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Old 19th July 2013, 04:48 PM   #11
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Yes. It was most helpful link I could find about basic operations of folded. I was getting confused about which element, the RL/CCS or Vref of cascode, that set the drain voltage of the gain device. I got it sorted now. I am not even trying to tackle modulated part.
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Old 19th July 2013, 07:16 PM   #12
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Here is an idea. Little unsure if it will work as setting gain is in question.
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Old 19th July 2013, 07:40 PM   #13
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Quote:
Originally Posted by buzzforb View Post
I need some help with folded cascodes. In a typical cascode circuit, the voltage seen by the gate of the cascode device sets the drain voltage of the gain device. In a folded cascode, the cascode device does not set the drain voltage of the gain device, which is now determines by either CCS or Rl. What purpose does the Vref on the cascode serve? What determines the output of the collector of the folded cascode. If not loaded by a resistor or transistor, isnt it functioning as a transimpedance amplifier.
Since we are in the Pass forum, why not use a Pass reference? The A75 has a folded cascode option, and the operation is explained https://www.passdiy.com/project/amplifiers/a75-part-1. To me it seems like a level shifter, a way to take the signal from near a rail down towards ground without adding anything to it. This is further supported by the description of the F5T cascode as "The choice of Q7 and Q8 is not critical for the most part they merely need not to break." I operate my A75s in folded cascode mode, it seems to sound better than when operated with more open loop gain/lower measured distortion.
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Old 19th July 2013, 07:47 PM   #14
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Thank you, Bob. I have studied the A75, but was looking at it from slightly different slant of either the folded cascode as the output device or feeding another fet in either common drain or common source. I did like the "sound" of the folded in a RIAA I recently built and have seen it in a lot of the newer CFB op amps. Seems to bring excellent stablilty and extreme bandwidth. It was a defining part of John Curls audio career, from what I have read. I am trying to stay away from the complimentary imput, as the J74 decvices are becoming harder to get and we should be able to achieve pretty good performance with LTP option. Add to this the excellent performance of the 369 jfet and something suitable should be within reach.

BTW,
old buddy Bill Fuss spole very highly of you.
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Old 19th July 2013, 07:53 PM   #15
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Aww, shucks.

I wasn't suggesting going to full balanced, just that the article provides some insight into folded cascode operation.

From what I see your circuit is exactly as described, folded cascode as the output device, all voltage gain in the differential pair and the signal shifted to no DC offset. Interesting.
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Old 19th July 2013, 08:10 PM   #16
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I do have a question about the A75. It is something that I have never understood. Why load both the output device and the negitive input with the same 10R resistor. I would tihnk this would affect the balance of the input pair.
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Old 19th July 2013, 08:13 PM   #17
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What is the part designation?
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Old 19th July 2013, 08:16 PM   #18
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Sorry, in later schematics in changed to 47R. The part is R7
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Old 19th July 2013, 08:37 PM   #19
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That's part of the do you want this a conventional or folded cascode flexibility. With low values of R21 the inverted signal is shunted to the signal ground (via the rail and decoupling caps) Other than a variable current source to set current in the stage and DC offset it is out of the audio chain.

Leave R21 open and the inverted signal is amplified by Q3 in grounded source mode and swamps the folded cascode signal. That's where the higher open loop gain comes from.
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Old 19th July 2013, 08:52 PM   #20
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THanks for the explanation.
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