Electronics > RF, Microwave, Ham Radio

Regarding BJT in parallel as Power Output

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JhonStan:
What do you think of the possibility of putting several BJTs in parallel, each of which has its own independent circuit? I know that it could be done with higher power components but I would like to know if one could work or not once built. Input and output of all BJTs are in parallel. What problems could there be and how to solve them?
I attach schematic image and .asc file for LTspice (right click and save).

Thank you!





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Tangent_Tracker:
Your biggest problem is going to be input impeadence... You need some sort of buffer or matching stage with a very high impeadence.

T3sl4co1l:
Considering you chose a 8GHz transistor, you will quickly find problems in layout as the reactance / transmission line length between ostensibly-parallel units is not negligible but indeed makes the whole thing scream at undetectable frequencies (unless you have adequate equipment to probe it).

More common is to build a distributed amplifier, which also acts to extend bandwidth, but BJTs are not good at this, for the same (above) reason.  Most often they are built with FET cascodes (or vacuum tubes if you're feeling particularly old school).  (Monolithic dist. amps, using FETs or PHEMTs, push into the 100s of GHz!)

Tim

JhonStan:
Ok thanks to both of you for the answer, I just point out that in simulation with this component, also given the low capacitance, it allows you to have an almost flat phase up to 100Mhz. I attach screenshots of the plot.



That said can I get the same result with dej JFet?. Maybe someone would be kind enough to make me a schematic with the same structure with some JFets if suitable? I just need a msolo scheme then I'll take care of putting it in parallel. With this simulator and very simple.

Wallace Gasiewicz:
There are plenty of MOSFET linear power amps that are used by CB people. I believe they are mostly illegal. 27 MHz stuff.
Having said that, there are lots of diagrams of these "Boxes" available. Look around for a few. Mostly they use IRF 520s and I do not know if those meet your needs for the frequency you desire.
Usually these things use maybe one MOSFET to drive 4-5 MOSFETS. They use RF transformers on input and outputs. Pretty simple designs.
Perhaps you could also look into HAM 144 MHz amplifiers for the 2 meter band. The designs in that range might be closer to your needs.

I would think that using MOSFETs would be cheaper than BJTs, I could be wrong of course.

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