Re: MOSFET PA RF Choke?


Leland L. Bahr
 

Yes, I agree with everything you said.  I also don't understand how 25 ohms could choke out RF from feeding back into the power supply. 

Lee

On 4/21/2019 5:20 PM, Curt via Groups.Io wrote:
Studying a 5 watt 40M Transmitter that uses an IRF510 for PA, what is puzzling is the RF choke is specified as a T68-2 with 10 turns.  By my calculations this presents an impedance at 7 MHz of about 20 ohms, which seems way less than the 30 times output impedance rule.  Am familiar with 10 turns on an FT37-43 which presents an impedance of about 1500 ohms, have encountered other IRF510 PAs that used that type choke.  How can the low impedance choke keep RF from leaking out of the output circuit to the power supply?

The same circuit uses a 2N3053 as a driver, the collector being coupled to the PA via a transformer that uses an FT37-43 with 14 turn primary ( 3000 ohms @ 7 MHz ) and a 4 turn secondary ( 246 ohms ). 

I am suspecting that the core types have been transposed.  If so, then the choke would be 1500 ohms and the transformer would be 49 ohm primary with a 4 ohm secondary.  These seem more what should expect?

Comments?

Curt KB5JO

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