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I'm trying to learn about RF amplifier design and build a common emitter stage with an BFR92P.
And now I'm struggeling with the simulation in LTspice, or maybe with reality. As you can see attached is a simple common emitter testcircuit, that I have tried to optimize .
.asc -file: https://drive.google.com/file/d/1T7y4zdWiqA7fs3u-jYscfxaS7kk1qGwp/view?usp=sharing

Stepping Re from 200 to 800 ohm there is a sudden change in amplification. 350ohm is maximum an 400ohm is completely broke down.
I can't see this from DC characteristics (also attached) or I don't unstand something fundamental
The spice model is from infineon.
Maybe someone can help me to understand what's happening and how get a maximum voltage amplification.
Many thanks

enter image description here enter image description here enter image description here

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  • \$\begingroup\$ The two things that stick out to me most are 1) Re is in the emitter AND being used on the collector. 2) For RF, you usually want an inductor on the collector instead of a resistor. google for "class A rf amplifier circuit" and you'll find lots of examples of the topology. \$\endgroup\$ – Aaron Jul 12 '19 at 23:03
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If you increase the value of RL, the bias point of the collector voltage drops. If it gets too low, the BJT goes into saturation operation rather than forward active. In saturation, the \$h_{fe}\$ parameter of the BJT drops dramatically so the gain of your circuit also drops.

If you're really going to use this in an RF context, you should include the load impedance in your model. It won't affect the bias because you've AC-coupled the output. But it will likely reduce the RF gain of your circuit substantially.

If you don't consider the load impedance, you're likely to choose the wrong value of RL for optimizing the circuit's gain in the final application.

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  • \$\begingroup\$ Many thanks! Increasing Re to 150ohm makes it much better to handle and it doesn't drop that fast. \$\endgroup\$ – betex Jul 12 '19 at 20:18

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