Konnichiwa,
Actually, on the principle "where does current flow" this folded cascode is very neat.
In a nomral cascode the signal output is effectively referenced to to positive supply rail but the following stage commonly amplifies the differences between some usually ill-defined "ground" and the input terminal. Under ideal conditions the supply rail and ground are the same in AC terms, in reality they rarely are.
Now the folded cascode as Werner shows:
1) The BJT's base is referenced (AC) to the positive rail, as is it's emitter and it's input signal, namely the current in the J-Fet.
2) The collector current of the BJT is controlled by the J-Fet current and pretty much little else, it, just like the J-Fet operats as variable current source.
3) A resistor (be it a direct collector resistor as Werners original shows or a virtual resistor as in my Vendetta inspired idea) then converts this AC current into voltage which is referenced to the so-called ground and which is fairly well independent from the supply rail noise and modulation (with ideal BJT's, J-Fet's and capacitors it would completely unperturbed by anything on the supply rail).
Now I call that very smart and tricky....
It might be fun to do a folded cascode "Pacific"!
Sayonara
Pjotr said:
Actually, on the principle "where does current flow" this folded cascode is very neat.
In a nomral cascode the signal output is effectively referenced to to positive supply rail but the following stage commonly amplifies the differences between some usually ill-defined "ground" and the input terminal. Under ideal conditions the supply rail and ground are the same in AC terms, in reality they rarely are.
Now the folded cascode as Werner shows:
1) The BJT's base is referenced (AC) to the positive rail, as is it's emitter and it's input signal, namely the current in the J-Fet.
2) The collector current of the BJT is controlled by the J-Fet current and pretty much little else, it, just like the J-Fet operats as variable current source.
3) A resistor (be it a direct collector resistor as Werners original shows or a virtual resistor as in my Vendetta inspired idea) then converts this AC current into voltage which is referenced to the so-called ground and which is fairly well independent from the supply rail noise and modulation (with ideal BJT's, J-Fet's and capacitors it would completely unperturbed by anything on the supply rail).
Now I call that very smart and tricky....
It might be fun to do a folded cascode "Pacific"!
Sayonara
Kuei Yang Wang said:
Hi Sayonara,
Ahhh… I see and R8 does not inject a bunch of PSU related noise into the emitter? Actually combined with R1 there is a gain of app. 2 concerning PSU related noise. Therefore C2 is needed to relax that, but it works only at higher frequencies.
Maybe replacing R8 by a current source will do better, but that needs to be very low moise then.
Cheers
Konnichiwa,
No, as the controlling voltage appears between base and emitter and thus C2 clamps this...
Well, the turnover for this RC combo is 1.6Hz, so at 16Hz noise is clamped down to 20db less and so on. Increeasing the capacitor to 1,000uF is trivial and increases the supply rejection for any given frequency by 20db and increasing nthe resistor values in the divider for the base by a factor of 10 gives another 20db, so 60db supply line noise suppression at 16Hz above your case of -6db should be non too difficult to attain....
Sayonara
Pjotr said:
No, as the controlling voltage appears between base and emitter and thus C2 clamps this...
Pjotr said:
Well, the turnover for this RC combo is 1.6Hz, so at 16Hz noise is clamped down to 20db less and so on. Increeasing the capacitor to 1,000uF is trivial and increases the supply rejection for any given frequency by 20db and increasing nthe resistor values in the divider for the base by a factor of 10 gives another 20db, so 60db supply line noise suppression at 16Hz above your case of -6db should be non too difficult to attain....
Sayonara
Kuei Yang Wang said:
Domo arigato. But as said, the credit goes to JC and that Kirchoff guy.
Too bad that the present prototype is housed together with its
multi-transformer PSU in one box. Baaad situation. I must rectify that. Which is kinda hard with my present corporate-life and daddy-life combined.
Pjotr said:
In fact the other part of the trick is that the original folded cascode allows the use of a simple emitter-follower regulator as supply (*), one per stage. This is of very low noise, and unperturbed by the actual signal as the folded cascode's current consumption is purely DC.
So the clamping capacitor is not even strictly required. Just nice to have.
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