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walid

Audioguru FM Tx

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Guest Alun

I notice that this is addressed to audioguru, so why is it posted here? We do have a personal message service you know. ::)

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Hi Walid,
There isn't any text. It isn't even a project.

Someone posted a very simple FM  transmitter circuit and said it didn't work.
I looked at it and saw that its audio amplifier transistor was incorrectly biased so that it worked only when the battery was from 7V to 8V. I also explained that the circuit had other problems:
1) The audio amplifier transistor didn't have negative feedback so the received sound would be distorted.
2) The circuit didn't have pre-emphasis  (treble boost) like radio stations use, so the received sound would have no treble and sound something like a stereo with the treble tone control turned all the way down.
3) Its radio frequency would drift all over the place when anything got near its antenna because it was connected directly to its tuned circuit, and the transistor's capacitance changes when the battery voltage runs down.

He asked how to fix it so I decided to improve it since I haven't built a transmitter for about 40 years. I built his circuit and it didn't work with a new battery nor with an older battery. I improved it in 4 stages with parts I had. It works pretty well.

Do you have questions about it?

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Guest Alun

audioguru,
If you have any spare time you could make it in to a propper little project, contact mixos and get it published on this site, then you'd be even more of a legend. 8)

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Hi Alun,
I don't have a scanner nor digi-cam to post the circuit's Veroboard layout. My son lost or sold his digi-cam and I don't think a pic from my cell phone would be clear enough.

Hey, there's a digi-cam in the pile of "unrepairable" electronic stuff my son brought home. I fixed a portable "i-pod with a hard-drive" kinda thingy called a Creative Nomad, so maybe I can also fix the digi-cam. See, I always have something to work on.

Yesterday I tested an "unrepairable" wireless phone with digital answering machine. The phone doesn't work but I recorded a message to my dog and it drove her nuts when I played it back. It didn't sound clear even though I spoke directly into the little holes labeled "Mic". When I looked inside, the holes were only dents, and the actual mic was on the bottom which was against my carpet.

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To Alun, I mean it, I want all the readers and you to see and share the discussion with Mr. Audioguru.   

To Mr. Audioguru:

Audioguru: "Do you have questions about it?"
Walid: Sure I have, I have a dozen of questions, lets start:

FM Tx is one of the important subjects that interests me and many of people, so I start to disinter it hopping that finishing it with the ability to make our special designs or at least to discuss with others any design we may faced.

(1)(2)(3)(4).......(10000) I'd ask you about every element value (resistors caps ...others) in your circuit, so, why not to tell us a brief summary about these elements.
At the meantime I'll test the stages using Pspice to enter deeply into the design.
I know i add weight to you but hope you bear us.
thank you very much.
yours, walid.

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Hi all my friends,
I write these commands for Pspice sim.:

Audioguru FM Tx 1st stage (Audio preamp), V- divider configuration
VCC 1 0 5V
Vmic 8 0 SIN( 0 XXX XXX 0 0 0 )
Rmic 8 7 XXX
R2 1 2 150K
R3 2 0 39K
Q1 3 2 4 2N3804
R4 1 3 10K
R5 4 0 470
RL 6 0 XXX
C1 7 2 330N
C2 2 4 100P
C3 3 6 330N
C4 4 0 100N
.MODEL 2N3804 NPN (Is=6.734f Xti=3 Eg=1.11 Vaf=74.03 Bf=416.4 Ne=1.259
+ Ise=6.734 Ikf=66.78m Xtb=1.5 Br=.7371 Nc=2 Isc=0 Ikr=0 Rc=1
+ Cjc=3.638p Mjc=.3085 Vjc=.75 Fc=.5 Cje=4.493p Mje=.2593
+ Vje=.75 Tr=239.5n Tf=301.2p Itf=.4 Vtf=4 Xtf=2 Rb=10).TRAN 1ns 5us
.OP
.probe 
.end

As you can see there are some xxx inside these commands:
The first two xxx are the amplitude and frequency out of the mic if we consider the MIC as a signal generator with Vs and internal R.
The frequency, i think, is between 300 and 3400 Hz (the human voice) and the amplitude may be few tens of mV. I can't make a decision, please help me in that.
The 3d xxx represents the Mic impedance taking into acount the supply resistance R1 in the above schematic and that the mic have typically 5k ohm so if i still remember Rmic = R1//5k = 10k//5k = 3.3k, correct me.
The 4th xxx represents the load impedance, that is the i/p Z of the second stage in the figure above. please calculate it for me to complete the commands.
when I finish it i'll show you many thing about this subcircuit.

