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    This is a 1 watt fm amplifier with a good design that can be used to amplify a rf signal in the 88 – 108 MHz band. It is very sensitive if you use good rf power amplifier transistors, trimmers and coils. It has a power amplification factor of 9 to 12 dB (9 to 15 times). At an input power of 0.1W the output will be 1W.

    You must choose T1 depending on applied voltage. If you have a 12V power supply then use transistors like: 2N4427, KT920A, KT934A, KT904, BLX65, 2SC1970, BLY87. At 18 to 24V power supply you must use transistors like: 2N3866, 2N3553, KT922A, BLY91, BLX92A. You may use 2N2219 at 12V but you will get an output power of 0.4W maximum.

    Calibration of the 1 watt fm power amp

    Do not connect any RF source, just apply the power supply and measure the voltage at point 1. Adjust R3 until you measure 0.7V. Replace the antenna with 2 x 100 Ω 0.5W resistors in parallel at the RF output. Now connect the rf signal that you need to amplify and connect this RF Probe to the output.
    Slowly adjust C1 in order to get the highest voltage value on the rf probe. Now adjust R3 again to get 0.7 V at point 1. Now adjust C5 and C6 for maximum output voltage (must be between 12V to 18V).

    Check the temperature of T1′s heatsink, if it is ok turn off the power supply, disconnect the 2 resistors of 100 Ω and connect the antenna (keep the probe connected). Apply the power and now adjust again C1, C5 and C6 for maximum voltage indication on the probe.

    You may use an ampermeter in order to check the current flow through T1. This must not exceed 150mA at 12V and 100mA at 24V or the transistor will burn. L2 and L3 coils must have an angle of 90 degrees between them. Don’t use the 1W rf fm amplifier if you find that you tv set is jammed and the laws of your country does not allow the use of FM transmitters.

    1 Watt fm amplifier circuit diagram

    1 watt fm amplifier circuit schematic

    Component values
    R1 = 100Ω
    R2 = 2.2KΩ at 12V and 4.7kΩ at 24V
    R3 = 10KΩ
    R4 = 100Ω
    C1 = C5 = C6 = 10 – 60pF
    C2 = C4 = 1nF
    C3 = 10uF
    D1 = 1N4148
    L1 = 20 turns of 0.2mm EnCo* wire over R4
    L2 = 7 turns of 0.8mm EnCo* wire with 6mm diameter on air
    L3 = 4 turns of 0.8mm EnCo* wire with 7mm diameter on air
    T1 = 2N4427, KT920A, KT934A, KT904, BLX65, 2SC1970, BLY87 (2N2219, output of 0.4W) at 12V
    T1 = 2N3866, 2N3553, KT922A, BLY91, BLX92A at 24V
    * EnCo = enamelled copper

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    46 Responses to "1 Watt FM Amplifier Circuit"

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    1. Ashutosh says: on June 16, 2010 at 11:03 am

      Hello Sir,

      Thank you for your earlier reply

      I am working on making an FM transmitter for more than 500 m range for my college on which i want to send songs using my laptop and transmit them to a large distance so i think i will be in a need of stereo transmitter in order to play songs using my laptop and transmitting to a good distance ……

      Waiting for your reply

      • hi….
        asutosh ji…

        i also want to know that did u succed in ur work??? if yes.. please tell me also, how to do that… my college official’s are also interested in this, but they are asking me to give a whole idea on this… so will you please help me in this…

      • This circuit is very odd for using 2n4427 and saying its only produces 400mW output. The base voltage is unnecessary for a 2n4427 and will lower its potential gain, Finally if you use a 2n4427 you need to add a 1.2uh choke from the T1 base to ground.

        This schematic is more suited for the 2n3553 and using a 1watt drive to give you a very nice 4Watt amplifier for FM Broadband applications, A 2n3553 does require a base voltage and the schematic is correct.

        15volts is ideal for this particular design. You could even remove C1 for a fixed Ceramic capacitor, a 10pf should be fine.

        Try it my way and you will get better results from a genuine Motorola 2n4427 and don’t use these component D1,R1,R2,R3 & C2 unless you are using a 2n3553

        Junction

      • Finally just to mention that I would recommend using 2x ferrite beads either side of L1 to limit any amplification ” Take Off ” This will slightly lower the gain but is essential for stopping unwanted harmonics

        Junction

      • But as you can see, you can choose different transistor for T1. 2N2219 is the only transistor that delivers 400mW.
        Can you send me the link where it says that 2N3553 gives 4W rf power? And finally why do you think that using a choke in the base will give enough voltage to open the transistor?

