another FM transmitter project

Discussion in 'Electronic Projects Design/Ideas/Reviews' started by hannobisschoff@gmail.com, Apr 14, 2011.

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Look at all the other higher power circuits. The use a tuned circuit feeding the antenna while mine uses only a coupling capacitor.
They have much higher average current than mine and mine is close to the maximum allowed current (100mA) of a 2N43904 transistor.

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Look at all the other higher power transmitter circuits. They use a tuned LC circuit to match the output impedance to the antenna impedance. Mine uses only a simple coupling capacitor.

They use much more average current than a little 2n3904 can handle. The maximum allowed current of a 2N3904 is 200mA and it works poorly above 50mA.

Last edited: May 7, 2012
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3. NepalimanMember

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As your equation, I calculated with 9V and 230 hFE. It will be 365mW output. But again, why your transmitter can produce only 20mW at same 9V? Transistors max P. Diss. is 625mW, if it produces 365mW in 9V, will it melt too? Why?

4. DaveNew Member

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A 2N3904 has a maximum allowed dissipation of 625mW. At 365mW it is quite warm but not too hot. But if its hFE is at the maximum of 300 then its current is 53mA and its dissipation with a new 9V battery is 477mW which is hot but still not too hot.

I told you that my simple output coupling capacitor does not match the high output impedance of the RF amplifier to the low impedance of the antenna. I told you that higher power circuits match impedances with a tuned LC circuit and use a higher current.

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6. NepalimanMember

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don't laugh at me,
- Actually what is impedances and how impedances affacts in circuits?
- What is hFE?
- What is gain?

Last edited: May 7, 2012

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Impedance is AC resistance. The parallel tuned LC circuit is a short to DC but is a high impedance at the resonant frequency. The antenna is a fairly low impedance of 50 ohms to 75 ohms.

hFE is the DC current gain of a transistor. The input is base current and the output is collector current when the collector to emitter voltage is what is stated. At low frequencies the hfe (AC current gain) is almost the same as the hFE. Fairchild's datasheet for the 2N3904 has a graph of the hfe at radio frequencies. It is typically 16dB (5.7 times) at 100mHz.

Gain is the amount of amplification. There is voltage gain, current gain and power gain.

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8. NepalimanMember

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Um..m... 16dB.... 5.7 times... Don't laugh at me again,
What is dB? Does it mean- amplification capacity??? (5.7 times) :|

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9. trashMember

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No 16dB is 40 times.

The formula is _dB = 10 log (p1/p2)

where p1 and p2 = power 1 and power 2 respectively.
For example if p1 = 5mW and p2 = 10mW
5/10 = 0.5
LOG(0.5) = 0.3
10 x 0.3 = 3dB (gain)

if we reverse the values.
10/5 = 2
LOG(2) = -0.3
10 x 0.3 = -3dB (loss)

To convert dB back to gain.
16db / 10 = 1.6
10^(1.6) = 39.8 (close enough to 40 times)

These examples are for power.
If voltage (or current) is used, then the formula is dBV = 20 log (v1/v2)

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Yes, for POWER gain. But hfe is current gain so the formula is dBV = 20 log (v1/v2)
then the result is 16dB that is about 5.7 times.

Nepaliman,
You have a lot to learn about electronics. Every time I answer you I am not laughing, instead I am crying that you do not understand.
I am not a teacher.

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Sorry AG but your maths is a bit off.
16db represents a voltage or current ratio of 6.31 rather than 5.7.

I can only agree with you AG.
Nepaliman is trying to run before he can walk, he needs to learn the basics before getting involved with transmitters and antennas.

JimB

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Hee, hee.
I lost my slide rule so I guessed (11% wrong was not too bad).

I can only agree with you AG.
Nepaliman is trying to run before he can walk, he needs to learn the basics before getting involved with transmitters and antennas.

JimB[/QUOTE]

13. trashMember

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yep, sorry guys. I just a misread and thought we were talking about power, not the current gain.
Thanks audioguru for correcting it with the voltage/current formula.

14. NepalimanMember

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Hahaha but i am laughing at you cause of your anger at me. My academic field is 'Linguistics' and 'Education'. I'm going to university to study about John Deway, Rousseau, Noam Chomski, B.F. Skinner and some philosophies. But I am too much interested in electronics. Due to some personal reason and education system of my country I couldn't face electronics university. So started to involve in another field. But now a days, I am being an 'micro expert' leraning fron you. I will never stop questioning to you to make irritate hahaha....D:

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15. NepalimanMember

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Ya, I cannot understand such equation but I can make transmitter from predesigned circuit as well as expert can. Equation is necessary to design circuit but not necessary to assemble parts according to designed circuit. So I have made three miniature FM Transmitters of around 5mW powered, before 3 years. And have made sound activated switch, sound trigger detector and 7642 based AM receiver. And going to assemble 1 watt transmitter soon.

16. trashMember

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17. NepalimanMember

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I found a few 1976 datasheets for the very old BF199 but they do not open.
It is not in my Philips 1968 databook but the BF197 and BF200 are there.

19. NepalimanMember

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Ya, it's old because these transistors has been old. So, i am trying to renew it with new similar transistor, (like 2n3904) CAN'T I ?????????????

CEO-25V
CBO-40V
EBO-4V
CC-50mA
hFE-38 (min)
TPD.-350mW

CBV-60
CEV-40
EBV-6
CC-200mA
PCC-300mA
TPD.-500mW

20. NepalimanMember

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Is your 3904 transmitter able to drive a dipole antenna with 1 meter coax?

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