How to Bais a tuned rf amplifier and compute I/O Impedances
The AC equivalent circuit would look like this, assuming that C1, C2, C3 have very low impedance (large capacitance values) at the frequency of interest:







simulate this circuit &ndash; Schematic created using CircuitLab
 There is a potential problem with this circuit. When Lin combined with Cin has a resonant frequency very close to the resonant frequency of Cout combined with Lout, it is a potential oscillator. Because Q1 has significant gain, its collector-to-base capacitance feeds a portion of the output signal back to the input: oscillations result.

R1 and R2 could be made small enough that input impedance is reduced to the point where oscillations don't occur. Or you could bias Q1 so that its input impedance is low. Similarly, the output impedance can be reduced by loading the tank with resistance. Such techniques are often used to make potentially-unstable amplifiers refrain from oscillation.
At much higher frequencies, gain of the transistor is so low, that potential oscillations are not a problem.