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walid

Audioguru FM Tx

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hi AN920 and Audioguru :)
hi AN920 i like your circuit :)
thanks :)
but still i have a question....
it will drift its frequency, can i use Audioguru`s circuit in the place of
your 100mW circuit.

and also i m thinking about using good RF transistors in the place of 2N3904.
i think it will do the job.
both of you plz think and any body who is reading this, and tell me about good transistors for using in the place of 2N3904.
thanks..

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The frequency of the oscillator changes when the supply voltage changes. I used a low-dropout 5V regulator so my circuit's oscillator 's frequency doesn't change as its 9V battery runs down to about 5.5V. But then the output level of my oscillator is much less than AN920's oscillator.

You can use RF transistors if you want.
You can use extra coils and capacitors for impedance matching and filtering if you want.
You can transmit with a high power output which might cause interference then get caught by the RF cops who send you to jail if you want. ;D

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Hi audioguru

after my last question, another friends take the subject to a more complex corner, so i back it to its simplist form again.

In our discussion about R1 (=10K) we conclude that most of mics drawing 0.5mA and have 1/2 vcc across them so R1 must be 5K or 4.7K, and you told me that your mic is from a telephone board and it draws only 1/4 mA so R1 =10k is ok
I understand you and all this is oK
I take these info and teach it to other friend.
today he asked me about this circuit:

fm_transmitter.gif


he asked me the following question:
why this circuit using 1k as R1 and not 9K ?
can u help me in answring this question
thank u

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Hi Walid,
An electret mic should have about 0.5mA through it. For its FET transistor to have some gain the load/powering resistance should be as high as possible but still allow a few volts across the mic. So with a 9V supply, a 0.5mA current and about 5V across the mic the powering resistor should be 10k.

But the mic in this simple circuit is driving the low input impedance of the transistor. Therefore the FET transistor in the mic won't have much more gain if R1 has a higher value. difference.

The values of R7 and R8 are too low and are creating an attenuator to the output of the 1st transistor.

I notice that the value of C4 is too high. It cuts high audio frequencies above 1.2kHz. An FM transmitter is suppost to boost audio frequencies above about 2kHz. 

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Hi guru
thank u very much

But the mic in this simple circuit is driving the low input impedance of the transistor. Therefore the FET transistor in the mic won't have much more gain if R1 has a higher value. difference.


lets discuss it:
1- is the mic is a constant current source that its current not effected by changes in R1 value. or this current increased by decreasing R1 value.
2- the sensivity of mic is increased when more current pass through it>
3-when mic circuit driving a next low impedance stage, R1 must be low value,why? beccause a low impedance stage wants more current than a high impedance one, so the mic's current must be greater.

What abou the circuit below, its Vcc is only 1.5 v!!!!

post-2833-14279143221904_thumb.gif

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1- is the mic is a constant current source that its current not effected by changes in R1 value.

Correct. The drain of a jFET is a high impedance current sink like the collector of a transistor. But the signal modulates its constant current.

or this current increased by decreasing R1 value?

The constant current becomes only a little higher when a lower resistance of R1 increases the voltage at the drain of the jFET.

2- the sensivity of mic is increased when more current pass through it?

No. The signal modulates its constant current. Therefore a high load resistance allows the jFET to amplify the signal more than a low load resistance, like a transistor. A jFET doesn't have as much voltage gain as a transistor so changing the load/powering resistance doesn't make a big difference.

3-when mic circuit driving a next low impedance stage, R1 must be low value,why?

No. The next low impedance stage is in parallel with R1 so increasing the value of R1 won't make much difference. R1 should be about 10k so that the mic's jFET can provide some voltage gain, and the input resistance of the next stage should be more than about 30k so that it isn't a low load resistance to the output of the jFET.

What abou the circuit below, its Vcc is only 1.5 v!!!!

If it works, it will have a very low RF output power so its range might be only across a big room or across the street.
Many electret mics don't work properly with such a low voltage and current.

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Hi audioguru

The constant current becomes only a little higher when a lower resistance of R1 increases the voltage at the drain of the jFET.

yes, it is very good point. i experiment with 3 mics, applying 9 volt dc to each and measure the drawn current with 1k then 10k ohm resistors.
i had the resuls shown in the table below.
look at it, changing R from 10K to 1K increasing the current only small amount, it is really a high impedance current sink.
Now I see by my eyes that decreasing R1 value will not increase the mic current segnificantly.
OK

Therefore a high load resistance allows the jFET to amplify the signal more than a low load resistance, like a transistor.

a high load resistance, u mean R1
you mean that R1 looks like RC with BJT, more RC value leads to more voltage gain.

