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Old 7th January 2013, 08:08 PM   #101
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Originally Posted by keantoken View Post
Maybe true for modular instrumentation amplifiers but this circuit will only be connected one way in one circuit. It has passed all the requirements so far so even if CMRR is low, there may be little to gain from increasing it with this particular implementation.

R7/R8 give the + input slightly extra gain. At a current gain of 18.54k at the Jfet sources, this calculates to a CMRR of 2.85db, which seems to agree with the simulation. R7/R8 need to be bootstrapped to the negative input or replaced with current sources.

I think in the first simulation since you bypassed the 1k resistor you caused major offset which saturated/clipped the output resulting in optimistic PSRR.
I was thinking of both, CCS and bootstraping when I started this lineamp, but do I need all that complication? I am preparing PCBs and after listening to it I will deside upon implementing some of those upgrades.
Thanks Kean I like how you prceive all that electronic staff, realy remarkable.
Damir
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Old 7th January 2013, 08:40 PM   #102
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Only for the purposes of CMRR measurement do you need the CCS. R7/R8 act like a 11k resistor parallel to R9, so they are relatively benign except for noise conducted from the rails. However if your rails are well filtered with low impedance I don't think this is a problem. How much current is going through your BC5xx C-multipliers? BTW, 4.7k/470k for the C-multipliers seems pretty reasonable.
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Old 7th January 2013, 08:56 PM   #103
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Originally Posted by keantoken View Post
Only for the purposes of CMRR measurement do you need the CCS. R7/R8 act like a 11k resistor parallel to R9, so they are relatively benign except for noise conducted from the rails. However if your rails are well filtered with low impedance I don't think this is a problem. How much current is going through your BC5xx C-multipliers? BTW, 4.7k/470k for the C-multipliers seems pretty reasonable.
C-multiplier disipation is les then 80 mW(65mA) and it is 47k/470k.

Attached is similar lineamp but with GNFB and sub ppm distortion. It will be easy to transform non GNFB one to the GNFB one, but I am not sure do I need that.
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File Type: jpg Gainwire-GNFB-FFT.jpg (204.0 KB, 199 views)
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Old 7th January 2013, 09:08 PM   #104
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The circuit without NFB is very inert. Potential reactive areas are the output CFP and Baxandall cascodes. They may need a small cap at their collectors or something else if they oscillate.

In any case, I've found inert circuits tend to have the most realistic imaging. The more positive and negative feedback paths, the more potential for adverse resonances and poor RFI tolerance.

It seems like a simple enough modification, maybe you can try it with an without feedback? You would have to bother with checking and compensating for stability though.
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Old 8th January 2013, 10:19 AM   #105
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The same distortion as GNFB citcuit I've got with CCS and bootstrap, but with the cost of four more transistors and four CCS(mosfet).
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File Type: jpg GaiWire-new-bootstraped-FFT.jpg (208.9 KB, 186 views)
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Old 8th January 2013, 02:31 PM   #106
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I`m working on something similar. What`s the nose level of input stage?
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Old 8th January 2013, 03:25 PM   #107
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Originally Posted by bogdan_borko View Post
I`m working on something similar. What`s the nose level of input stage?
Noise is directly proportional to the R1 value, lover R1 lover noise, but higher distortion.
After the listening test I will decide what metters more, noise or distortion.
Attached two noise simulation.
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File Type: jpg Noise-high.jpg (183.8 KB, 170 views)
File Type: jpg Noise-low.jpg (194.9 KB, 157 views)
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Old 8th January 2013, 05:27 PM   #108
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thanks!
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Old 9th January 2013, 12:11 PM   #109
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CMRR for Gainwire with CCS and bootstrap is here.
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File Type: jpg CMRR-gainwire-CCS-bootstrap.jpg (237.7 KB, 141 views)
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Old 9th January 2013, 07:24 PM   #110
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I'll bet you could replace M1 and M2 with resistors without losing anything. Q17/18 will not improve PSRR, and bias loading here is relatively benign.
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