1.3W Power Amplifier (Part III)
This part III is basically an RF amplifier.
It will amplify the RF signal about 10 times 100mW in to 1.3W out.
All contribution to this page are most welcome!
Some amplifier is based around a RF transistor.
In my case I will use a common NPN RF transistor called 2SC1970.
You can also use other transistors like 2N4427 and some others.
Check datasheets to se how much power the transistor can handle.
2SC1970 will handle 1.3W and 2SC1971 up to 6W.
Most transistor has a power gain of 10dB meaning 10 times power amplification.
This is the reason why you must use several stages to achieve a strong transmitter.
In this construction we are getting higher power then ever and you must be careful with the transistor.
A 50 ohm dummy load MUST be used while testing, else the final transistor will break.
An antenna can be used but the antenna must be properly made else the transistor will break.
More about that later.
Hardware and schematic
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 that…
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.
Above you can download a (pdf) filer which is the black PCB. The PCB is mirrored because the
printed side side should be faced down the board during UV exposure.
To the right you will find a pic showing the assembly of all components on the same board.
This is how the real board should look when you are going to solder the components.
It is a board made for surface mounted components, so the cuppar is on the top layer.
I am sure you can still use hole mounted components as well.
Grey area is cuppar and each component is draw in different colours all to make it easy to
identify for you.
The scale of the pdf is 1:1 and the picture at right is magnified with 4 times.
Click on the pic to enlarge it.
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.
As you can see there are several variable capacitor and coils in this construction
and it must be so.
I advice you to put all in middle range before you apply power.
To tune this amplifier you can either use a wattmeter, SWR unit or you can
do as I do using a RF field meter.
Good grounding is very important in a RF system.
I use bottom layer as Ground and I connect it
with the top with wires to get a good grounding.
Make sure you have some cooling at the transistor.
In my case I put the 2SC1970 close to the PCB to handle the heat.
With good tuning the transistor shouldn't become hot.
Low pass filter
Some of you might want to add a low-pass filter at the output.
I have not added any extra low pass filter in my construction because I don't think it is needed.
You can easy find several homepages about low pass filter and how to build them.
I will not go future into this subject.
This part II describes the FM PLL controlled VCO unit.
Again, this is a strictly educational project explaining how a RF amplifier can be built.
According to the law it is legal to build them, but not to use them.
Next part will be the 7W (or maybe more) Power amplifier unit.
You can always mail me if there is anything unclear.
I wish you good luck with your projects and thanks for visit my page.