A jFET doesn't have as much voltage gain as a transistor so changing the load/powering resistance doesn't make a big difference.

as a role always take R1 = 10K

The next low impedance stage is in parallel with R1 so increasing the value of R1 won't make much difference. R1 should be about 10k so that the mic's jFET can provide some voltage gain, and the input resistance of the next stage should be more than about 30k so that it isn't a low load resistance to the output of the jFET.

If the next stage i/p impedance  is in parallel with R1, and the next stage i/p impedance  is 30K, then 30//10 = less than 10
and 30// 5 = less than 5, so it is not true to say: the value of R1 won't make much difference.

** the last question
If we conclude that decresing R1 will not increase the mic current, and cosequently will not increase the sensivity of the mic, and it is prefered to make R1 big value (10K), then:
1) how the designers increase the sensivity of the mic, what they do?
2)Why someone see many different values for R1 in simple FM Txs.

guru, thank you very much for everything u learned me.
walid


post-2833-14279143222741_thumb.jpg

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please don't skip the above questions, i forgot to ask this important one:

what the optimum impedance value that mic like to connect with, that is R1// Zin of the next stage?

thank u again

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Hi Walid,
The jFET in an electret mic makes a pretty good constant current sink but it isn't extremely high impedance. I measured my mic with 10k as R1 and their total resistance was 3.33k ohms. Therefore the mic alone is 5k ohms.

The preamp transistor Q1 in that transmitter has a total input impedance of only 3.2k ohms. So increasing R1 to 10k for more gain from its jFET won't make much difference.
Increasing the total input impedance of Q1 won't make much difference.
R1 and the total input impedance of Q1 are the load for the jFET and both must be increased.

I think the total input impedance of the mic preamp should be 30k or more and R1 should  be 10k if the supply voltage allows it. Then the jFET in the mic can have a reasonable voltage gain.

post-1706-14279143223012_thumb.png

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Hi guru

thank you for your efforts to explain

First, you skip my last questions with a colored table.

So increasing R1 to 10k for more gain from its jFET won't make much difference.

with 1k the total = 762 ohm as u said
with 10k the total = 2424 ohm, big difference

I think the total input impedance of the mic preamp should be 30k or more and R1 should  be 10k if the supply voltage allows it.

I calculate Zin at the base the Ic=0.1 mA, Zin = 52K
to make it 30K, Ic must = 0.2 mA
If i do so, what are the side effects on the circuit

another general  equestion:
how the designers increase the sensivity of the mic, what they do?

thank u guru

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Hi Walid,
I think that R1 should be 10k so that the jFET in the electret mic gets about 0.5mA and a few volts.
I think that the input of the preamp shoud have a total resistance of 30k or more, so the values of the biasing resistors should be high and either have the transistor operate at a very low current to increase its internal Re or have an unbypassed emitter resistor, then its input resistance will be high enough.

If R1 is 10k and the total input resistance of the preamp transistor is 30k then the jFET has a total load of 7.5k which is much higher than only 762 ohms or your calculated 2424 ohms. Then it will have a reasonable amount of voltage gain.

The jFET in an electret mic has a certain value of transconductance which determines its voltage gain. Therefore if a jFET with a higher transconductance is used, the mic's sensitivity will be higher. Transconductance doesn't change very much from one jFET to another but the difference will be measurable. This jFET operates at about only 0.5mA and at a low voltage. jFETS that operate at 10mA and more and at higher voltages have a much higher transconductance.
If the jFET has too much gain then it will clip when the sound pressure level is high.

I have never made an electret mic but I think if its diaphraghm spacing is reduced then it will be more sensitive but then its max sound pressure level will be less. 

