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Design of a simple double tuned BJT amplifier

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arebee

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Hi.

I've been studying stability and alignment of simple amplifiers with tuned circuits (LC tanks) at the input and output (e.g. a simple common emitter with a grounded emitter) and I have a few questions.

I noticed in simulations that the input capacitance Cbe is proportional to the bias current and varies alot between devices. This makes it hard to design the input LC filter since the capacitance depends on the transistor. Also, as the collector current varies, so will the center frequency of the tank.
Is there a way to minimize this interaction (assuming the transistor is unilateral and alignment is not an issue)?

Can anyone help me with this problem?

Best Regards
 

You design with the external capacitor in the tuned circuit very large so that the variation in the transistor capacitance has little effect. You also select the bias network for very high stability (small change in collector current with changes in transistor parameters.)
 

Thats what I thought, but that leads to some pretty big capacitors. I was hoping there was a more elegant solution.
 

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