Starting to like the MOSFETS - am I wrong?

[Captain Kilowatt]

They are just fine for a 50 ohm load. How many cb'ers can tell you their feedpoint impedance? Most don't have a clue, hence the problem.

Actually using a proper MOSFET allows the impedance to be virtually anything from a short circuit to an open circuit. There are MANY videos showing such devices operating into an open circuit with the antenna removed as well as showing a screwdriver being placed directly across the output. It's all about using the proper device and there is no reason manufacturers cannot do that.

 

[BBB]

Agreed, but for CB they don't and won't. They still put IRF 520 POS devices in them. Don't think they will change as it is a price point issue.

The broadcast use NXP brand LDMOS RF specific devices you may be referring to are a whole other puppy that I would definitely embrace when and if the prices on them come down. Start collecting 50VDC x 50 amp power supplies now before everybody wants one





He just sold this one on eBay a few weeks ago

 

[rabbiporkchop]

Actually using a proper MOSFET allows the impedance to be virtually anything from a short circuit to an open circuit. There are MANY videos showing such devices operating into an open circuit with the antenna removed as well as showing a screwdriver being placed directly across the output. It's all about using the proper device and there is no reason manufacturers cannot do that.

No such thing as "proper" mosfet.
LDMOS however is killer technology, albeit the 50 volt limitation.

 

[Captain Kilowatt]

No such thing as "proper" mosfet.
LDMOS however is killer technology, albeit the 50 volt limitation.

Yeah there is such a thing as a proper MOSFET.......the PROPER device for the application. IOW use a regular MOSFET meant for RF applications and not a device designed for switching power supply applications like what are being used in CB radios today. Not all MOSFETS are 50 volt devices either and not all RF types are LDMOS. The MRF5015 is a 15 watt device designed for operation on 12 volts. Sounds about perfect for CB applications. It is good to 512 MHz so it will have lots of gain at 27 MHz. The 2SK2975 is another MOSFET that operates on 12 volts but with lower power suitable as a driver stage. The MRF5007 is another. The majority of high power MOSFETs dooperate on 50 volts but they are a great many that are designed for 28 volts that will work great with a dual battery supply like someone would use in a motor maul. There are also more that operate on 12 volts that produce typical output levels of what CB.s and most exports use. If manufacturers would use a PROPER (there's that word again) MOSFET instead of the switching types then the price would be a bit higher but reliability would be GREATLY increased.

 

[543_Dallas]

Reminds of the time I said sweep tube amplifiers belonged in a land fill. I got called out because I use a tube amplifiers too....

 

[StrangeBrew]

Yeah there is such a thing as a proper MOSFET.......the PROPER device for the application. IOW use a regular MOSFET meant for RF applications and not a device designed for switching power supply applications like what are being used in CB radios today. Not all MOSFETS are 50 volt devices either and not all RF types are LDMOS. The MRF5015 is a 15 watt device designed for operation on 12 volts. Sounds about perfect for CB applications. It is good to 512 MHz so it will have lots of gain at 27 MHz. The 2SK2975 is another MOSFET that operates on 12 volts but with lower power suitable as a driver stage. The MRF5007 is another. The majority of high power MOSFETs dooperate on 50 volts but they are a great many that are designed for 28 volts that will work great with a dual battery supply like someone would use in a motor maul. There are also more that operate on 12 volts that produce typical output levels of what CB.s and most exports use. If manufacturers would use a PROPER (there's that word again) MOSFET instead of the switching types then the price would be a bit higher but reliability would be GREATLY increased.

Any thoughts on how hard it may be to modify a radio that uses something like 520's to take these parts?

 

[Captain Kilowatt]

Any thoughts on how hard it may be to modify a radio that uses something like 520's to take these parts?

Honestly I have no idea whatsoever to do that. It could certainly be done but is it worth the cost? That is the deciding factor I would think. The MRF5015 is a 15 watt device and costs $34.90 from RF Parts while the IRF520 costs just $1.95 from RF Parts. It's a no-brainer what the manufacturer is going to use as long as he can make it produce power regardless of the reliability under high SWR conditions.

 

[Robb]

Bought ten International Rectifier IRF-520's for less than $10 from Mouser. Don't think they carry that brand any more; think they have Vishay now for about the same price.

 

[Captain Kilowatt]

Bought ten International Rectifier IRF-520's for less than $10 from Mouser. Don't think they carry that brand any more; think they have Vishay now for about the same price.

Yeah the IRF520's I priced from RF Parts are Vishay.