You 'know' wrong then, a condensor mike requires power, usually 48V - the little inserts are 'electret condensor' mikes, and have a built-in FET amplifier to buffer the condensor element.

Essentially the electret mike has a permanent charge stored on the active element, this removes the requirement for an external supply - but adds the requirement for a buffer, which then needs a supply itself.

Check Wikipedia! http://en.wikipedia.org/wiki/Electret

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My my my. This is gonna be a fun place...lol. What is piezo-electric material? Does it require electricity to produce electricity? You go to Wikipedia for electronic advice?....LOL That's too much...LOL. That one takes the cake!!!! A condensor mike requires no power! Period! The teacher has spoken.

 

Sorry, what has 'piezo-electric' got to do with condensor mikes?, piezo mikes are a completely different beast, and you are quite right that piezo mikes don't require any power.

Those who can do, those who can't teach!

And even teach absolute rubbish!

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Because condensor mikes have piezo-electric crystals in them.

 

Nope. Read about the electret mic and about the condensor mic in Google.
A condensor mic needs a 48V power supply to allow the capacitive voltage divider action of the diaphragm. An electret mic has the 48V built-in on its electret material. An electret mic doesn't have an amplifier, it has a FET transistor impedance converter which doesn't have much voltage gain.

No they don't! Besides, we are using an electret mic, not a condensor mic and not a piezo crystal mic.

The RF oscillator's transistor's collector/emitter changes its capacitance a tiny amout with the modulation. A hyper-abrupt varactor diode has a wide capacitance change and is usually used to tune the entire band of frequencies.

There are many FM transmitter circuits on the web. Most use just the transistor's small changing capacitance to produce the FM. Some use ordinary diodes or transistor junctions as varactors.

Attached Images

Electret mic.PNG (18.5 KB, 12 views)

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Uncle $crooge

 

Whatever. I'm not gonna go on and on about a condensor mike.

Well first off. The same varactor diode that is used in a PLL to shift the frquency is also the chosen modulation device in most applications today. Even though theoretically it is PM (Phase Modulation) it does the same thing and is detected the same way. Now if you consider the circuit, and to be honest with you, I have not seen the circuit. Just considering what you said above, what do you think you are doing by applying audio to the collector of the transistor? You are applying an Audio voltage in reverse bias to a P/N junction. A transistor is just two diodes connected back-to-back. So, when you apply this audio voltage to the collector of the transistor, you are causing the depletion layer of one of the diode junctions in the transistor to change in size by the amount of reverse bias applied to it. This causes the P and N material to act as plates of a capacitor which will vary with audio frequency and amplitude. Personally I find this to be very trashy design and I would never do this myself.

 

My RF oscillator is just a Colpitts circuit. It is Amplitude Modulated a little by the audio signal and its changing collector/emitter voltage changes its capacitance (as shown on the datasheet) which changes its frequency, creating FM. An FM radio's detector rejects the AM.

My FM transmitter circuit that is talked about in this thread sounds perfect! I was amazed when I tried it. It has pre-emphasis (treble boost) like FM stations have and very low distortion. It has a voltage regulator and an RF buffer amplifier to keep its RF frequency from drifting all over the place like simpler designs do. Its range is over 2km line-of-sight to a very good quality FM radio.

Attached Images

FM.GIF (11.6 KB, 16 views)

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Uncle $crooge

 

Another reason I don't like that design is: Notice I said it will vary with frequency and amplitude. That means the amplitude of the audio signal will effect the collector current thereby producing AM components as well. In this case you will need a limiter circuit to get rid of the AM. Now you run the risk of audio distortion. I would much rather modulate the tuning section of the oscillator. Then you won't have any unwanted AM components.

Edit after I saw your design:

I assume your pre-emphasis is C9. I don't have a problen with that but where is the buffer amp you speak of? A buffer consists of an emitter follower or darlington pair amplifier and yes you are right, buffers a very good idea! I use buffers all over the place. Especialy following the all important oscillator.

 

The AM from my FM transmitter is fairly small.
The IF amplifiers of all FM radios have a limiter that rejects AM.
I can't hear distortion from my FM transmitter on my very high quality stereo tuner nor on my cheap clock radio.

If I wanted to be an FM broadcast station operator then I would have made this frequency-synthesized MICROMITTER one with a PLL and varactor diode. It is stereo and also has pre-emphasis.

Attached Images

Micromitter schematic.gif (147.2 KB, 14 views)

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Uncle $crooge

 

C9 is a supply bypass capacitor.
C4 provides pre-emphasis (boosts the high audio frequencies). C4 has a different value for the different amounts of pre-emphasis around the world.
Transistor Q3 buffers the oscillator so that things near the antenna don't change the oscillator's frequency. It also amplifies the RF power.

Attached Images

fm tx mod4.GIF (46.2 KB, 13 views)

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Uncle $crooge

 

OK buddy! I don't want to get off on the wrong foot here. It's just that I got attacked about the condensor mike and I have done too many designs with them, but like I said, I'm not gonna go there with you. Now there is something to be said for cheap and dirty. I have designed PLL FM transmitter using PIC16F84 and Motorola MC145149's. I also have designed stereo multiplexers with discrete components because at the time a common chip (the RHOLM BA1404) was not redily availble. You simply run your left and right adio into the MUX and you get a composit signal that will modulate the tranmitter oscillator or in my case the VCO with a stereo signal. I would post the cotton picker but I did it so long ago it is only in Orcad SDT which runs in DOS.

The only IC is a divider used to obtain the 19KHz pilot and carrier for the 38KHz double sideband signal.

 

Here's one of my FM broadcaster designs:

Crap! can't figure out how to add an attachment.

 

The Rohm BA1404 was full of problems and has been replaced by Rohm's BH1417 that is used in the Micromitter that I posted.

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Uncle $crooge

 

It looks very faint.
What are the functions of transistors Q7 to Q159?

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Uncle $crooge

 

What are you talking about? I didn't even attach anything. It won't let me.