PA emitter area - RF Cafe Forums
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Post subject: PA emitter area Posted: Fri Nov 18, 2005 7:52 pm
The issue for power output is current density in the emitter-base junction - you can't increase that without limit. So the more area, the more power, given a limit on current density.
What determines Ft is (among other things) the width of the base junction. Since junctions are usually the result of diffusion, smaller emitters often result in narrower base junctions.
Nf, on the other hand, is largely due to the resistivity in the base region.
Hope this helps.