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Old 14th April 2006, 06:39 PM   #851
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John,

Referring to earlier posted schematic shown below, and assuming that we use an attenuator at the outputs of the DC servos (100:1 or 10:1), would their be a more apropriate and better connection-point possible for the servo +/- outputs and where in the schematic should this be applied?
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Old 14th April 2006, 10:22 PM   #852
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Your servo has too much gain, but you can scale the servo resistors to change the gain, instead of adding anything extra.
 
Old 14th April 2006, 11:58 PM   #853
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John,
Allow me to rephrase my question...if there were a way to squeeze an ideal CCS between the JFET differentials--I know there's not enough room, voltage-wise, but let's pretend--would that decrease the circuit's sensitivity to real world power supply imperfections?
The higher Zout of a CCS (since we're pretending, let's go ahead and pretend that the Zout of the CCS is symmetrical [read: high] for both the N and P differentials) should increase the CMRR of both N and P differentials, thus making them less sensitive to power supply problems.
Yes, it's possible to use separate CCSs for each differential, anchoring each on the opposite rail, but that would no longer be elegant and would likely entail more noise. I like the elegance of the resistor(s) between two differentials and am willing to make the compromise, as complicated power supplies don't intimidate me. Better there than in the signal path. I've been doing odd and intricate power supplies for years, as long as it allows me to get things out of the signal path.

Grey
 
Old 15th April 2006, 03:33 AM   #854
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Franklin, to work properly the DC-servo compound must include 2 time constants; the average servo must act before the differential one, otherwise in certain conditions the outputs +and could be near each other, but not close to the ground.
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Old 15th April 2006, 07:47 AM   #855
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4 good current sources would significantly improve the power supply rejection ratio, but why not just make a better supply?
 
Old 15th April 2006, 07:56 AM   #856
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And why not both, John ?
 
Old 15th April 2006, 11:41 AM   #857
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I'm in agreement, just making sure that I understand the behavior of the circuit.

Grey
 
Old 15th April 2006, 12:28 PM   #858
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...........it might be tough to fit a current source between the differentials because of the voltage drops, maybe a current diode would work...........but if you look at the circuit the differentials are acting like current sources (attaching the gates and sources).....if that is true, why would you want to current source a current source.............I can agree with using current sources for the loads (drain resistors as Mr.Pass has mentioned)

Grey, I knew that you would come over to the Dark Side.

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Jam
 
Old 15th April 2006, 06:43 PM   #859
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The challenge is to make a 'perfect' current source that doesn't add its own distortion and NOISE. A resistor is a nearly 'perfect' device, by comparison.
 
Old 15th April 2006, 10:27 PM   #860
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Jam,
John has said precisely the same thing that I have said in many posts here over the years; some even before he became a member here. The point of my question was to make sure I understood why the circuit was, perhaps, more sensitive to power supply-related noise etc. than other topologies. It was just a thought experiment, nothing more.
I had already noted in my previous post that there's not "room" between the differentials for a current source. Again, just a thought experiment.
The front end JFETs aren't acting like current sources--especially not for each other. The Gates are tied to each other, not to the Sources. The differentials also react against a virtual ground at the midpoint of the Gate connection and another at the midpoint between the two pairs of Sources. The Source "ground" can either be made explicit by attaching it to a real ground or left floating, which does to some extent increase the apparent impedance seen by the Sources.
Given that John's goal, as I understand it, is a 0dB NFB circuit, using current sources as active loads would generate a lot of unwanted and unnecessary gain that would be a burden if you don't intend to burn it off with NFB. As such, I imagine that the idea of an active load is completely antithetical to John's intentions. And that's not to mention frequency-related compliance problems, noise, etc. Give me a good ol' resistor. Given that others here seem to feel that infinite NFB would lead to the ultimate sound, I'll let them follow their muse. For myself, the last line stage I built used a little over 7dB NFB. I'd like to get down to 0, but I'm finding it a bit of a challenge, given that I insist on at least 250kHz bandwidth in a small signal circuit. I need more experience and better parts, but I'm getting there.
Me join the Dark Side?
As Luke said..."Never!"
I will become a Jedi or bust...

Grey
 

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