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MRF455

roadrage

roadrage

Active Member
Aug 3, 2011
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In the middle of nowhere. Central/northern Maine
I was looking at the spec sheet of the MRF455s and want to confirm. The output was 60W @12.5V. What I am curious about is the input. The Toshiba spec charts list a max output and a max input level on their transistors The manufacturer of the 455s don't. I am assuming that the 60W is the max recommended output. There is no spec listed for the input level aside from the graph. On the graph it looked like about 2W in produced 60W out @12.5V, but that can't be right. Can the input drive that low produce such a high output or am I reading it wrong?

http://www.macomtech.com/datasheets/MRF455.pdf
 

I think the input on those is 4 watts max on those, normally they don't like a high dead key from the amps I have owned. 1-2 watts at most is what I would dead key into an amp with the listed transistors, but I think max is 4 watts but don't hold me to the flame on that.
 
Look at the specification sheet again, scroll down to the graph in figure 2
This is the typical power curve at both 12.5 volts and 13.6 volts
Read input watts on the bottom and output on the left hand side.
For example, in a test jig, 2 watts input@12.5 volts = 60 watts out.


73
Jeff
 
Last edited:
You'll also notice the test circuit uses narrow band tuned input and output circuits. They tend to be more efficient at matching the impedance of the transistor than the broadband input and output transformers. This makes the manufactures gain specifications look good with the use of the most efficient matching to the device.

The 2 watts drive can easily produce 60 watts output with the LC tuned circuits where it could require significantly more drive to reach the same output in a broadband transformer circuit. The best transformers have about an 80% efficiency rating as I recall.
 
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We have said here before that the transistor Spec sheets do not tell the whole story, they are a starting point.
They are only testing a single transistor, in a special jig set-up only for this purpose and show "typical" or sometimes min gain etc.




73
Jeff
 
Captain Kilowatt said:
If you take the power out of 60 watts and divide that by the gain of 13 dB you get a drive level of 3 watts.
Click to expand...

TAAAHHH DAAAAAAAAHHHH!!!! There you have it ;) I remember seeing on old Palomar and amplifiers 4 WATTS INPUT AM and I would laugh and shake my head and think "well.... these are illegal so I suppose common courtesy is also out the window and non existent"
 
It's like Jeff (Audioshockwav) said, those figures are a guideline and a good starting point. Some people like to quote drive levels at the point the amplifier has about 1 dB of compression while others like to quote the point where the signal flat tops while still others like to use the output versus gain figures like I did in my previous post. in all cases the circuit itself will have an effect on the final outcome.
 

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