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 Home » Radio Frequency » UHF TV Linear Amplifier (450-800MHz)
UHF TV Linear Amplifier (450-800MHz)    
This structure is an amplifier RF, covering the range of UHF and intended for small TV transmitters, working in the channel UHF. The amplifier can enhance any video signal to the region UHF (450-800MHz). 
The circuit operates in class A and uses the known transistor BFQ68. The construction, though simple, we can say that is quite critical, due to the nature of the high frequencies. The construction is a double-sided printed circuit with conventional materials, although we used only two capacitors surface mount. We preferred not to use it anywhere a surface support, to enable easier the reader to make the construction. The strengthening of the transistor is 13dB, for an operating frequency of 800MHz, voltage emitter and collector Vce = 15n and current func tioning 240mA. If you give him a driving force 100mW, then the output will providing a power of about 2W. 

If you give him 200mA, it will take about 3W. The increase in output does not mean that will devote time and clean video signal Video. The output will be distorted, full of lines, no color and unclear.
This problem is created because of intermodulation derivatives of the transistor and overdriving. The transistor to work, needs, and any construction, a stabilized power supply with very good stabilizing output voltage of 25V and can supply at least 3A.
Transistor BFQ68 manufactured by PHILIPS intended for applications amplifiers central television systems, average power. At the same time, it is very likely video output stages for low-power television transmitters or driving. The most effective point of the transistor class A is the 15v supply voltage and collector current 250mA.
The transistor is available in ceramic shell SOT-122. If for any reason, the transistor is wrong with some damage, should not be abused, like any other. The base, consisting of beryllium oxide (BeO), a substance extremely toxic and carcinogenic. The capacitor C1 is used to isolate input voltage from the driving stage, and there are still two variables capacitors, C2-C3 (4-30pF) to coordinate the input circuit of the transistor.
The adaptation of the transistors in row output of the 50R, made with the help of variables C4 and C5 and inductor L5. In the circuit we have chosen to build coils to make it easier to coordinate the strengthening step. The transistor, to work on class A, need an extra circuit for biasing the base.
The polarization voltage is governed by the potentiometer, which has a value of 1 KO. In the circuit shown the necessary two-sided printed circuit, which will place the materials. The photograph of the construction will help you in how to mount the material on the circuit, since the placement of the material is the "air".
The transistors have a large heat sink to cool sufficiently. The circuit of the amplifier to stabilize operating point, needs a panel, consisting of the resistors R1 = 560 R2 = 220 R3 = 820.
To network. This, together with the decoupling capacitors, to stabilize the operating point of the transistor to 0,75 V. Inductor L6 is Ten spirals wire 0,3 mm, wrapped in diameter 4mm, and the two capacitors C6 and C7 are surface mount, without this means that their position can not be used classical capacitors, the same price of course.
The coils L1 and L4 consists of a coil, wrapped in diameter 4mm, with wire 1 mm, silver. To L5 consists of 2 spirals, wrapped in diameter 4mm.

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