th June 2011, 02:42 PM   #13059
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Quote:
Originally Posted by ThorstenL View Post
Hi,



I like the use of the Rush Cascode. In order to gain more control about the transconductanceone might deliberatly add some DC between the two gates of the Rush Cascode (tapping the resistors) to either minimise or maximise transconductance, however, I not necessarily see this as being appreciably superior to John Curls original topology shown below.

Ciao T
I don't particularly like the asymmetry of the second stage, what goes up and what goes down are no longer one in the same.
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Last edited by scott wurcer; 26th June 2011 at 02:45 PM.
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Old 27th June 2011, 02:37 AM   #13060
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Some sad news about two gentlemen whose names have appeared in this thread. I didn't know them personally, but I'm sure many of y'all did. I'm sorry for the loss.

Analog guru Jim Williams dies after stroke - 2011-06-13 15:26:32 | EDN

http://www.edn.com/article/518568-An..._car_crash.php
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