Qeustions about fm transmitter i just buid.

I recently just build a simple 10 component fm transmitter:
and it works perfectly well, although i'm having some trouble understanding how it works.

as a example, the 1-33 pf capacitor connected to the audio plug, doesn't it create ac-like waves?
I heard that ac should never be used with electrolytic caps..?

what exactly is the purpose of the transistor?

I know i'm asking a lot here but if anyone could help i'd greatly appreciate it.

C1 can be 1uF for telephone sound quality without much bass sounds or it can be 33uF for deep bass sounds to be passed, or any capacitance value in between.
This capacitor does not create anything, instead it passes the audio signal but blocks the DC.
The audio source is probably 0VDC and the base of the transistor is POSITIVE 0.7V so the electrolytic capacitor is properly polarized. This capacitor always has 0.7VDC across it but both ends swing up a little (positive) and down (towards ground) with the audio signal.

The transistor oscillates in the FM broadcast band group of frequencies. Its frequency is changed slightly creating Frequency Modulation (FM) because the audio signal varies its conduction and its datasheet shows that is capacitance changes when its conduction changes. When its capacitance changes then the frequency of the oscillator also changes creating FM.

Audioguru said:

C1 can be 1uF for telephone sound quality without much bass sounds or it can be 33uF for deep bass sounds to be passed, or any capacitance value in between.
This capacitor does not create anything, instead it passes the audio signal but blocks the DC.
The audio source is probably 0VDC and the base of the transistor is POSITIVE 0.7V so the electrolytic capacitor is properly polarized. This capacitor always has 0.7VDC across it but both ends swing up a little (positive) and down (towards ground) with the audio signal.

The transistor oscillates in the FM broadcas
band group of frequencies. Its frequency is changed slightly creating Frequency Modulation (FM) because the audio signal varies its conduction and its datasheet shows that is capacitance changes when its conduction changes. When its capacitance changes then the frequency of the oscillator also changes creating FM.

Click to expand...

The transistor actually oscillates, mechanically?
So the transistor works as you would put a oscillator there?

rata478 said:

The transistor actually oscillates, mechanically?
So the transistor works as you would put a oscillator there?

Click to expand...

The transistor is in an electronic oscillator circuit with no moving parts. The electrons go back and forth.

Audioguru said:

The transistor is in an electronic oscillator circuit with no moving parts. The electrons go back and forth.

Click to expand...

just a suggestion

feed back for the transistor to oscillate is between collector and emitter.

Audioguru said:

The transistor is in an electronic oscillator circuit with no moving parts. The electrons go back and forth.

Click to expand...

Ah, alright.
That was quite a stupid qeustion, i should've known better.

THINKER_KAM_MAN said:

just a suggestion

feed back for the transistor to oscillate is between collector and emitter.

Click to expand...

What exactly would that do and why if i may ask?

rata478 said:

Ah, alright.
That was quite a stupid qeustion, i should've known better.



What exactly would that do and why if i may ask?

Click to expand...

well if there is no +ve feed back to the input the
transistor just wont oscillate.
now at the frequency of resonance given by the value
of the coil in (H) and the Value of the capacitance at the
collector+stay (C) to any one of the supply rail.
where by F= 1/{2pi*sq root(L*C)}=frequency of oscillation.
the voltage wave form will
be large sin wave and depends on the Q of the circuit. (that's if there is not too much feed back)
you will notice that the emitter of the transistor is not decoupled, so the internal
capacitance of the transistor Cce+stray+c5 will cause some of the collector ac voltage to be fed back to the emitter, so consider under this condition that the transistor is working in common base, thus causing the circuit to oscillate at the resonance frequency.
the modulation applied to the base changes
the capacitance at the collector of the transistor
which then changes the frequency, which then gives the FM modulation, you will also get some AM modulation.