I remember playing around with folded cascodes many years ago when I was only just starting to get into audio electronics, thinking that it seemed like a good way to achieve a 2-stage power amplifier. I never managed to make much of them so I forgot all about it. Seeing the subject crop up a few times here recently got me thinking again, so I fired up the simulator and tried to make a fully symmetrical one, just because I can.
I've attached a pic of the circuit I came up with. It doesn't simulate too badly; maybe I'll try building it sometime. In case you're wondering, the current sources are modelled as J510 3.6mA constant current diodes, the BJTs are all BC556/546, the zeners are 20V and the MOSFETs are ECF20N16/20P16. The output stage has a little gain because of the limited output voltage swing of the cascode stage. C2 provides some frequency compensation to (hopefully) keep it stable.
With an extra output stage and a rearranged feedback network, the signal would be balanced throughout, which could be beneficial.
Comments on possible problems would be nice, since I am but a novice compared to some of the designers here.
I'll attach some pics of the sim results down below...
P.S. sorry the images are all rotated 90 degrees, but the forum doesn't like them the other way around.
I've attached a pic of the circuit I came up with. It doesn't simulate too badly; maybe I'll try building it sometime. In case you're wondering, the current sources are modelled as J510 3.6mA constant current diodes, the BJTs are all BC556/546, the zeners are 20V and the MOSFETs are ECF20N16/20P16. The output stage has a little gain because of the limited output voltage swing of the cascode stage. C2 provides some frequency compensation to (hopefully) keep it stable.
With an extra output stage and a rearranged feedback network, the signal would be balanced throughout, which could be beneficial.
Comments on possible problems would be nice, since I am but a novice compared to some of the designers here.
I'll attach some pics of the sim results down below...
P.S. sorry the images are all rotated 90 degrees, but the forum doesn't like them the other way around.
Attachments
Folded kaskode... well there is several symetric designs.
Abt your outputstage config: The resistors R10 and R12 is in series emtying the input cap on the mosfets(R7 and R11 for the upper halve). Thus off time is much higher than on time and the bias will bee kind of sliding ( at least sliding in transients). I belive there is som high current flowing thru bouth outputs as they are shifted from on to off.
The output stage gain ideally ((R13+R14)/R14) it is a must to have this gain or reduced feedback when using cfp mos output. The good thing is increased slew.
Your implement of folded kaskode: kolectors of Q10 and Q5 floating? This point has the opposit signal of the Q9 Q4. And is many times seen used as output for bridge or Fully symetric designs.
I dont mean to criticise your design, only let you have som "input"
ragards.
Abt your outputstage config: The resistors R10 and R12 is in series emtying the input cap on the mosfets(R7 and R11 for the upper halve). Thus off time is much higher than on time and the bias will bee kind of sliding ( at least sliding in transients). I belive there is som high current flowing thru bouth outputs as they are shifted from on to off.
The output stage gain ideally ((R13+R14)/R14) it is a must to have this gain or reduced feedback when using cfp mos output. The good thing is increased slew.
Your implement of folded kaskode: kolectors of Q10 and Q5 floating? This point has the opposit signal of the Q9 Q4. And is many times seen used as output for bridge or Fully symetric designs.
I dont mean to criticise your design, only let you have som "input"
ragards.
Yes, I can see some cross-conduction occuring with very fast signals. How do I stop that?Konrad said:
I did mention that in passing, that it could be made fully balanced, but I thought I would start simple just to see how well it works.Konrad said:
Not at all! That's why I posted it here.Konrad said:
Mr Evil said:
Faster off, a tiny litle help from a transistor. Here in the att: Q5 and Q8.
My first thougts abt your scematic:
The current gain in your org sirc is rather low , i mean vas current gain idealy =1 . The way your zeners is implemented psu ripple is directly set into base rail voltage for your folded circ. swapping current sorces and zeners would bee better.
I dont have the models for your output choise or have sim this sirc so there is still a looooong way before its you can call it ready for som real trial.
Hi mr.evil !
You might have a look here:
http://www.diyaudio.com/forums/showthread.php?s=&threadid=39502&perpage=10&pagenumber=19
But good that you opened a thread for folded cascode...
It's a very interesting topology ! And my prototype of a symetrical
folded cascode is already running !
Mike
You might have a look here:
http://www.diyaudio.com/forums/showthread.php?s=&threadid=39502&perpage=10&pagenumber=19
But good that you opened a thread for folded cascode...
It's a very interesting topology ! And my prototype of a symetrical
folded cascode is already running !
