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Old 17th December 2011, 10:05 PM   #1
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Default Simple Circlotron with power Mosfets

Hi guys,
I want to show you this simple Circlotron circuit. In the simulation with LT-Spice it works very well. The signal will be taken off from this circuit at the drain of the MOSFET. It is therefore a flawless transconductance amplifier.
This is the circuit diagram:
Click the image to open in full size.
This is the FFT simulation:
Click the image to open in full size.
These are the frequency responses, open loop an closed loop.
Click the image to open in full size.

Ready to play
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Old 23rd January 2012, 11:09 AM   #2
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I have simulated a little bit.
And I found out that the current availability is extremely important in this topology. With the 2SK1058 the distortions increase sharply at higher powers. The IRFP240 provide significantly better results.
Now I know why Thorenz use in their Circlotron such strong mosfets.

Now I'll build this circuit to test:
Click the image to open in full size.
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Old 23rd January 2012, 11:27 AM   #3
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Hi moschfet

I like your circlotron sch, awaiting for real circuit performance.

If possible avoid input opamp, better use double j-fet unity gain buffer, IMHO it is much more transparent than OPA604.
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Old 23rd January 2012, 11:28 AM   #4
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This design is interesting. But, what about security in case of a MOSFET shortcicuit failure? I work with electronic and almost all times MOSFET and transistor fail shortcircuited. Keep in mind some fusing in the load???
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Old 23rd January 2012, 11:51 AM   #5
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Quote:
If possible avoid input opamp, better use double j-fet unity gain buffer, IMHO it is much more transparent than OPA604.
Perhaps, but as a prototype, it is easier and I get a good distance DC.
1kohm is very low, can a JFET buffer drive it with low distortion?

Quote:
Keep in mind some fusing in the load???
Yes, but a simple fuse enough for now.
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Old 23rd January 2012, 02:32 PM   #6
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Default Very nice!

Hi moschfet,

Nice to see that driver stage connection getting some use. It works quite well in practice; I've had similar designs running for some time (see US patent 6747513). You can easily drive it from a single differential voltage gain stage, if you like.

Also, as you probably know, the idle currents in the IRFP240 output MOSFETs will tend to increase as they warm up over time. If you provide compensation for that, you may not need source resistors in the output stage.

Best of luck with your design!

Joe
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Old 23rd January 2012, 04:49 PM   #7
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Hi Joe,

thanks you I will thermally couple the current source at Q10 with the Mosfets.
There I will also set the bias.

I've just changed the OPA604 against a JFET buffer. The simulation also seems to work with very little distortion...but with higher DC-offset
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Old 23rd January 2012, 11:06 PM   #8
alex mm is offline alex mm  Romania
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Default PCB first try .....

..... My contribution in this thread ...
Alex.
Attached Images
File Type: jpg FET CIRCLOPHONE.JPG (911.2 KB, 1521 views)
Attached Files
File Type: pdf FET CIRCLOTONE.pdf (91.7 KB, 319 views)

Last edited by alex mm; 23rd January 2012 at 11:13 PM.
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Old 24th January 2012, 07:19 AM   #9
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Hi Alex,
very nice.
I have seen your layouts and I like them very much, but you are too fast.
I have changed some details. I think the Mosfets must be thermally coupled with the current source Q10. So I plan to put Q11 on the heatsink. R17 is a pot to set the bias. I estimate, the circuit at the bias must be tuned in reality.
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Old 24th January 2012, 11:04 AM   #10
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Default Re: DC Offset

The DC offset problem noted earlier can be addressed by adding a current path from each output terminal to the -50V rail, to recapture the current supplied by Q10 from the +50V rail. Resistors will work, but an NPN current sink can be made to provide thermal tracking against the Q10 current source (though such precision may not be needed), and will better isolate the outputs from any noise on the -50V supply rail.

Also, speaking of 50V supply rails, I notice that the front end does not need nearly this much voltage to operate. It should work fine on 25V rails, or perhaps less.
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