Hi DigiTan,
Somebody else on this site had an FM transmitter circuit that didn't work. I wanted to help fix it but I don't have a good sim program. Since I knew that the capacitance between tracks on a breadboard are maybe 10 times higher than the circuit's lowest value cap, and that the "audio" transistor that is used as the 100MHz carrier oscillator hardly has any gain at such a high frequency, I built the circuit on a Veroboard.
I planned the layout and connections to be close and tight. I cut all extra track length. I used proper supply decoupling caps unlike your circuit, wound my own coil using heavy enamelled wire from an old transformer, and it worked!
Sort of.
It was nearly identical to your circuit and had all the problems of the person who complained about it:
1) No audio. Very similar to your project, with a new 9V battery, its audio preamp transistor was saturated and therefore didn't amplify and didn't even pass the mic's signal.
2) So I lowered the supply voltage and it worked for a moment but the audio preamp transistor became cutoff with a supply voltage of about 7.5V or less. When I touched the audio preamp transistor with my finger to warm it, it worked for a few seconds until it cooled.
3) With a supply voltage of about 8.2V the circuit would transmit my voice to my FM radio but was very distorted and muffled without any treble. When I touched the audio preamp transistor with my finger to warm it, it became saturated.
A wonderful circuit!
I re-designed the audio preamp, biasing it properly and using DC negative feedback to keep it immune from temp changes. I added AC negative feedback to reduce its distortion and added pre-emphasis so that my normal radio produces treble sounds from it. I also added a low-dropout voltage regulator so that it works when the 9V battery's voltage runs down to 5.3V.
It sounded great!
But it had more problems:
1) It changed its RF frequency a lot if I or anything conductive came close (10cm) to its antenna.
2) It changed its RF frequency a lot as the battery's voltage ran down. Connecting its RF oscillator to the voltage regulator easily fixed that.
3) It changed its RF frequency a lot if its temp changed.
4) It had a short range of only about 30m without walls or anything in its path.
So I added a tuned RF buffer stage to its RF oscillator to keep its antenna away from its oscillator's tuned circuit, which also improved its range enormously. I mounted it in a metal box and it works great!
Prime quality (not factory rejects, I don't know why people buy the cheap thingys) but inexpensive 2-wire electret mics are all about the same sensitivity. I have tried "normal-size" electret mics from toys and a tiny one from a cell phone with my transmitter and they sound the same. Since they have a FET transistor in them at their output they work a little when connected with backwards polarity but not very well.
They require only 0.1mA to 1mA so the value of its load/powering resistor (R6) in your circuit is low with a 9V supply. I always use 10K with them and measure about 4.3VDC across them. They produce an AC output level of about 10mV if you shout and eat them, which you probably can't measure with a multimeter. Since they taste bad they need a preamp. :lol: