Various FM transmitter circuits

[deepak007]

hello

i want to substitute the last two transistors, in the circuit given below , with 2N3866, and want to use a 9v supply. could anybody tell me the biasing resistor changes?

thanks in advance

 

[RF-OM]

Re: transistor bias

The voltage swing across the emitter resistor of the second transistor is close to 2V. This means that the third transistor will work in class C and may produce a lot of RFI. Increasing the voltage and therefore power may create a problem with interference. If you increase the voltage on emitter of the second transistor you need to be careful not to exceed the allowable base-to-emitter voltage for the third transistor, or you may kill it. Probably going to 9V may be a wasting of battery power. It looks like it is a bug circuit and it was design for low voltage operation.

 

[Audioguru]

transistor bias

The very old 2N3866 has such a wide range of current gain that some will work and others won't work in that circuit. The value of the base resistors need to match the amount of current gain.

 

[zlatkoMM]

Re: transistor bias

This is my favorite simple Fm transmiter:

 

[Audioguru]

Re: transistor bias

This is my favorite simple Fm transmiter:

1) Its antenna is connected to the oscillator's tuned circuit so its frequency will change if anything gets near it or moves away.
2) It doesn't have a voltage regulator so its frequency will also change as the battery voltage runs down.
3) It doesn't have pre-emphasis (treble boost) like FM radio stations so the de-emphasis in all FM radios will make reception with the treble frequencies cut like your stereo with its treble tone control turned all the way down.
4) It is mono, not stereo.

 

[deepak007]

Re: transistor bias

thanku mr.audio guru! could you give me a perfect FM transmitter circuit?

 

[Audioguru]

Re: transistor bias

thanku mr.audio guru! could you give me a perfect FM transmitter circuit?

The Micromitter is an FM stereo transmitter project in an Australian magazine. It uses a modern IC that uses a quartz crystal in a PLL frequency synthesizer circuit for perfect frequency stability and stereo. It has pre-emphasis so it sounds perfect.
The project has an attenuator at its antenna output to meet the law in Australia but the range is much farther when the attenuator is removed. A kit is available.

The BH1417 IC selects the frequency with DIP switches. The BH1415 and BH1416 select the frequency with a micro-controller that also displays the frequency.

The Silicon chip magazine wants you to pay for their article. It is here for free:
https://electronics-diy.com/schematics/BH1417_fm_transmitter.htm
https://electronics-diy.com/BH1417_PLL_Stereo_FM_Transmitter.php

 

[zlatkoMM]

Re: transistor bias

Look very well...thank Audio...but this circuit had some sirius
limitations.
First of all range is only available in two ranges covering from 87.7-88.9MHz and 106.7-107.9MHz in 0.2MHz steps.
So you can listen this on digital tuner but not in all fm radios.
Second how powerful is this circuit i think not so much.
And how much cost this IC?

 

[Audioguru]

transistor bias

The BH1417 has only 8 frequencies that are selected with DIP switches. The BH1415 and BH1416 have any frequency selected with a micro-controller.

They are designed for legal low power output to transmit the sound from an MP3 player in a car to the car radio a few meters away. A single transistor can amplify the output signal if you want it to be illegal and maybe cause interference.

Digikey sell one BH1417 for $3.92US, about the same as a cup of coffee at a restaurant.

 

[RF-OM]

Re: transistor bias

To zlatkoMM,

With 200kHz step any FM radio will receive this signal well if the SNR is enough. Model you link and you will get the maximum distance for given SNR. In most cases of FM transmitter usage on broadcasting band it is wise to keep your TX power as low as it possible. If it is not possible, better to not use it at all.

 

[zlatkoMM]

Re: transistor bias

Here is very weird shematic but circuit work very well with very good
frequency stability for such a simple circuit.

 

[zlatkoMM]

Re: transistor bias

Ups i find error in shematic , here is right:

 

[RF-OM]

Re: transistor bias

To deepak007,

Check this link

http://www.discovercircuits.com/R/rftrans.htm

There are a lot of FM transmitter circuits.

 

[zlatkoMM]

Re: transistor bias

I apologise i realy mess up schematic , this one is 100% right.

 

[Audioguru]

transistor bias

The BC547 has its collector and emitter shorted together.

 

[zlatkoMM]

Re: transistor bias

Yes it is short conected and what you mean what is this? ,there is no error.

Added after 2 minutes:

This is question for all what is transistor BC547 in this case?
real chenge....
where you are experts....

 

[Audioguru]

transistor bias

Your FM transmitter circuit does not make sense.

 

[Amrikove]

Re: transistor bias

I don't get why the emitter and the collector are shorted together. The transistor can't work that way

 

[zlatkoMM]

Re: transistor bias

Oh my transistor in this case is replace for capacitor(varicap) diode,
you can replace transistor with BB105 and anoda is where is base of transistor
try and must work
regards
zlatko

 

[Audioguru]

transistor bias

The base of the BC547 does not have a DC voltage so it cannot be a varicap diode.