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
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