Audioguru FM Tx

[walid1]

Hi guru

That circuit is nearly the same as mine, except its frequency will drift as the battery voltage runs down because it doesn't have my voltage regulator.

yes exactly, i noticed this. I'll redesign it after your help, and with u makeing its range more.
I know that this circuit is not better than yours, but I have a chance with this.
when I build a circuit and didn't work it is more easy to me to build anthother new one than searching for the problem in the first, it is very complex and make headache. it is hard to me, after 4 hours of work and organize, to look again for the causes, especially if i use a used parts from my junk, the possibility is large.

It also won't boost treble audio frequencies which are cut in FM radios.

please explain this statement.

The easiest way to increase its range is to use a 9V battery instead of only 3V. But then the 1st transistor will probably saturate and will need its base resistor's value increased or its collector resistor's value decreased.

If I increase RB or decrease Rc, then I'll have more voltage gain!

The microphone's powering resistor is connected directly to the battery and the supply bypass capacitor has a low value which is good for RF, so the circuit might "motorboat" until you add a 100uF supply bypass capacitor.

u mean "motorboat"  after using 9 v batt

The output transistor is connected without any negative feedback to control its operating point.

also your FM Tx has no -ve FB

Transistors have a wide range of current gain so some have low gain and will be cutoff in this circuit and others will have high gain and the transistor will be saturated.

I need more explaination to this point.

The simplest fix is to change the value of the 150k resistor to 68k then connect it to the collector instead of to the supply.

to make the last stage a collector feedback configuration, but doind this adds what to the circuit.

thank u guru

 

[audioguru2]

please explain this statement.

Broadcast FM is designed so that the stations boost the treble audio frequencies and FM radios cut the treble down to normal and at the same time cuts down any hiss noise caused by a weak signal. C4 in my circuit does the treble boost. It is a different amount of boost for different countries. Cheap simple transmitters don't boost the treble so a radio receiving sounds from them cuts down the treble and it sounds like the treble is missing.

If I increase RB or decrease Rc, then I'll have more voltage gain!

No. Rb provides lots of DC negative feedback but not much AC negative feedback because of its high value and the low value of the mic's load/powering resistor. So increasing the value of Rb won't increase the voltage gain very much.
When Rc is decreased then the voltage gain is decreased.

u mean "motorboat"

 

[walid1]

Hi GURU

Quote from: awash on August 14, 2006, 10:16:14 AM
C1 and C3 = 330N I got only one 330N cab I replace the other one with like 470 n or should I have make the same capacitor for both please help me to solve shortage of one 330N cap.
They are just audio coupling capacitors. A higher value will pass lower audio frequencies. 330nF passes frequencies down to about 40Hz, 470nF will pass audio frequencies down to 28Hz and 220nF will pass audio frequencies down to 60Hz, with the impedances of the circuit. Their values don't need to be the same. Use 100nF if you don't like deep bass.

From the above values of C1 and C3 and the corresponding values of the freq they can pass, I deduce that u assumed or calculated the i/p impedance of the first stage and the second stage to be 12K.

For the 2nd stage: you did these calculations:

{The input impedance of the 2nd stage is determined when the emitter current is determined:
a) The emitter voltage is 2.25V (a guess).
b) Therefore the emitter current is 2.25V/220= 10.23mA.
c) The hFE is 230 so the base current is 44.5uA.
d) The voltage across the 47k base bias resistor is 44.5uA x 47k= 2.09V.
e) As a check, the voltage across the 47k resistor is 5V minus
[the emitter voltage of 2.25V plus the Vbe at 10mA of 0.72V]= 2.03V. Pretty close.
f) Part of the transistor's input impedance at 10mA= 500 ohms from its graph.
g) The input impedance of the emitter resistor times the hfe of 180 (from the graph)= 39.6k.
h) The total input impedance of the 2nd transistor = 40k.
i) The 47k bias resistor in parallel with the transistor's total input impedance= 21.6k.}

So u concluded that Zin = 21.6K
RC filter freq at that Zin = 1/(2 pi R C) = 22 Hz and not 40 as u said>

For the First stage u concluded that the i/p Z is about 10K and so the cutoff freq = 33Hz and not 40

NOTE: I am not looking for mistakes to say that this is a mistake, I feel the forest when I find a contradiction. I studying this discussion often without any comment and every time I understand more things

Thank u guru for everything u teached me

 

[audioguru2]

Hi Walid,
I said "330nF passes frequencies down to about 40Hz".
I did not say that 40Hz is the -3dB cutoff frequency.

I design many audio circuits to have a flat frequency response down to a certain frequency, then the cutoff frequency must be much lower, 1/3 to 1/10th of the frequency where the response begins to drop.

The input impedance of the first transistor stage is the transistor with its emitter resistor, in parallel with the two base bias resistors. The output impedance of the electret mic circuit is also in series with the coupling capacitor and further reduces the cutoff frequency. I measured my electret mic in parallel with the 10k resistor that powers it as 3.3k ohms.
So the cutoff frequency from the microphone to the first transistor stage is about 19.5Hz.

The output impedance of the first transistor is its collector resistor of 10k. It is in series with the coupling capacitor and futher reduces the cutoff frequency.So the cutoff frequency between the first and second transistor stages is 15.3Hz.

Both coupling capacitors reduce the output at low frequencies and their losses add. The output is fairly flat down to about 40Hz and is -6dB down (half the signal voltage or one-quarter of the power) at about 17.4hz.