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audioguru fm transmitter


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hi again.. so i'm in need to make such transmitter(and i will design it on smd ;) ) but i want to ask how sensitive is that transmitter for voice?huh? bcoz i need to put it in domestic AC socket(TŪŪ HIDE) and hear whisper in another end of big room, btw i don't get  eg. if my transistor Hfe is eg. 200 and i'm using eg. 10k collector resistor(for simple voltage amp) when what will be difference if i'll use 400Hfe tran. with 5k collector resistor? i thik voltage gain would be the same,and also i want to ask if transistor's collector cutoff current is 50nA,when why almost everyone design bias current for 1mA, i think 10uA would be greatly enough. ok maybe i talked not about transmiiter,but i will probably need this in this transmitter, and what do U think about darlington transistor for audio pre amp? i think when transmitter would be superb sensitive. huh?

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how sensitive is that transmitter for voice?huh? bcoz i need to put it in domestic AC socket(TŪŪ HIDE) and hear whisper in another end of big room.

My transmitter's mic is sensistive enough to pickup voices in a room where people are about 4m away. Enclosed in an AC jack it might not be sensitive enough.

if my transistor Hfe is eg. 200 and i'm using eg. 10k collector resistor(for simple voltage amp) when what will be difference if i'll use 400Hfe tran. with 5k collector resistor? i thik voltage gain would be the same

Hfe (current gain) has nothing to do with a transistor's voltage gain. Since your collector resistor's value is half then the voltage gain is half.

if transistor's collector cutoff current is 50nA,when why almost everyone design bias current for 1mA, i think 10uA would be greatly enough.

With a collector current of 1mA in a 10k collector resistor, a 20V supply can be used and the collector will swing nearly 20Vp-p. With only 10uA then the collector resistor's value will need to be 10M and its output impedance would be much too high. Base to emitter capacitance would cut high frequencies.

what do U think about darlington transistor for audio pre amp? i think when transmitter would be superb sensitive. huh?

No. The voltage gain of a darlington transistior is the same as a regular transistor if the collector and emitter resistors are the same. The darlington has a very low input bias current.

The voltage gain of a transistor (and a darlington) is the ratio of the collector resistor (and load in parallel with it) to the unbypassed emitter resistor (plus the transistor's small internal emitter resistance).
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so Y'ur transmitter voice gain is about 16times,am i right,and i would like to increase it to about ~100times, so i'll need to use base resistor(between base and collector) about 1M,yes?

What are you talking about? A resistor from the transistor's collector to its base makes NEGATIVE FEEDBACK which reduces voltage gain.
Add another transistor or use an opamp if you want more audio gain.
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ar Usure it's output power is only about 200mW,bcoz if Q3 capacitor tuned well with  coil,when load resistance in Rf for Q3 should be only about 60Oms,and if suplly is really 9V when it should be even about 1W, oh.. but i just thinked that collector resistor doe's not let to saturete for Q3, yeap.. probably...

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  • 1 month later...

audioguru I am trying to build ur fm transmitter i have one prob i coudnt find more than one 2N3904 (in my place ) but i Have 3 pices 2N2222 . does it make  much different with the ur project if i use  one 3904 and the rest 2222 ? please help me thanks .

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Increase your battery's voltage to 9V and the range will be much farther. Also, decrease the value of the 470 ohm resistor to 220 ohms.

The antenna should be about 80cm long, in the same direction as the radio's antenna. The radio should also have an 80cm long whip or 160cm long folded dipole antenna or rabbit's ears and be very sensitive.

The difference between using 12pF instead of 10pF is nothing.

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Thanks again Audioguru please coudnt get this “15:1 attenuator with 2 (mono) or 3 (stereo to mono) resistors" could you show me the picture and I will try to find from my old junks. 
Or
I got some circuit on the net to change Dynamic microphone to electret microphone
Do u think it works perfect? And I don

post-21211-142791429659_thumb.gif

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The transmitter needs only 10mV of audio from an electret microphone. The output from a dynamic microphone is a little less, so it needs a preamp to increase its level.
The output from your CD player is about 150mV so needs an attenuator (voltage reducer) to reduce the 150mV to 10mV.
Two resistors in series make an attenuator. Two resistors connected together convert stereo to mono then a 3rd resistor makes them an attenuator.
R1 in the project powers the electret microphone and will need to be removed.
Here is an attenuator:

post-1706-14279142965946_thumb.png

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Increase your battery's voltage to 9V and the range will be much farther. Also, decrease the value of the 470 ohm resistor to 220 ohms.

The antenna should be about 80cm long, in the same direction as the radio's antenna. The radio should also have an 80cm long whip or 160cm long folded dipole antenna or rabbit's ears and be very sensitive.

The difference between using 12pF instead of 10pF is nothing.


hi audioguru
i want to know that what will be the out put power of this circuit in  mW  when we do these changes.
thanks..


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  • 2 weeks later...

yeap that wasp circuit gived about 20m range for me too,and it consumed wery little.. in fact the only thing i was happy about that circuit is frequiency stability,the voice sensitive was wery bad,it really would be suitable only for hand-held microphone... 20m in the same room(not tru walls)unsensitive mic so u wont get oscillation with U radio... hey it's not a bug...o and btw when i count tank circuit to work at about 100mhz and calculating tank circuit only with tank circuit capacitor/tuned capacitor values i found my wasp work at about 60Mhz...

yes my post is about that yellow circuit given somewhere around in this topic

bla bla try too kill Uself

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when i count tank circuit to work at about 100mhz and calculating tank circuit only with tank circuit capacitor/tuned capacitor values i found my wasp work at about 60Mhz...

It is inaccurate to calculate LC values for a tank that operates at VHF or above.
How much is the stray capacitance across the coil, trimmer capacitor or other parts connected to them?
How much is the capacitance inside the transistor?
How much is the inductance of each piece of pcb track or wiring?
How tightly wound is the coil?
Etc?
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in recent post U had said that  capacitances,i dont remember exactly but it was calculated as i said not counting other values ;) i said that for others to know,at lowest frequiency windings were tight,anyway o dont care about that transmiiter anymore becoz i sold it :) and buy ..em... U know and Im think i had learned enough from that transmitter. o btw does chip voltage stabilizator consumes any power itself?(i'm not about voltage drop trough it)

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  • 4 weeks later...
If Q3 is operating in class-C then the output power is more.

I connected R8 to ground instead of to the positive supply but I didnot get good results, then I connected R8 to positive then the circuit gave good output.
why the circuit hadnot given more output when i operate Q3 in class-C?
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The oscillator signal is small, it comes from the oscillator's emitter.
When Q3 is a class-A amplifier then it is biased and ready to amplify it.
When Q3 is a class-C amplifier then the input signal must have an amplitude of at least 0.7V before it amplifies it.
I didn't try it but Q3 might be a good class-C amplifier if its input is larger from the oscillator's collector. Then a reversed diode across the base-emitter of Q3 would be needed to stop the coupling capacitor from charging.

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