[NLRS] MRF658 replacement for MRF648 in a 222 MHz amplifier?

[Dr. Gerald N. Johnson]

I found both data sheets. The 648 is not in the RF data book Rev 5, but 
the 658 is. there is a table in that edition of transistors that should 
not be designed in and the 648 is not mentioned.

648 is in Rev 3 and 4 while 658 is in Rev 4 and 5.

Both transistors are characterized only from 400 to 512 MHz. The 658 
takes 13 watts drive to put out 50 watts at 512 and the 648 takes 15 
watts drive. The input and output impedance real parts are about 15% 
lower for the 658 while the reactive parts are very similar in that 
frequency range. The Cob (output to base capacitance) for the 648 is 125 
to 150 pf and is 170 to 220 pf for the 658. They both will take 20:1 SWR 
at the collector. The test circuits are slightly different, but the 
board parameters for the 648 in the board impedances aren't listed so 
its hard to compare them. The shunt capacitors at the base and collector 
are different, but the input tuning caps in the PI are the same while 
the variable at the collector of the 658 is not present.

I think the differences between their data sheets is smaller than the 
differences between two transistors with the same part number off the 
same production line. They both have the same power dissipation rating 
while the 658 is rated to put out a bit more power, 65 vs 60 watts.

73, Jerry, K0CQ

On 2/11/2013 7:28 PM,  Jim Froemke wrote:
>
>
> I have an MRF658 that I'd like to use as a replacement for a blown MRF648 in
> a 222 MHz amplifier however:
> - The MRF648 "brief" data sheet says "applications up to 512 MHz".
> - The MRF648 data sheet only lists specs. from 400 to 520 MHz.
> So, has anyone used the MRF658 at any frequency below 400 MHz and in
> particular at 222 MHz?
>
> Comments would be appreciated.
>
> 73, Jim
> K0MHC
>
>