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Author Topic: SD2933 idle bias for class AB operation  (Read 4344 times)
K9AQ
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Posts: 55




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« on: January 18, 2013, 08:08:44 AM »

I am in the process of building a 1KW amplifier using (4) SD2933's.  I am curious if any one knows what bias idle current is used in the Tokyo Hy Power amplifiers, or similiar SD2933 amplifiers.

Don
K9AQ
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KO4NX
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Posts: 179




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« Reply #1 on: January 18, 2013, 01:02:36 PM »

.Hello Don:

I am not sure if this is what you are looking for, but the following App Note seems to address this issue:

http://www.st.com/internet/com/TECHNICAL_RESOURCES/TECHNICAL_LITERATURE/DATASHEET/CD00269521.pdf

On page 8 of 12, it appears as though you will need to set 900mA per device or 3.6A total.  Hopefully you have a method of switching the bias on and off before and after transmit.


73

Rich, AJ3G
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K9AQ
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« Reply #2 on: January 18, 2013, 02:31:35 PM »

Thanks Rich,

Yes I have seen this data sheet but I am looking for another confirmation as this is considerably higher than other recommendations that I have gotten.  I do have the bias switching so that it is turned off when the amp is not in transmit.

Don
K9AQ
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KO4NX
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« Reply #3 on: January 19, 2013, 05:49:25 AM »

Hi Don:

While I cannot tell you how THP has their amplfier setup, you may want to call Steve at HRO in Woodbridge. From what I understand, he is pretty knowledgable about these amplfiers, and may be able to get the information you need if he does not already know it off the top of his head.

I agree 900mA per device seems like alot, but without further information I would be inclined to trust the application note intially, and then take it from there.

Rich
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N3QE
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« Reply #4 on: January 19, 2013, 07:33:56 AM »

I agree 900mA per device seems like alot, but without further information I would be inclined to trust the application note intially, and then take it from there.

I have not used these particular transistors, but based on other 48V type MOSFETs, 900 mA seems entirely appropriate for some class AB operation point with very high (-25 dBc) IMD. Lower bias will give even worse IMD unless lots of negative feedback is being used (you will note that lots of negative feedback via transformer windings is a common theme in many of the lower IMD MOSFET amps.)
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