This circuit is an amplifier for small transmitters operating in the UHF band, specifically in the 450-800MHz range. The amplifier works in class A and we used the well-known Philips BLW33 transistor for its manufacture. The construction, although simple, can be said to be quite critical due to the high frequency singularity. To achieve this, a double-sided printed circuit is required, where the lower face is used for grounding with conventional materials. Is preferred not to use many surface support materials so you can make the amplifier as easily as possible. The amplification of the BLW33 transistor is 8.5dB at 860 MHz and 11.5dB at 470MHz.
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So if we give it as a driving power of -60dB, then its output will deliver a power of 120mW at 470MHz and 900mW at 860MHz.
Increasing the output power as a function of the input power does not mean that it will deliver at the same time a clear signal. The output signal from one point on will be distorted. This problem arises because of intermodulation derivatives.
The theoretical circuit
The transistor to work needs, like any construction, a stabilized power supply with very good stabilization, output voltage 25V and current capacity of at least 0.5A.
The most efficient point of class A transistors is the 24V power supply and the 33 Ohm collector current. Under these operating conditions it can pull out at least 1.2W. The transistor is available in ceramic shell SO122. If for any reason the transistor suffers some damage, you do not have to mistreat it, as Of course any other. Its base is made up of beryllium oxide (BeO), a substance that is extremely toxic and carcinogenic. The theoretical circuit is shown in the figure.
As we can see, there are three fixed surface mount capacitors C1, C4 and C8. C1 is to isolate the constant voltage that can be displayed by the drive circuit while the others are used for decoupling.
The adjustment of the transistor to the 50Ω output line is done with the help of the C10 and C11 variable capacitors, and the power lines at the output circuit respectively. Similarly, the input of the amplifier is adapted via the variable capacitor C2.
In the circuit is preferred the transfer lines instead of coils to make construction easier. The transmission lines have a more stable behavior than the coils. The transistor, in order to work in class A, needs an additional polarization circuit for its base.
This circuit consists of a transistor, T2 (BD136) and a voltage divider with the resistors R1, R7 and R8 polarizing the base of T2 and the triplet R6, which regulates the base current of the BLW33. The polarization circuit is shown in the figure above.
R1,R3,R5 = 10Ω
R2 = 33Ω
R4 = 220Ω
R6 = 220Ω trimmer
R7 = 150Ω
R8 = 1,8ΚΩ
C1,C4,C8 = 100pF
C2,C10,C11 = 1-4pF
C3 = 100nF
C5 = 470nF
C6,C7 = 6,8pF/63V
T1 = BLW33
T2 = BD136
D1 = BAW62
L1 = 1μH
The first image shows the theoretical circuit of the circuit and the printed double-sided PCB, in which you will place the components. The BLW33 transistor needs a small heatsink to cool. Coil H has a value of 1μH. For the polarization circuit, there is not PCB, since it is customary to be placed outside the amplifier circuit for protection purposes and to avoid any interference it may cause to the circuit.
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