Tom Cathey wrote:
> BTW,
>
> Used as an IPA:
>
> Is it possible to clean up the N4LTA 6M MRF-492 brick by idling it at
> several amps (instead of 100ma) - closer to class A1 and adding some
> negative feedback via an emitter resistor? [Assuming extra air and
> heatsinking keeps it within its ratings]
>
> Here's the schematic:
> http://www.qsl.net/k0kp/hbrew/6m_amp/schematic.html
>
> Right now the emitter goes directly to ground and the bias is obtained
> through the base using R1 and D2. An extra -10db of 3rd order
> improvement would be all that's needed if it worked.
If you want a nice, linear, driver don't waste time with 12V transistors. 28
or 50V makes a world of difference. Something like MRF173 or MRF150 (fets)
from RF Parts won't break the bank but will be massively better. Acrian
VMIL80FT or Point9 Technologies D1005 are more linear if you can find them
retail.
To run true ultra linear class A bipolar, you need a device rated for about
4x the wanted output when in class C (200-250W in this case), then run it
with dc dissipation 2x the rf output with an active closed loop bias
circuit, such as in this datasheet:
http://www.alldatasheet.co.kr/datasheet-pdf/view/MOTOROLA/TPV597.html
You will need to re-calculate the resistor values to suit. Typically you
should expect 3rd order IMD at -50 or so - goss overkill, but good forthe
soul.
Steve
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