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Simple High Performance DC Coupled Class A HPA with sub PPM THD
Simple High Performance DC Coupled Class A HPA with sub PPM THD
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Old 18th November 2018, 01:26 AM   #301
bkdog is offline bkdog
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Simple High Performance DC Coupled Class A HPA with sub PPM THD
RFI filtering and protection of the input FETs, as well as bandwidth limiting so that it does not oscillate. You can short out the 100R and it may work fine, but you may have RFI problems and it may oscillate...
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Old 18th November 2018, 04:45 AM   #302
Harry3 is offline Harry3  Australia
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I understand the filtering part.
I am thinking that a 2K2 resistor at the gate would be enough or perhaps Iím missing something.....
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Old 18th November 2018, 05:30 AM   #303
bkdog is offline bkdog
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Simple High Performance DC Coupled Class A HPA with sub PPM THD
Lookup "FET gate stopper"...
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Old 18th November 2018, 07:22 AM   #304
Harry3 is offline Harry3  Australia
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Perhaps I didn't explain it clearly in my previous post.

I am proposing to move the 2K2 resistor to the gate, as a gate stopper. In front of it, the two capacitors will act as high pass and low pass filters.

Is there an issue doing this with a FET input?
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Old Yesterday, 05:43 AM   #305
xrk971 is offline xrk971  United States
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Simple High Performance DC Coupled Class A HPA with sub PPM THD
Harry3,
Can you mark up the schematic with what you are proposing?
Click the image to open in full size.

Do you mean make R13 2k2 and get rid of the 100R? The 2k2 acts with C4 as a filter, whereas the 100R is a gate stopper, which probably isn't needed as the 2k2 is already there. However, it matters where the node to the filter cap is located. I think you can leave R13 where it is but ok to get rid of the 100R at R4.
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Last edited by xrk971; Yesterday at 05:48 AM.
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Old Yesterday, 05:59 AM   #306
Harry3 is offline Harry3  Australia
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X thanks for the response.
see diagram attached.

Also do you need regulators for the cap multiplier?
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Old Yesterday, 06:07 AM   #307
Harry3 is offline Harry3  Australia
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OK. I now see what you are saying about where the node should be.
I will leave the 2K2 resistor in it's original position.
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Old Yesterday, 11:37 AM   #308
xrk971 is offline xrk971  United States
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Simple High Performance DC Coupled Class A HPA with sub PPM THD
Yes, I use the TI TPS7Axxx ultra low dropout ones and set for +/-18v following cap
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