Here the load resistor
has been replaced by a
tuned circuit, C4 and
L1.
At resonance, the tuned
circuit is high
impedance.
At all other frequencies
it is a low impedance.
Therefore only signals
at the resonant
frequency will be
amplified
Since the tuned
amplifier works at radio
frequencies the
capacitors can be much
smaller in value than
those used at audio
frequencies
