I always use 10K with them and measure about 4.3VDC across them. 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. Since they have a FET transistor in them at their output they work a little when connected with backwards polarity but not very well. 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. 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 mounted it in a metal box and it works great! 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. Connecting its RF oscillator to the voltage regulator easily fixed that.ģ) It changed its RF frequency a lot if its temp changed.Ĥ) It had a short range of only about 30m without walls or anything in its path. I also added a low-dropout voltage regulator so that it works when the 9V battery's voltage runs down to 5.3V.ġ) It changed its RF frequency a lot if I or anything conductive came close (10cm) to its antenna.Ģ) It changed its RF frequency a lot as the battery's voltage ran down. I added AC negative feedback to reduce its distortion and added pre-emphasis so that my normal radio produces treble sounds from it. I re-designed the audio preamp, biasing it properly and using DC negative feedback to keep it immune from temp changes. When I touched the audio preamp transistor with my finger to warm it, it became saturated. When I touched the audio preamp transistor with my finger to warm it, it worked for a few seconds until it cooled.ģ) 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. 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.Ģ) 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. It was nearly identical to your circuit and had all the problems of the person who complained about it:ġ) No audio. I used proper supply decoupling caps unlike your circuit, wound my own coil using heavy enamelled wire from an old transformer, and it worked! I planned the layout and connections to be close and tight. 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 wanted to help fix it but I don't have a good sim program. Somebody else on this site had an FM transmitter circuit that didn't work. Just check your antenna length which according to me should be around the length of a half pencil. one cm short or long & you should not expect anything from it. The length of the antenna at these frequencies is highly unforgiving. Also an RFC(5-6 turns) can be included between C3 & C4. I would recommend you to feed the output coming from C4 directly to C3 input, eliminating the first stage. By amplifying it, you are really going out of the active region on both sides. Since you are feeding your audio to the base of the second stage you really should not amplify it. Hence you will get clipping in your audio.īut the biggest Mistake in the given circuit is that the FIRST STAGE is unnecessary. audio amplifier stage is close to saturation, if you can plot the Ic-Vce characteristics, you will come to know. It will work properly, no doubt about that.
0 Comments
Leave a Reply. |
AuthorWrite something about yourself. No need to be fancy, just an overview. ArchivesCategories |