Mike
Ahah! That really works, thanks.Konrad said:
I do like using unusual topologies. If they happen to work well too, then that's an added bonus.MikeB said:
Are you sure? Q9/10 and Q4/5 are connected in common-base configuration. They follow after Q13/14 and Q2/6, which are CE. CB following CE is the definition of a cascode, if I recall correctly.bocka said:
Hi Mr. Evil,
I was refering to the circuit Konrad has attched. Your design is a folded cascode.
Mike:
Welcome to the folded cascode topology club! To my opinion it has several advantages. When you cut the folded cascode into an upper and a lower half, it contains only one amplifying transistor in each half (electrically these two transistors are paralled). This is the main reason why the folded cascode sounds better (to me) than any other topology. The first diff amp only produces K2 and K3 and this signal ist not fed into another amp stage which distorts and produces high order distortion. If you compare it to SE triode tube design, it has a similar topology. Unfortunately "PNP" tubes not exist.
Because it has only one amplification stage the folded cascode is theoretically stable into any load. Practically it needs some work to do so but I build amps which are extraordonary stable with a very low output inductor and drives high capacitive loads up to several uf. When you want to "colorise" the sound try to use highest quality feedback resistors as well as micas for the feedback and compensation caps. The folded cascode is really worth it and it shows the differences of these components very clearly. Also try to increase the driver and bias currents. The more the better.
I was refering to the circuit Konrad has attched. Your design is a folded cascode.
Mike:
Welcome to the folded cascode topology club! To my opinion it has several advantages. When you cut the folded cascode into an upper and a lower half, it contains only one amplifying transistor in each half (electrically these two transistors are paralled). This is the main reason why the folded cascode sounds better (to me) than any other topology. The first diff amp only produces K2 and K3 and this signal ist not fed into another amp stage which distorts and produces high order distortion. If you compare it to SE triode tube design, it has a similar topology. Unfortunately "PNP" tubes not exist.
Because it has only one amplification stage the folded cascode is theoretically stable into any load. Practically it needs some work to do so but I build amps which are extraordonary stable with a very low output inductor and drives high capacitive loads up to several uf. When you want to "colorise" the sound try to use highest quality feedback resistors as well as micas for the feedback and compensation caps. The folded cascode is really worth it and it shows the differences of these components very clearly. Also try to increase the driver and bias currents. The more the better.
I'm new to the folded cascode, and it looks an interesting topology.
If you have the supply rails available though, a conventional cascode stage followed by a current mirror to change the output to be ground referenced (rather than referenced to the supply rail) seems to give better performance. At least SPICE says so!
Any comments regarding folded cascode versus conventional cascode + current mirrors?
If you have the supply rails available though, a conventional cascode stage followed by a current mirror to change the output to be ground referenced (rather than referenced to the supply rail) seems to give better performance. At least SPICE says so!
Any comments regarding folded cascode versus conventional cascode + current mirrors?
Hi bocka !
Yes, i look forward to finetuning the circuit, i hope i get the same
performance for mids like my other symamp. For trebles the folded
cascode definitely outperforms all amps i have access to.
At this moment my amp sounds too "dry", but it's the first version
and there are many possibilities for optimizing sound. I haven't
really adjusted the feedbacknetwork yet, in my experience the most
important thing for good sound...
Mike
Yes, i look forward to finetuning the circuit, i hope i get the same
performance for mids like my other symamp. For trebles the folded
cascode definitely outperforms all amps i have access to.
At this moment my amp sounds too "dry", but it's the first version
and there are many possibilities for optimizing sound. I haven't
really adjusted the feedbacknetwork yet, in my experience the most
important thing for good sound...
Mike
i am curious ...
I haven't played with "official" folded cascodes in a power amp yet. I have played around with circuits simular to what Konrad posted, also shown on Borbely's web site. Overall, I've been pretty happy with the results.
I have read a number of comments here and elsewhere about sonic benefits of folded cascodes topologies. I am curious if anyone can pose sound theories as to why folded cascodes have not caught on in a bigger way among the "I make a living at it" audio design community (Nelson Pass and Jeff Rowland excluded, of course, there may be others?).
mlloyd1
I haven't played with "official" folded cascodes in a power amp yet. I have played around with circuits simular to what Konrad posted, also shown on Borbely's web site. Overall, I've been pretty happy with the results.
I have read a number of comments here and elsewhere about sonic benefits of folded cascodes topologies. I am curious if anyone can pose sound theories as to why folded cascodes have not caught on in a bigger way among the "I make a living at it" audio design community (Nelson Pass and Jeff Rowland excluded, of course, there may be others?).
mlloyd1
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