Note: the trasistor model is found in its datasheet.

thank you all
walid.

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Hi Walid,
1) You entered the wrong transistor number into Spice, it should be 2N3904.

2) The frequency response of the preamp is flat from 60Hz to about 600Hz and is boosted above about 1.5kHz due to the selectable pre-emphasis capacitor.

3) My electret microphone measured about 5k ohms so you are correct.

4) I made the mic preamp sensitive enough to pickup sounds in a room. Try 5mV as its output (I didn't measure it) and the transmitter should have full modulation of 75kHz.

5) 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.

6) C2 shorts the base to emitter at RF frequencies to prevent RF pickup at the mic from being rectified by the transistor which would upset its bias. ;D

Let's see what Spice says about my circuit!

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All the time I ask one question and with your reply it become 100000 questions, I don't know why.
Other friends in this community just look leaving me alone with you, also i don't know whyyyyyyyyyy!!!

In your last reply You said:

Audioguru:"a)The emitter voltage is 2.25V (a guess)."
Walid: I understand b) to i), but guessing VE=2.25V not, lets calculate it:
i/p loop: Vcc-R6 IB-VBE-VE=0, VBE = 0.72V as you said,
          5-4700 *IB-0.72-(hfe+1)*IB*R7=0
          IB = 77.4 uA
          IE = (hfe+1) * IB = 17.9mA, so,
          VE = 17.9m * 220 = 3.9 Volt and not 2.25V it is far from it.
What would you say?
What I want to say is that I can't build my solution to a guess when someone asking me the same qestion.

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Hi all my friends

Audioguru said: "4) I made the mic preamp sensitive enough to pickup sounds in a room. Try 5mV as its output (I didn't measure it) and the transmitter should have full modulation of 75kHz."
Walid: Two questions needing direct answers
1) How u made the mic preamp sensitive enough to pickup sounds in a room. I guess through R1, the resistor feeding Vcc to the MIC. You said once a day that the rule u use when choosing this R1 is that when Vcc = 9V, R1 = 9K, and When Vcc = 3V, R1 = 3K and so on.
  I think that R1 is not the only thing you take into account to to make the Mic is sensitive! What you do?
2) When we suppose that 5mV AC is out of Mic, the transmitter should have full modulation of 75kHz. What if this 1mV and what the advantages and disadvantages by this?
Thank you. 

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Hi Walid,
I calculated the circuit starting from my guess of 2.25V at the emitter and ended-up with 43.19uA through the 47k resistor which is supposed to be 44.48uA. Don't forget that the voltage across the 47k base bias resistor isn't 5V.

post-1706-14279142475885_thumb.png

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I'm sure u OK but:
1) tell me where the error in my calculations
2) What was the first the circuit with its resistors value known, or the whole circuit is not known.
I mean when you finished the 1st stage and move to this VCO stage, you sure put your assumptions, is VE=2.25 is one of them then u calculate the other values according to it.
why not to tell me the full story in numbers and I'll be very thankful for your favor.
Yours, Walid. 

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1) How u made the mic preamp sensitive enough to pickup sounds in a room. I guess through R1, the resistor feeding Vcc to the MIC.

An electret mic works well when operating at its spec'd 0.5mA. Half the supply voltage across the mic determines the resistor's value.
The circuit's gain requirement depends on the sensitivity of the mic, how much the Q1 transistor circuit loads-down the mic, the gain of the Q1 circuit, how much the 2nd transistor circuit loads-down the collector of Q1 and the sensitivity of modulation for the oscillator.

You said once a day that the rule u use when choosing this R1 is that when Vcc = 9V, R1 = 9K, and When Vcc = 3V, R1 = 3K and so on.

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Hi Walid,
Good point! I guess I don't follow my own advice. :-[
The FET in the mic isn't a perfect current source, its current drops a little when its voltage is reduced. My tiny electret mic came from a cell phone and has a current of only 0.25mA in my circuit. Most electret mics draw 0.5mA and R1 should be 4.7k. ;D

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So if I want to build this Tx using a mic from a radio/cassette, then i must put R1 = 4.7K and not 10K, or whay you say?

