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1 watt fm amplifier circuit schematic

1 Watt FM Amplifier Circuit

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

54 Comments

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  • Michael

    The circuit as published is a class “A” linear amplifier, which means it is biased so that the collector is sitting at approx. 1/2 of the supply voltage at quiescence. This is very critical for linearity, such as amplitude modulation. For FM we can use a class “C” amplifier, which means that the device is turned off completely. To make this into a class “C” amplifier, one removes all of the base bias circuitry, then the device is exclusively suited for FM use.

  • Amal

    Anybody please tell how can you tweak the circuit to suit the 2.4Ghz frequency?

  • radioman

    i would make some chnages to that or you could have some nasty harmonics. it’s quite easy to do just add a couple of bits around the base of the transistor and a low pass filter.
    Also try something like a 2sc1947 transistor.

  • Daniel

    Use 2n5109. It is cheap, and I have got 40v on the output (antenna) with 10v power supply and 2 V RF input. Great circuit!

  • Matthew

    I’m looking at the frequency range specified for this, 88 – 108 MHz band.

    I’d like to build a small amplifier to put inline with my 2 Meter HAM radio, to give it a bit of a boost. (2 Meter : 144-148 MHz)
    Radio output power levels:
    – Low power: 1w
    – High power: 5w

    This circuit looks like it’ll give me the small boost I’m looking for, and it’ll be small enough to put into a bag and keep with my handheld system.

    Since I’m a neophyte with this level of electronics, is it possible that someone could respond and give me a bit of information/direction so that I could take it that direction?

  • Jonas

    and what kind of resistor do I need? is 5% too much?

  • Jonas

    hey, do i have to connect – of power source with – of Transmitter?

  • lewis

    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

  • Bholenath waghmode

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

  • issa ssebayinda

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

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