      • I’m going to start to make one tomorrow for someone who need a 4watt band 1 TX on 62.5Mhz when its done I’ll take a picture with my digital camera and show you it working on the Power Meter.. 2n3553 with 1 Watt drive..4 watt output, also I don’t use free standing air coils, as Toko’s are the best and the L1 can be a 3.5uh Choke instead. 2n3553 will deliver 7 watts with 1 watt drive, but will only last seconds until it smokes and burns out.

        2n3553 has always been rated at 3watts, thus why we always run output transistor at half there dissipation.

        Junction

      • I can see where it mentions the 2n2219 as 400mW, When I first read this article I must have thought it said the 2n4427 was 400mW :)

        Anyway the 2n4427 unlike to 2n3553 does not require any voltage applied to the T1 base as the voltage from the “collector” goes down through the “emitter” will power the 2n4427 and then also allowing a small amount of voltage to pass back through the “Base” with the 1.2 uH Choke, The RF Current flows in the opposite direction to the voltage and back up out the “collector”

        like to old saying to describe how a Transistor works

        “Base is the Goes-Inter and the Collector Goes-Outta”

        “Emitter” = (Current) also Goes-Inter (Voltage) Goes-Outer”

        Junction

      • I see. Ok, now let’s wait and see the pics :)

      • Junction says: on March 1, 2012 at 2:10 pm

        Here’s the 1Watt driver I will be using that I made, Sorry I haven’t got time to finish it 2day as I’ve spent most of lunchtime finding a way of uploading a picture with a Link for you to see.

        More Coming later.

        If you can see on the picture on the right is a 2n4427 producing just over a 1watt identical to the way I said to make it earlier.

        http://ubuntuone.com/57z5ObGMkSVUkwiuOLDFMi

        Junction

      • Junction says: on March 1, 2012 at 2:47 pm

        Picture link isn’t great sorry, I used a Blackberry phone as it was the easiest way to transfer it.

      • What is the schematic you used for this one? Is it a complete transmitter, with oscillator and buffer?

      • Yes this is a complete Broadband FM Transmitter with OCS, Twin Buffer and driver stage, I will be publishing all my Schematics on my website which is currently under construction .
        It will be done very soon!!!

      • Ok, please tell us when your website is ready :)

      • Function: you seem to know your beans, just wondering why you’re spider wiring and not using proper pcb’s? I got a good link for cheap ones.
        Do you regularly make rigs?
        Do you pll your low band (1) exciters? Which device (pll) do you use.
        Get in touch.

      • Also: if you do bias a device into b or a/b operation (like junction says the 3553) then I would suggest d1 configured that way isn’t going to help you much.
        Also, as junction says for a 4427 you don’t need the base bias, and that device would probably be happier in class c with the base a ground DC potential.

      • radio head says: on September 4, 2012 at 6:58 pm

        Does the 2n3866 require a base voltage? I have several of them and I’m dieing to use them.

        Junction, you seem very knowledgeable, Thanks for your input.

      • DJ Adam Eve says: on September 5, 2012 at 12:12 pm

        That all depends on whether you want to operate it in class C or class B (or AB)?!
        The 3866 works pretty well either way. Do a little research on difference between the two and then decide. There are better (more specific) designs than this for the 3866 online, just search 2n3866 class c amp in google. What frequency range do you need? If it’s 100MHz then you could use this circuit as a basis: reduce the output coil by about 1 turn, swap the resistor/20 turn coils for a choke (e.g. ferrite bead with a few turns, if it’s easier for you). For the variable caps you can use phillips/vishay 65pf foil types. At the base (input) do away with the resistors/diodes, connect the base to ground through a low value resistor (or even a choke). If you ahve trouble tuning try an extra variable capacitor down to the 0v rail after C1. Alterantively you could match the base to your previous stage using an RF transformer, very common at VHF.

        Good luck!

      • radio head says: on September 5, 2012 at 8:45 pm

        So basicly thats a yes, get rid of r1 r2 r3 d1 and c2? The low value resistor going to the base, do you mean a 100ohm? I’m assuming I can use open air variable capacitors 0-65pf?

        Who would of thought there would be so many knowledgeable people in a comments board, this is awesome! thank you thank you thank you!

      • radio head says: on September 7, 2012 at 5:28 pm

        I didn’t realize there is such a big difference between brands.

        2n3866 Motorola – 5w out max
        2n3866 Philips – 3.5w out max
        2n3866 API – 1w out min (no max listed)

      • radio head says: on September 30, 2012 at 4:02 pm

        I just got this thing on the air. I’m not sure I’m running at 1watt, I haven’t built the rf meter yet. I’m using a .5ohm resister from the base to ground. I might try a choke instead and see if I get more gain. Maybe I’ll through in that cap too.

        - Thanks again from out here in radio land!

