RF Circuits

Hardware and schematic - Link
The schematic show you a RF amplifier with very high gain. The feeding RF signal enter C9 to transistor Q1 which has a self biased working point. The gain and working point is set with the two resistors R1 and R2. FB1, C5, C6 works as filter for rejecting RF to power line. Q1 has a gain about 15dBm. The output signal can be found a the collector which then enter a second amplifier stage Q2. This stage also has a self biased working point. The gain is set by the resistors R3//R4 and R5//R6.

Why do I have 2 parallel resistors like that?
It is because I want to be able to change the gain of the amplifier. On the PCB below you will see that I only have 2 pads for the resistors. When I want to resistors I solder the two resistors R5 and R6 on top of each other and the same with R3 and R4.

I advice you to start building without R3 and R5 and test the unit. If you want you can then add R3 and R5 later to obtain max gain of this stage. Q2 has a gain of 12 dBm. FB2, C7, C8 works as filter for rejecting RF to power line.

The last amplifier stage is based around the transistor 2N3866. This transistor has low input impedance. I match it by using 2 capacitors (C11, C12) and the inductor L1 to about 50 ohm. The transistor has an output impedance match, (C13, C14, and L3) to get best performance for an 50-75 ohm antenna.

# The inductor L1 is made by a wire 2 turns with 5mm diameter.
# The inductor L2 is made by a wire 7-9 turns with 6.5mm diameter.
# The inductor L3 is made by a wire 4 turns with 6.5mm diameter.

L4 is a Axial Lead Bead, which reject RF very good and has low resistance. You can use almost any choke or large inductor for L4, it is not a critical component. The FM transmitter require 2 AAA batteries and consume about 38mA. To get rid of batteries, I have added a voltage regulator IC1, to the PCB which deliver 3.3V to the FM transmitter unit.



Printed Circuit Boards (PCB) Download
Above you can download a (pdf) filer which is the black PCB. The PCB is mirrored because the printed 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 copper is on the top layer.

Grey area is copper 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.

Soldering and testing:
The soldering of this unit is pretty basic.
Connect all parts and make sure you have no soldering bridges on the PCB. soldering wick and rosin are good tools to have handy while soldering.
When testing the amplifier I advice you to use a 50 ohm dummy resistor as load or a proper antenna (more info about antenna below).
Make sure you use a non-inductive resistor. Before you switch on power you should set the variable capacitors C13 to max capacitance and C14 to min capacitance.

DC testing:
To make sure that both transistors Q1 and Q2 has good working point I advice you to measure the DC voltage at the junction R1 - FB1 and R4 - FB2.
I measured with NO input RF signal:

DC volt FB1 = 3.7 to 3.9 V
DC Volt FB2 = 7.1 to 7.4 V

Now your unit is ready to be tuned for best performance!

You may want to continue reading : FM Transmitter Antenna