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Old 5th September 2012, 02:03 AM   #711
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Originally Posted by coluke View Post
....THD is now 0.0013% @ 0 dBV into 30 ohm in the 100 Hz - 10 kHz range, low order only (2nd & 3rd).....
coluke, what kind of distortion figures/spectrum do you get OPEN LOOP?
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Old 5th September 2012, 02:23 AM   #712
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OK, here's one more. Folks asked about subing PNP's for the PFET's, this is one choice using the Rush cascode. The diode connected transistors could be substituted by diodes in most cases. Again use any output stage you like Dick's CF pair or triple-darlington or whatever. The ccomp and the degeneration in the input can be varied for different applications. I think with care this can do anything the OPA627/37 can do. With BF862's we have about $2 in devices.
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Old 5th September 2012, 02:37 AM   #713
RNMarsh is offline RNMarsh  United States
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Originally Posted by scott wurcer View Post
OK, here's one more. Folks asked about subing PNP's for the PFET's, this is one choice using the Rush cascode. The diode connected transistors could be substituted by diodes in most cases. Again use any output stage you like Dick's CF pair or triple-darlington or whatever. The ccomp and the degeneration in the input can be varied for different applications. I think with care this can do anything the OPA627/37 can do. With BF862's we have about $2 in devices.
The jFEts with 100 Ohms in the sources -- is that for CMRR improvment?
Thx - RNM
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Old 5th September 2012, 02:44 AM   #714
bcarso is offline bcarso  United States
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Originally Posted by scott wurcer View Post
OK, here's one more. Folks asked about subing PNP's for the PFET's, this is one choice using the Rush cascode. The diode connected transistors could be substituted by diodes in most cases. Again use any output stage you like Dick's CF pair or triple-darlington or whatever. The ccomp and the degeneration in the input can be varied for different applications. I think with care this can do anything the OPA627/37 can do. With BF862's we have about $2 in devices.
Me likey
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Old 5th September 2012, 03:15 AM   #715
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The jFEts with 100 Ohms in the sources -- is that for CMRR improvment?
Thx - RNM
That's DC bias, sets the current.
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Old 5th September 2012, 06:27 AM   #716
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I like your common mode tricks with diffpairs Scott!
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Old 5th September 2012, 08:54 AM   #717
tvi is offline tvi  Australia
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Originally Posted by RNMarsh
source resistors - makes matching more critical but THD and Noise is lowest - purist approach
That what this Pioneer patent is said to address
US4356453.html
"Accordingly, an object of this invention is to provide a grounded-source type field-effect transistor circuit in which the source resistance is minimized to minimize the thermal noise in the circuit and to increase the circuit gain."

Adds more devices though.

rgds
james
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Old 5th September 2012, 01:40 PM   #718
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Originally Posted by tvi View Post
That what this Pioneer patent is said to address
US4356453.html
"Accordingly, an object of this invention is to provide a grounded-source type field-effect transistor circuit in which the source resistance is minimized to minimize the thermal noise in the circuit and to increase the circuit gain."

Adds more devices though.

rgds
james
It can be seen in Pioneer A09 amp, excellent amp of late 80s and 90s. Some seem to think the idea is new.
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Old 5th September 2012, 02:07 PM   #719
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Quote:
Originally Posted by tvi View Post
That what this Pioneer patent is said to address
US4356453.html
"Accordingly, an object of this invention is to provide a grounded-source type field-effect transistor circuit in which the source resistance is minimized to minimize the thermal noise in the circuit and to increase the circuit gain."

Adds more devices though.

rgds
james
Looks like they used two current sources to increase the drop in the source resistors, i.e. more offset range with smaller resistors. Smaller resistors = less noise. The current sources are not in the signal path and can be made to not contribute noise (heavy degeneration). This trick also works in output stages to make the Vas current and output bias independently trimmable.

On another point, if you look at the ideal JFET equations you will see that the cancellation of the quadratic term requires both Vp AND Idss to be the same. This would require exact complimentary pairs not just Idss selection.
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Old 5th September 2012, 03:15 PM   #720
dimitri is offline dimitri  United States
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the cancellation of the quadratic term requires both Vp AND Idss to be the same. This would require exact complimentary pairs not just Idss selection.
There is another way
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