As I understand, class-C amplifier can only generate signal of constant frequency and amplitude. So the question is, how can it be useful and what are its applications?
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1\$\begingroup\$ Amplitude doesn't have to be constant, and frequency can vary within the passband of that LC filter. So, AM radio and FM radio transmitters. \$\endgroup\$– user_1818839Feb 9 '21 at 0:18
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\$\begingroup\$ @BrianDrummond How can amplitude and frequency vary? \$\endgroup\$– Rodion DegtyarFeb 9 '21 at 0:26
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\$\begingroup\$ They both come from the input signal so of course they can vary. \$\endgroup\$– user_1818839Feb 9 '21 at 0:45
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\$\begingroup\$ @BrianDrummond: Class C. The output amplitude is largely determined by the supply voltage. But -- still good for FM, or on-off keying, or PSK. \$\endgroup\$– TimWescottFeb 9 '21 at 0:52
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2\$\begingroup\$ The output is driven by the transistor -- the tank circuit is passive, it can only be driven. Typically, the tank circuit has some bandwidth over which it works; as long as the signal that's being amplified is within the tank circuit's bandwidth, the amplifier will work. \$\endgroup\$– TimWescottFeb 9 '21 at 1:06
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Narrow band RF applications are where the class C amplifeir is usefull .FM ,PM,FSK,PSK schemes dont rely on amplitude linearity so the good efficiency and simplicity of class C is an advantage .AM can be implemented by modulating the power supply voltage which is also simple.
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\$\begingroup\$ @Rodion Degtyar Classic AM modulator for C-class amps adds AC (for. example an audio signal) to the power supply DC. The secondary winding of a transformer is in series with the DC supply. The method has a drawback: As much AC power is needed from the transformer as there's needed DC power to generate the AM carrier. I the power of the AM carrier is say 5W in a walkie talkie, a 5W audio amp is needed to feed the modulation transformer. \$\endgroup\$ Feb 9 '21 at 1:11