Broadband antenna amplifier for TV signals. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Antenna amplifiers

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For repetition by radio amateurs, a relatively simple antenna amplifier for television signals in the MB and UHF bands is proposed

The amplifier, the schematic diagram of which is shown in fig. 1, consists of two stages, assembled according to the OE scheme and covered by a common negative feedback circuit (NFB) for direct current through the resistor R2. OOS for alternating current is negligible.

The main amplification of the signal is provided by the first stage. The second stage, covered by parallel (through resistor R1) and serial (through resistors R4 and R5) OOS. serves to equalize the total amplitude-frequency characteristic (AFC) of the amplifier. Capacitors C4 and C5 in the emitter circuit of the transistor VT2 (at the specified capacitance values) correct the frequency response in the lower and middle frequencies of the passband.

The feature of the amplifier is that the resistor R1 performs three functions: it serves as a load for the transistor VT1. creates a parallel OOS of the transistor VT2 and together with the transistor VT1 forms a voltage divider to power the base of the transistor VT2. This allows a few decibels to increase the gain compared to traditional amplifiers [1. 2] and reduce the number of applied elements.

On fig. 2 shows the frequency response of three amplifiers assembled according to the considered scheme, but with different transistors: 1 - on KT3101A-2; 2 - on 2T657A-2; 3 - on KT368A. In the case of using KT368A transistors, a resistor R2 with a resistance of 4 Ohms and a capacitor C390 with a capacity of 4 pF are installed in the emitter circuit of the transistor VT30 (elements R5 and C5 are absent). The studies were carried out in a 50-ohm path using an XI-43 frequency response meter. The standing wave ratio (SWR) of the amplifiers on the input side was within 2...2.5.

An analysis of the characteristics shows that the maximum operating frequency of the amplifier reaches 0.2 ... 0.25 of the cutoff frequency of the transistors used. Therefore, in UHF amplifiers, it is desirable to use low-power microwave transistors designed to operate in cascades assembled according to the OE circuit (two emitter leads) and having a cutoff frequency of more than 3 GHz. Only for the MB range, RF transistors with a cutoff frequency of at least 1 GHz are suitable.

The amplifier uses MLT resistors and K10-17 capacitors. but KM and KD will do. All elements must have a minimum lead length.

The amplifier is assembled on a foil fiberglass board, the drawing of which is shown in Fig. 3. Resistor R1 is soldered to the terminals of the transistor VT2, placing it above it (if the transistor has lamellar terminals).

The voltage between the collector and emitter of transistors VT1 and VT2 is approximately 4 V. This is achieved by selecting the resistor R2. With powerful interfering signals at the input of the amplifier, filters are installed, the parameters of which are indicated in [2. 3].

Literature

1. Red E. Handbook on high-frequency technology: Schemes, blocks. 50 ohm technique: Per. with him. - M.: Communication. 1990.
2. Nechaev I. Combined amplifiers of TV signals - Radio. 1997. No. 10. p. 12. 13.
3. Nechaev I. Adders of television signals. - Radio. 1996. No. 11. p. 12. 13.

Author: N. Turkin, St. Petersburg

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