      • Jim Keith Jim Keith says: on September 30, 2012 at 4:29 pm
          member

        You can approximate the power out by multiplying the DC power in by approx. 0.7 (assuming 70% efficiency). Easy to do.

      • radio head says: on October 24, 2012 at 4:03 pm

        I think I’m running at 0.2watts.

        Can you run two of these amps in series?

    2. May i use SS8550 transistor for this schematic?

    3. I use 2N2219 transistor and get out about 1.3W.
      I drive it on 17V nad its take 350mA.

      Ineed use fan to cool it :D

      • P. Marian P. Marian says: on December 11, 2010 at 8:35 pm
          member

        2N2219 is a 400 mW transistor so there is no way that it will output 1.3 watt.

      • Try out, you get.

      • excuse me, 2n2219 can give a maximum power of 3watts, 1.3watts is only almost a half..

      • dear sir, to do transsmiting at 1.3W you must change the value of R1 to 10 ohom and the value of R2 to 500 ohom and the variable resistor shuld be 5k, the transistor i use is 2N4427 to get 1.3 watts but the problem is ,,5 minites only then proken …that is mean all kind of transistors that you use in this ciruet operate at list 4mW,exept the 2sc1971 will give us 1w maximume

      • See comments below about using 4427

      • ninzibarrientos says: on October 6, 2011 at 7:50 am

        it seems interesting.. do you have schematic diagram for this? or just simply use the diagram above ^

      • Yes, same schema is usable, real output power for 2N2219A is 400mW, after that, it start saturate and make harmonics and signal quality drops.

        Safe Max out is about 800mW for 2N2219 whit GOOD cooling!

        2N2219A gives 3Watts, but after 20sec, its explode!

        If you read somewhere about 3W 2N2219A, it’s fake!

        Here is real datasheet for it where you can find specs and collector power.
        http://www.pci-card.com/2n2219a.pdf

      • It’s even less than that. 800 mW is power dissipation and not output power. If a transistor could output the same amount of power it dissipates it would have a 100% efficiency, which is not possible.

    4. I am working on making an FM transmitter for more than 300 m range for my college on which i want to send songs using my laptop and transmit them to a large distance so i think i will be in a need of stereo transmitter in order to play songs using my laptop and transmitting to a good distance ……

      Waiting for your reply

    5. Hey I was wondering about L1 and R4. It says L1 over R4 is that litteral (wind magnet wire arround R4) or are you speaking of L1 in parallel with R4 as being “over”?
      Thank you for your time!

    6. Hi! I’ve got a kf622 and 2n3553, but I’d like to use that first one. Will it be a good replacement?

      Greets from Krakow!

    7. Finland says: on July 10, 2011 at 6:58 pm

      Now playing whit BLY87 transistor, it’s give about 7.6Watts whit 28V and 2.5Amp powersupply.

      Drive power is only 2Watts.
      Datasheet is here http://www.datasheetcatalog.org/datasheet/philips/BLY87C.pdf

      • I think the BLY87 transistor is recomended to be powered from a 12V supply.

      • Finland says: on July 17, 2011 at 10:42 am

        This text find in datasheet:
        RATINGS
        Limiting values in accordance with the Absolute Maximum System (IEC 134)
        Collector-emitter voltage (VBE = 0) peak value VCESM max. 36 V

        Put it’s work :D

    8. Use the cheap s8050 transistor to get around 300 meters range

    9. Rajtronix says: on May 21, 2012 at 6:51 pm

      As Junction’s saying, is it suitable circuit to use 2n3866 like 2n3553? If I use 24 voltage suitable transistors instead of 12V, will power dissipation be high or not?

      • 2N3866 at 100 MHz and 28V can deliver up to 1.5W and has a total gain of 100 times. That means that you need only 15mW to get a full 1.5W. The power dissipation depends on the transistor that you use, the input power, hFe and voltage of the power supply.
        R2 value must be 4.7K&Omega at 24V.

      • rajtronix says: on July 13, 2012 at 11:16 am

        …and what happens 2n3866 in 100MHz at 12 Voltage?

    10. i really want to know how to measure/calculate. uh,pf,NF,values so pleas i need help my email is (rehema12345@gmail.com)

    11. Bholenath waghmode says: on October 28, 2013 at 1:41 pm

      Not only we want circuit diagram but also whole design..can you give It ? can we make Fm transreceiver for 27 mhz fre…

    12. Now adjust C5 and C6 for maximum output voltage (must be between 12V to 18V).

      Measure the voltage where? From the probe, or point 1?

      Also, is a 1n4148 diode OK for the probe, it’s all I have.

      I have built this, and it makes almost no difference to the range. why?

      Thanks
      lewis

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