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Hi guru

I have never made an electret mic but I think if its diaphraghm spacing is reduced then it will be more sensitive but then its max sound pressure level will be less. 

hehehehe
I'm sure u have never made an electret mic.

somewhere i read that his mic is very sensitive and can pick up all voices inside the room, like u said about your FM Tx.
others said that you must talk near it....
from that, one deduce that it is posible to increase or decrease mic sinsivity by, say, increase Zin of the next stage, decrease R1...etec
I want your help in this point
thank you very much

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Hi Walid,
His electret mic has a total load of only 762 ohms so it won't have any voltage gain.
His preamp transistor has a total load of only 2k ohms so its voltage gain is only 35.
The preamp doesn't have any negative feedback so it will be very distorted.
The 0.1uF capacitor at the input of the oscillator cuts high audio frequencies and the de-emphasis treble cut network in an FM radio cuts the high audio frequencies more.
That circuit is a mess!

My circuit has 10k for R1 and the input resistance of the preamp is high so the jFET in the electret mic has good voltage gain.
My preamp has a high resistance load so it has high gain.
My preamp has negative feedback at lower audio frequencies so it has low distortion.
My RF filter at the input of the oscillator is only 470pF so it doesn't cut high audio frequencies.
My preamp has pre-emphasis (treble boost) so that a radio cuts the boost down to normal.
My FM transmitter is sensitive and sounds great.

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Hi guru
please read the following:

"The sensitivity of the WASP depends to a large extent on the value of the load resistor on the electret microphone. We have used 39k in the project, as the microphone we supply is a very sensitive type. If you wish to increase the sensitivity to super performance, the resistor can be decreased to 33k but don't go any lower otherwise the circuit may 'oscillate' or 'motor-boat.' "

The-Wasp_Circuit.gif

this from:  http://www.talkingelectronics.com/Projects/Wasp/Wasp-P1.html

please guru, comment.
thank u

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Hi Walid,
The jFET in an electret mic should have a higher voltage gain when its load/powering resistor value is higher, not when it is lower.

At first glance I thought they are talking backwards. Then I thought about the tiny amount of current the jFET in the mic is conducting and how small is its voltage.

If the mic has 1V across it then its current is (3V-1V)/39k= 51uA. Maybe the jFET is conducting as much as it can and its voltage is only 0.5V then its current is (3V-0.5V)/39k= 64.1uA. I think it is very distorted.

Yours and my electret mics don't operate with such a tiny current. Maybe their "very sensitive type" does.

I think its jFET is saturated when it has a 39k load/powering resistor and is not as saturated with 33k. It will probably work better with 3.3k.
They talked about "motorboating". It will occur when the mic gain is high and the battery has a high internal resistance. It is fixed by adding a 100uF bypass capacitor across the battery.

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Hi guru
for the circuit shown below, someone ask my the following question:
Can I increase the range of this transmitter by using power transistor like BLY89 and decreasing the RE?

I attached the datasheets of BLY89
i look at that datasheets and see it max freq = 175 MHz
i think it can handle the 88MHz FM freq
i know it is can not be used in stead of that small signal transistor
but i need your explanation to make the answer reasonable.
thank u guru

post-2833-14279143234253_thumb.jpg

bly89.pdf

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Hi Walid,
That RF power transistor has a typical power gain of only 6.6dB at 175MHz in that circuit. Then for 25W output into a 50 ohm antenna its output voltage is 35.4V RMS or 100V p-p, when powered from a 13.5V supply. It needs an input power of 5W at 175MHz.
It is used in our Linear FM 30Watt Amplifier project with a higher supply voltage. Its RF input power is 3W.
It is also used in our Linear 50Watt Amplifier project with a 12V supply. Its RF input power is 16W.

I have never seen a power RF oscillator. Usually a low power ocillator is used then it is amplified. Some FM transmitter circuits use many RF amplifier stages. Our 3W FM transmitter has 4 RF amplifier stages.

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Hi guru
I build this circuit:
The-Wasp_Circuit.gif
I followed exactly their instructions
I put parts very close to each other and very close to the board
it work fine
i built it in aboard with holes, and its area was 15 holes X 5 holes, it is very small and crowded with parts
i put it in my hand, so i toutch every part including antenna, and this did not affect the freq at all
I use a mic from an old tape recorder national panasonic, and i use the 39k resistor and the voice was fine
i omit only the variavle cap and depend on the coil length to slightly change the freq
the only problem is its range
in an open place its range about 50 meter only
can y help me to increase the range of this very good and very stable Tx.
thank you guru

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

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.
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.

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

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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
index.php?action=dlattach;topic=4741.0;i

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

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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"

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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

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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.

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