This amplifier is based on the transistor 2SC1970 and 2N4427.The output power is about 1.3W and the input driving power is 30-50mW.You can use other transistor as 2SC1971 and get much more output power.1.3W will still get your RF signal quit far and I advice you to use a good 50 ohm resistor as dummy load.Make sure it can take up to 5-10W, else it will be a hot resistor.You MUST use an antenna or 50 ohm dummy resistor while testing else you burn up the transistor.
In all RF system and specially in RF amplifiers, it is very important to have a stable power supply and making sure you won’t get any RF out on the power line. The Capacitor C12 and C13 will stabilise the DC power supply. L1, C10, C11 and L3 with C8, C9 will also prevent RF from leaking out to the powerline and cause oscillation or disturbances. L1 and L3 should be ferrite chokes or inductance’s about 1 to 10 uH.
Transistor Q1 will act as a buffer amplifier, because I don’t want to load the previous stage to much.The input RF signal is passin C1 and F1 which is a small ferrite pearl where the wire just passing through.F1 with C2 will act as an impedance matching for Q1.F1 can be substituted with a coil as L4, but in my test I found that the ferrite pearls gave best performances.L2 is nit a critical component and any coil from 2-10uH will do the job. Q1 will amplify the input signal from 50mW to about 200mW.Q1 can amplify much more, but It doesn’t need to do that because 200mW is good for the final transistor.If you want higher power you can decrease the resistor R2.
If you look at Q2 you will also find a ferrite pearl F2 at the base to emitter. This ferrite pearls is to set the DC voltage to zero and be a high impedance for RF signals. I wounded the wire 4 times around this small ferrite pearl. You can substitute it with a coil of 1uH or more.C4, C5 and L4 forms an input matching unit for the transistor. Not much we can do about thatâ€¦At the output of the final transistor Q2 you will find 2 coils L5 and L6.
Together with C6 and C7, they form an impedance unit for the antenna and also for the transistor.