Try both. The 4.7k might be slightly better. You might not notice the difference, it will be very small. ;D

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I want now to analyze the 1st stage of this Tx according to the discussion in autir's BJT biasing formulas:
VCC=5V, RC=10K, RE=470, RB1=160K, RB2=30k

The voltage gain Av=RC/(re+RE), but since RE bypassed ==> Av=RC/re, where re = 25m/IC.
VB=5*RB2/(RB1+RB2) ==> VB=0.79v
assume VBE=0.65V, so, VE=Vb-VBE=0.14v ==> IC=0.14/470=0.3mA
If hfe= 230  ==> IB =0.3mA/230= 1.3uA

Idiv = 5/(160k+30k)=26.3uA and the ratio = 26.3/1.3 = 20 not about 10 so i'm wrong.there are 2 possibilities:
1) hfe is lower than 230, it must be 1/2 230 to make that ratio ok
2)VBE is less than 0.65V, i don't know if it may be 0.51v

If any of these is true, then we have IB=2.6uA and the ratio ia about 10:1.

Back to Av;
re=25/0.3= 83.3 ohm
Av=10000/83.3 =120 it is a big gain!!!
   
Zin = 160k//30k//(hfe*re)= 25.3k//(230*83.3)= about of 10.9K
Zout= RC = 10K.

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Hi Walid,
You should never assume Vbe in a transistor circuit that has a low emitter voltage. It is typically 0.62V on my datasheet.

I used the parts I had. I didn't have 320k for R2 nor 60k for R3 so the divider current to base current ratio is 20:1 instead of 10:1 which better controls the effect of different current gain in different transistors.

My Mod-3 version of the circuit had an extra emitter resistor to raise the emitter voltage so that different Vbe in different transistors wouldn't affect the collector voltage too much. But I didn't have space for it and a large bypass capacitor for it.

The pre-emphasis at 15kHz in North America is about 7.2 times the gain at lower frequencies. With its lower frequencies gain of about 18 then the max gain of the transistor of about 130 results in a pre-emphasis which just makes it and sounds perfect.

You have used hFE (DC gain) of 230 in your calculation for Zin instead of Hfe (AC gain) of 100 so the actual impedance is less. Since the pre-emphasis capacitor affects only high frequencies then the correct value for input impedance occurs only at about 30kHz and higher. At lower frequencies the input impedance is much higher. ;D

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Hi audioguru
because I have 4 figures, I'll make this question into two.

From What you said:"You should never assume Vbe in a transistor circuit that has a low emitter voltage. It is typically 0.62V on my datasheet."
I can understand:
1) Are u tell me to go to datasheets only when determining VBE and never to assume it?
You teach me before that it is not different in electronic calculations. What the problem if it 0.6, 0.7 or 0.8 volts if we know that resistors are 5% error.
2)From the beginning of this discussion till now you frequently said datasheets datasheets datasheets datasheets datasheets datasheets datasheets datasheets datasheets datasheets...
I finally look at these datasheets of FAIRCHILD 2n3904 transistor.
Fig.1 and 2 are the relation btn VBE and IC, one in saturation and the other in ON.
What I can understand is that u choose the 25 degree curve.
I dont know the difference btn them or from any of them u take your values.

post-2833-14279142478833_thumb.jpg

post-2833-1427914247901_thumb.jpg

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In fig3 seems to be that figure u use to determine hfe vs the operating IC.
Figure 4 is to determine hie.
tell me how exactly u use them, and if VCE or frequency is defferent from specified in the curve, how can I use them.
thanx.

post-2833-14279142479083_thumb.jpg

post-2833-14279142479163_thumb.jpg

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audioguru: "I didn't have 320k for R2 nor 60k for R3 so the divider current
            to base current ratio is 20:1 instead of 10:1 which better
            controls the effect of different current gain in different
            transistors."
walid: who said that u use R2=320k and R3=60k, and if u do so the divider
current to base current ratio will be 5:1 and not 20:1.
And Why say that, I can't understand u!

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Hi audioguru;

you said: My Mod-3 version of the circuit had an extra RE to raise VE so
          that different Vbe in different transistors wouldn't affect the
          VC too much.

The question: Are u mean a bypassed RE added in series to a not bypassed RE?

you said: But I didn't have space for it and a large bypass capacitor for it.
walid: I don't know what do u mean!

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Hi audioguru,

u said: The pre-emphasis at 15kHz in North America is about 7.2 times the
        gain at lower frequencies.With its lower frequencies gain of about
        18 then the max gain of the transistor of about 130 results in a
        pre-emphasis which just makes it and sounds perfect.

My questions:
(1) What the meaning of The pre-emphasis from the view of electronic?
(2) What kinds of audio at 15KHz?
(3) Why u relate the 15kHz to North America region only?

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