Broadband PA for television broadcasting. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Телевидение

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Broadband power amplifiers (NOIS) are used in ultra-wideband radar and communication systems, in the construction of tunable oscillators, in the creation of panoramic impedance meters and laser radiation modulators. The proposed NOISE is designed to work as part of a cable television system that provides local broadcasting of 5 ... 10 TV broadcasting channels in low-lying residential areas. This amplifier is a modification of the amplifiers described in J1]. Its advantages are ease of manufacture and adjustment, high gain, manual gain control and output power level indicator.

The amplifier (Fig. 1) contains five amplification stages on transistors VT2, VT4, VT6, VT8, VT10.

(click to enlarge)

All amplifier stages operate in class "A" mode with a fixed operating point and quiescent currents of transistors VT2, VT4, VT6. VT8, VT10 equal to 0,08; 0,12; 0.3; 0,4 and 0,4 A, respectively. Stabilization of the cascade quiescent currents is achieved through the use of an active collector thermal stabilization scheme [1, 2]. The quiescent currents themselves are set by selecting resistors R6. R11. R16, R21. R26. Reducing the resistance of these resistors leads to a decrease in quiescent currents, and vice versa.

In all stages of the amplifier, except for the output, reactive interstage corrective circuits of the third order are used [1, 3], where the reactive component of the input impedance of the transistor is used as one of the elements of the corrective circuit [4]. The output stage is made according to the scheme with voltage addition and provides summation in the load of signal voltages given by transistors VT8 and VT10 [1.5]. When assembling the amplifier, the length of the circuit connecting the VT8 collector to the VT10 emitter should be minimized. This is due to the fact that the presence of an inductive component in this circuit leads to incomplete addition of the signal voltages given off by the transistors.

The printed circuit board of the amplifier (Fig. 2) with dimensions of 180x80 mm is made of double-sided foil fiberglass with a thickness of 2 ... 2,5 mm. The location of the elements on the PA board is shown in Fig.3.

(click to enlarge)

The dotted lines indicate the places of metallization of the ends. Metallization is necessary to eliminate parasitic resonances and ground the desired sections of the printed circuit board.

The amplifier uses non-inductive capacitors of the K10-42 type in the high-frequency path and of the K10-17 and K50-29 types in the filter circuits.

The amplifier housing (Fig. 4 and 5) is made of duralumin and is installed on a small radiator during long-term operation. All amplifier transistors are attached to the base using thermally conductive paste. To improve the thermal contact of transistors VT2 and VT4 with the amplifier case, they are pressed to the base with a fiberglass plate (Fig. 4).

Setting up the amplifier consists of several stages. First, using resistors R6, R11. R16, R21, R26 set the quiescent currents of transistors VT2. VT4, VT6. VT8, VT10 To do this, these resistors are alternately replaced by potentiometers, the voltages across resistors R8, R13 are measured. R18, R22, R28 and the required quiescent currents of transistors VT2, VT4, VT6, VT8 are determined. VT10. All elements of the high-frequency path are soldered, with the exception of capacitors C12, C17 and C22.

Note. Capacitors C3C, C8 and C27 are not shown in Fig. 3, their role is played by metallized pads, to which the bases of transistors VT2, VT4 are soldered, and the PCB track leading to the NOISE output.

When the amplifier is turned on without capacitors C12, C17 and C22, its frequency response in the small signal mode is uniform up to frequencies of 400 ... 500 MHz with a further slow decline, amounting to about 800..4 dB at a frequency of 7 MHz. By connecting capacitors C12, C17, the frequency response is leveled in the frequency range of 500 ... 800 MHz.

Moving from the small signal mode to the limiting mode (output stages), by selecting the capacitance C22, the maximum output power of the amplifier is achieved in the operating frequency range.

The output capacitance of the VT10 transistor turns out to be connected in parallel with the load, which leads to a decrease in the maximum output power of the amplifier with increasing frequency. To eliminate this drawback, elements 18 and C27 are installed at the output of the amplifier, which together with the output capacitance VT10 form a low-pass filter [1]. Therefore, by varying the inductance L8 within small limits, the maximum output power of the amplifier is equalized in the operating frequency range. And finally, selecting the quiescent currents of transistors, it is necessary to find such current values ​​at which the amplifier delivers the required power to the load with a minimum power consumption. Manual gain control is implemented on the potentiometer R1 and provides a depth of regulation of 12 dB in the range of 400...800 MHz with a gradual increase in the level of regulation up to 30 dB with a decrease in the signal frequency to 45 MHz.

To indicate the output power level of the amplifier, a directional coupler (NO) of the incident wave is installed at its output. The directional coupler is made in the form of a piece of fiberglass about 4 cm long with one-sided metallization, placed above a strip of a long line going to the amplifier output. Together with a VD1 diode detector and an M4761-M1 pointer indicator, a directional coupler allows you to control the output power level in the operating frequency range with an error of 4..Ts.5 dB.

Literature

1. Titov A.A. Transistor power amplifiers MB and UHF. - M.: SOLON-PRESS, 2006. - 328 p.
2. Titov A.A. Calculation of the active collector thermal stabilization circuit and use in amplifiers with automatic control of the consumed current. - Electronic equipment. Ser. microwave technology. 2001. No. 2. P.26.
3. Titov A.A., Ilyushenko V.N. Circuit design of ultrawideband and bandpass power amplifiers. - M.: Radio engineering. 2007. - 208 p.
4. Petukhov V.M. Transistors and their foreign analogues: a Handbook. In 4 volumes. - M.: RadioSoft Publishing Company, 2000.
5. Babak L.I., Dyachko A.N. Powerful nanosecond video amplifier according to the circuit with voltage addition. - Devices and technique of experiment. 1981. No. 3, p. 127.

Author: A.Titov, Tomsk

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