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help me understand fm transmitter

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youngboss

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long-range-fm-transmitter-circuit.GIF
can some one explain how C5 L2 is acting as a RF amplifier?
and also whats R5 doing?
i'll be grateful for any knowledge or references that u can look up
 
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electret mic has built in amp which is powered though R1 also acting as load resistor.
R3 is used to mathc impedance. C1 is used to isolate DC between Mic and Q1 but allow audio frequencies to pass (hence large value).
C3 and C4 are "grounding" capacitor for RF but not for audio frequencies (small value). R2 is biasing resistor for base of Q1.
C4 and L1 are tuned circuit that should be adjusted to some FM band frequency 88-108MHz. C6 is used as feedback to turn Q1 stage into oscillator. without it, this would be only amplifier. R4 is input resistor for Q1 for RF frequencies. (recall that base of Q1 is "grounded" for RF through C2 and C3; Q1 is therefore operating in Common Base mode, emitter is input, collector is output and C6 takes small part of output signal and brings it to input - when this arrives in phase you got oscillator).

rest of the circuit is the same, except that Q2 is in Common Emitter mode.
R5 is biasing resistor, there is LC tuned circuit and antenna is connected to collector.
 
Elaborating a little further, Q2 is the active element in a common emitter amplifier configuration. The combination of C5 and L2 forms a tuned collector load for the amplifier. They serve to provide a high impedance collector load at the operating frequency, also reducing harmonic content at the amplifier's output. R5 is providing bias for the amplifier to maintain Q2 in the conduction region. Without R5, the input from C7 may not be of sufficient magnitude to cause Q2 to conduct current.
 
The simple circuit is missing a few important things:
The missing voltage regulator for the oscillator will cause the RF frequency to change as the battery voltage runs down.
The missing pre-emphasis (all FM radio stations have pre-emphasis) will cause sounds transmitted to an FM radio to be muffled.
 
My FM transmitter has a preamp transistor Q1. Pre-emphasis is made with a low value capacitor C4 parallel to the emitter resistor R6. Then high audio frequencies are amplified more than low audio frequencies.
 

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