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MOSFET PA RF Choke?


Curt
 

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


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


nm0s_qrp <ai9e_qrp@...>
 

Whose design is this?  Is there documentation anywhere that others could see?


nm0s_qrp <ai9e_qrp@...>
 

It is difficult to speak intelligently on this design without seeing a schematic, but it is not at all unusual to have a low-inductance choke between the drain of the PA MOSFET and the supply, _if_ it is resonated against a parallel capacitor at the operating frequency to create a high impedance at the drain.  This capacitance may exist in parallel to the drain-source of the MOSFET, and have a return through a larger RF decoupling capacitor in parallel with the supply voltage.  The low value choke may let RF current pass, but as long as it is a high Q circuit, there is no RF power being dissipated.

73 Dave NM0S