[Amps] 'heat spreader' sizing ??

[Dr. David Kirkby]

On 02/ 8/11 05:40 AM, K8RI (Roger) wrote:
> On 2/6/2011 12:21 PM, Steve Thompson wrote:
>>> Both Freescale and NXP discuss mounting the latest high power
>>> devices in application notes. For example, see NXP #10800 para. 2.5.
>> Sorry, that should be AN10858.
>>
>> www.nxp.com/documents/application_note/AN10858.pdf
>>
> To me they look kinda impractical for linear techniques and particularly
> at HF.
> A 1200 watt transistor to get an average of 200 out?
>
> 73
>
> Roger (K8RI)
>> Steve

If you take a look at the data sheet, you will see it says:

"Typical pulsed performance at frequency of 225 MHz, a supply voltage of 50 V 
and an IDq of 40 mA, a tp of 100 μs with δ of 20 %:" Then it shows 1200 W. 
Everything on the data sheet refers to "pulses".

So this is designed for pulse use. With 100 uS pulses, and what I assume is a 20 
% duty cycle, it gives 1200 W, one would probably get about 1200/5 = 240 W 
continuous. So the fact it's a 200 W amp continuous amp is not unreasonable.

Note is should be very linear for a 200 W transistor amp. The 1 dB compression 
point of that design is 950 W.

I think for almost any ham amp, the 1 dB compression point would be less than 
the rated output - here is is almost 5x the rated output.

That said, the transistors are pretty damm expensive for 200 W out.

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Dave