audioguru fm transmitter

[sssaaa]

thank you very much audioguru

 

[sssaaa]

hi audioguru
i measure your transmitter with 1 metre distance between both antennas.
field strength metre and your transmitter.
and other one or two transistor fm transmitter circuit i have.
but your circuit is generating more RF power as compare to them.
and stability is good also.
but i judge that during transmitting Q3 is producing very littile amount of heat is it right?
i want to tune it around 80 MHZ, how many turns of coil do i make?

as you know my trimmers are 1-27pf.

 

[audioguru2]

hi audioguru
i measure your transmitter .... during transmitting Q3 is producing very littile amount of heat is it right?

Yes, my transmitter's Q3 produces a little heat with a brand new 9V alkaline battery. I can't remember exactly but the supply current was about 55mA so the power from the battery was 495mW. The preamp and oscillator doen't use much supply power so most must be used by Q3, 230mW as heat and 230mW as output I guess. If Q3 is operating in class-C then the output power is more.

i want to tune it around 80 MHZ, how many turns of coil do i make?

A calculation would probably be in error due to unknown stray capacitance. Try 11 or 12 turns on the coils.

as you know my trimmers are 1-27pf.

 

[sssaaa]

If Q3 is operating in class-C then the output power is more.

plz give diagram how to oprate Q3 in class-C.

 

[audioguru2]

The RF amplifier transistor Q3 might already operate a little in class-C if the oscillator signal is strong enough. The base-emitter junction could act like a rectifier and charge C12 so that Q3 is cutoff a little.
If R8 is connected to ground instead of to the positive supply then Q3 will operate in pure class-C because it will be cutoff and some of its base signal turns it on.

View attachment 38834

 

[steven2]

i think one of those simple to biuld peaking circuits would be handy for tuneing it for peak performance . look at the talking electronics site  they use the peaking circuit  for some of the fm transmitter bugs

 

[Silicon]

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?

 

[SM2GXN]

I would prefer the lower class C circuit unless the drive is to low, the upper class AB circuit need less drive.
Class AB will drain the battery faster and lower the efficiency, power amplifiers for fm transmitters doesn't have to run linear.

 

[audioguru2]

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).

 

[Silicon]

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?

 

[audioguru2]

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.

 

[Silicon]

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...

 

[shahzad-h]

Hi audioguru
I calculated the frequency of the tank ckt of ur transmitter in which

 

[audioguru2]

Hi audioguru
I calculated the frequency of the tank ckt of ur transmitter in which

 

[awash]

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 .

 

[audioguru2]

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

 

[awash]

Thanks for fast replay audioguru . As u said I did try with the other fm transmitter with 2n2222 it does work well . The prob is I couldn

View attachment 39365

 

[audioguru2]

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.

 

[awash]

Thanks so much your sweet help. u are generous .I am thinking to change voltage to 6 v camcorder battery (rechargeable) I hope it doesn

 

[audioguru2]

I am thinking to change voltage to 6 v camcorder battery (rechargeable) I hope it doesn