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FM transmitter (mod4)

Discussion in 'Electronic Projects Design/Ideas/Reviews' started by bananasiong, Oct 26, 2006.

  1. chingyg

    chingyg New Member

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    What you mean by adding a heat sink? the 2n3386 got a built in heat sink doesn't?
    Do you replace both Q2 and Q3 with it or just Q3?
     
    Last edited: Aug 14, 2009
  2. Hero999

    Hero999 Banned

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    Ah it'll go into thermal runaway.

    Adding a 68Ω emitter resistor should prevent this, at 50mA it should drop 3.4V which will take the voltage across it back down to below 9V.

    A more efficient solution would be to bias the transistor in class C so it doesn't dissipate quite so much power.
     
  3. audioguru

    audioguru Well-Known Member Most Helpful Member

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    A 2N2222A has different capacitance than a 2N3904 but should work. Try it.
    But the frequency might not be in the FM broadcast band without changing the coils.
     
  4. dave

    Dave New Member

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  5. transistor495

    transistor495 Member Forum Supporter

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    Like this I mean:[​IMG]
     
  6. chingyg

    chingyg New Member

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    Would you perfer transistor495's 2N3866?
    maybe I should stick to 2N3904?
     
  7. chingyg

    chingyg New Member

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    ok I am going to try to see if I could find one.

    No luck finding one. If I do use it, I'd probably just wrap it in thick foil or something.
     
    Last edited: Aug 14, 2009
  8. chingyg

    chingyg New Member

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    How would you do that?
     
  9. Hero999

    Hero999 Banned

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    Well removing R8 would do that but the duty cycle will be low.

    Here's how I'd do it.

    The transistor is baised just below the point where it turns on and stays biased in this manner regardless of the power supply voltage.

    You'll need more filtering on the output otherwise it will be littered with harmonics and interfere with other stations.
     

    Attached Files:

  10. chingyg

    chingyg New Member

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    ok, would this work?
    mod4.GIF
    Q3 is replaced by 2N3386, Q3 has 10R resistor on emitter.
    Q1 is omitted, audio signal is from PC.
     
  11. Hero999

    Hero999 Banned

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    That will work but you'll need to do the pre-emphasis on the computer.
     
  12. audioguru

    audioguru Well-Known Member Most Helpful Member

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    Q3 will operate in class-C if the 47k base resistor connects to ground instead of to +9V. I simulated it with a 12V supply and the output power became 0.09 times the power when it was the original circuit with a 9V supply.
     
  13. Hero999

    Hero999 Banned

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    That's true.

    I was wrong when I talked about removing R8, it's still required to pull the base down.

    That's because the duty cycle would be very low. My circuit will give a duty cycle of just under 50%.
     
  14. chingyg

    chingyg New Member

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    what about a amplifier like this
    chimage.php.png

    C1 68pF
    C2 10pF
    C3 omited
    L1 0.82uH
    L2 0.47uH
    RFC 10uH
    Q1 2N5109
     
  15. chingyg

    chingyg New Member

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    What simulator do you guys use?
     
  16. Hero999

    Hero999 Banned

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    Yes, I've seen that schematic on the Internet before.

    It will work but needs a fairly strong input signal.

    It could be connected to the output of the origional mod 4 transmitter.

    I use LTSpice; most people do around here.
     
    Last edited: Aug 14, 2009
  17. chingyg

    chingyg New Member

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    that what I was thinking, so input is where the aerial is. and can the output be a 70cm long of wire (aerial) or should I connect a cap in series with it? I must construct it on vero board and keep leads as short as I could. then should I place every thing I a box and cover it with foil that is connected to the ground of the power input? and can I just use regulated 12V for the amplifier (it is 15V on the schematics)?

    and for the 0.82uH and the 0.47uH inductor, do you use homemade aircore inductors. using the aircore inductor formula?
    LTSpice?
    I've got MultiSim 10 and proteus 7
     
    Last edited: Aug 14, 2009
  18. chingyg

    chingyg New Member

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    and I also run a coil calculator I got from the internet. It shows that the coil shown on the first page. the one with 9 turns, 5mm diameter and 10.16mm length has got a inductance of 0.23uH. Should I use that coil, or wind my own one that has got 0.1uH of inductance as specified on the diagram.

    0.23uH (5-35pF) -> 148.4 Mhz - 56Mhz
    0.1uH (5-35pF) -> 85Mhz - 225Mhz

    These are from the LC equation, I dont know if it is correct or not, since as audioguru said that the stray capacitance got a huge influence on the behavior of the circuit.
     
  19. Hero999

    Hero999 Banned

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    Yes, use the air coils and calculate their dimensions using the inductance formula.

    The frequencies will be slightly lower due to the parasitic capacitance.
     
  20. chingyg

    chingyg New Member

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    So I should use the ones audioguru is using, in his circuit? his design worked after all.

    BTW for those TO 39 transistor, Can I improvise the heat sink by wrap foil around it or glue a plate of aluminum on top?

    About that Class C amplifier, I posted earlier, RF out can just be an antenna right? no need for another capacitor?
     
  21. chingyg

    chingyg New Member

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    This is what I would use.
    mod4.GIF
     

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