مكبر الترددات العالية rf .. كتاب التلفزيون الملون والعادي

E-3 High Frequency RF Amplifier:

It amplifies the signal of the required channel only, thus raising the useful signal-to-noise ratio, which may appear on the screen in the form of rain or snow dots, and reduces the possibility of interference and the appearance of ghosting.

25 It is useful for isolating the local oscillator from the antenna, so that it interferes with neighboring devices. In old devices, three or five valves are used by the radiator or common base method, or a double triple valve connected by the (Cascode circuit) method, which combines good amplification and a small value of the noise in the valve (Fig. Integrated circuits.

The signal enters from the antenna to the feeding cable and then to the compatibility circuit at the voter entrance, so the impedance is transformed from (280) - 300 ohms balanced to (70 - 75 ohms) unbalanced in order to reduce the loss in the antenna and cables - the circuit includes isolation capacitors that isolate the device from the antenna, thus preventing electrification The antenna or the entry of an electric current is dangerous from the antenna of the device, while these capacitors allow the entry of the required frequencies.

The high-frequency amplifiers in television are tuned so that the carrier wave of the image and sound signal falls on the two edges of the response curve of the radio frequency amplifier (Fig. 7). In the color device, the luminance signal carries all the information so that the image appears in black and white. In the region (4) - (5) MHz, which is (4.43) in the PAL and SECAM systems, and 358 MHz in the NTSC system.

The luminance signal covers the entire frequency range of the TV channel, which is (7) MHz - the value of the additional color carrier frequency was chosen in the upper part of the channel frequency because the power of the visual signal or luminance information is the least possible in this part, while the lower part of the channel frequency It carries the largest capacity and information about the image. As in Figure (5-6).

5-4 The use of the transistor in the RF amplifier: The common emitter or common base method can be used, which is similar to the method
The grounded cathode or the grounded network in the valve amplifier circuit.

Grounded Radiator Amplifier Method


In this method Figure (5) - (11) the capacitive resistance between the base and the collector represents a path for the negative feedback that weakens the gain in the amplification because the difference of the collector signal page is 180 from the signal of the base entrance.

Therefore, a neutralizing capacitor must be placed between the secondary of the transformer and the base, so that the signal is compatible in the page and cancels the negative feedback via the capacitance between the base and the collector.

It should be noted that the AGC system has a positive voltage if the transistor is a type (NPN) and its voltage is negative if the transistor is a type (PNP). Also, the AGC voltage is more effective if it reaches the circuit of the RF amplifier than if it reaches the circuit
Intermediate frequency IF.

It is noted that the tuning of the RF circuit occurs by switching the primary coil of the transformer T through the channel switch, so it is tuned to the frequency carrying the channel.