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Old 18th December 2012, 08:33 PM   #3701
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In my opinion would be 100 mA enough to the end Fets and 15 mA to the Cascodes. What value of Bias would be useful for the driver Fets M9 and M10?
I can change the current through the cascode, by two Ways: First: I change the value of Resistors R9 and R12 (270 Ohm). second: I change the value of the Cascode -Resistors R16 and R18 (10 Ohm). What would be the better way?
Best regards Gerd
You better listen, in order to set the best power bias :-) On my preference, it is often around 150ma.
About the cascode, just set-it for enough current to feed the parasitic capacitance of M9/M10. I believe 14ma is enough for a good slew-rate.
About the way to change the current in the cascode, i use 2 methods, first to change the current of the input stage. It will change the voltage across R9 - R12. Changing the values of R9 - R12 do the same, but will change the gain of the first stage, so the open loop gain, right ? The best is to simulate for the best distortion. Takes some times playing with this or that, not so easy...
I am curious to see the answers of LC to those questions.
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Old 18th December 2012, 08:52 PM   #3702
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Hi Esperado,

150 mA for the FET drivers M9/M10 to ?
I am also looking forward to answering LC
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Old 18th December 2012, 09:49 PM   #3703
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150 mA for the FET drivers M9/M10 to ?
I am also looking forward to answering LC
Just verify there is at least 1/4 of the current of the output devices ?
Caution, I believe there is a mistake at output of the M9 on your schematic: Don't it has to be connected to the common path of the 47 Ohms ?
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Old 18th December 2012, 10:14 PM   #3704
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I am curious to see the answers of LC to those questions.
Quote:
Originally Posted by coolewater View Post
I am also looking forward to answering LC
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Old 19th December 2012, 12:28 AM   #3705
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Can-you comment ? (means: How do-you manage your choices of gain/current of the different stages, specialty input and VAS ?).
Well seen about the temp compensation that i had not noticed: Laterals have a negative temp behavior, so such a stage is not needed and worsen the characteristic.
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Old 19th December 2012, 10:56 AM   #3706
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Hi LC, thank you. A Question to reach the 15 mA to the Cascode. First: What would be the better way: I change the value of Resistors R9 and R12 (270 Ohm). Or I change the value of the Cascode -Resistors R16 and R18 (10 Ohm). What would be the better way?
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Old 19th December 2012, 11:18 AM   #3707
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Hi coolewater

Please make all changes marked with red on sch I posted.

Regarding 15 mA VAS (cascode) bias current, you must not change any of mentioned resistors, since they're in direct relation to OLG (open loop gain).

15 mA VAS bias current you can set with adjusting (trimming) 10 mA current source/sink in input stage.

BR LC
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Old 19th December 2012, 07:38 PM   #3708
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Any comment about my question, a problem i'm always asking myself when i design a power amp. How to manage the relative gain in the various stages of the OLG and their currents. I have spend a lot of time simulating different solutions, no evidences yet(assuming current are enough for class A, of course) .
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Old 20th December 2012, 02:49 AM   #3709
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Edmund Stuart's SuperTIS is an example of reducing the gain of the VAS and reworking compensation to great effect.
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Old 24th December 2012, 12:36 AM   #3710
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Hmmm...what this circuit could be for? So many elements just for soft start? Naaaah
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