Antenna amplifiers for broadband antennas. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Antenna amplifiers

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The article discusses in detail the device and principles of operation of the amplifying modules of broadband television antennas.

The amplifier in the television receiving antenna is designed primarily to increase the sensitivity limited by noise, and secondarily to compensate for the loss of the received signal in the coaxial cable. TVs themselves have a very large margin of their own amplification, i.e. have high sensitivity limited by gain. They have somewhat worse sensitivity, limited by synchronization. And finally, the lowest is the sensitivity limited by noise. Therefore, the factor that determines the long-range reception should be the level of the noise floor of the linear path, and not the gain.

The influence of noise is estimated by the signal-to-noise ratio, the minimum value of which is taken equal to 20. For TV sets of the third - fifth generation, the sensitivity limited by noise is 50-100 μV. However, at a signal-to-noise ratio (S/N) of 20, poor image quality is observed and only large details are intelligible. To obtain a good quality image, a useful signal should be applied to the TV input about 4 times larger, i.e. ensure the s / w ratio is about 80.

Currently used cables with a wave impedance of 75 Ohm, depending on the design and quality of the dielectric, have a specific attenuation of 0,07 - 0,18 dB / m in the meter and 0,25 - 0,6 dB / m in the decimeter wave range. With a cable length of 2 ... 4 m, the total attenuation can be 1,2 - 2,4 dB. In this regard, the amplifier should have a gain of about 3 dB for typical reception conditions. A margin of 12 ... 14 dB is added to it to amplify weak signals, which is necessary due to the low efficiency of broadband small-sized receiving antennas.

Any amplifier has its own noise, which amplifies along with the useful signal and degrades the signal-to-noise ratio. Therefore, the most important parameter of the amplifying element should be considered its noise figure K_ш.

For a unified assessment of the noise of a multistage path, there is an indicator of the reduced noise figure K_ш, which is equal to the output noise level divided by the total gain, i.e. To_ш = K_w.out / TO_У. Since the output noise level K_w.out depends to the greatest extent on the noise level of the first transistor, amplified by all subsequent stages, the noise of the remaining stages can be neglected. Then K_w.out= K_w1К_У, where K_w1 is the noise figure of the first transistor. Therefore, we get K_ш= K_w1, i.e. the reduced noise figure of the amplifying part is mainly determined by the noise figure of the first transistor. This leads to the conclusion that the use of the active part can give a positive result when the noise figure of the first transistor of the amplifier is less than the noise figure of the first stage of the TV. The noise figure also depends on the quality of matching at the input of the amplifier and the operating mode of the first transistor.

The frequency range of the amplifier should provide signal amplification in the frequency band of broadcast television f = 48-790 MHz. To increase the dynamic range, the amplifier must have negative feedback.

Figure 1 shows a diagram of a single-stage amplifier with a transformer input and an open asymmetric output, which makes it possible to remotely power the amplifying module via a signal cable. This single-stage circuit is highly stable and easy to cascade.

Rice. 1. Single stage amplifier

The antenna excitation points are connected directly to the balance section of the transformer Tr1, which provides broadband matching of the antenna input with the input of the amplifying stage. The amplifying element VT1 is connected according to the scheme with a common emitter. This allows you to realize more bandpass gain and better noise properties of the circuit compared to other switching options. The effects of the influence of the cutoff frequency of the transistor on the slope on the change in the gain and input resistance in the operating frequency range are compensated by using combined frequency-dependent feedbacks of parallel and series type in the circuit. Parallel feedback is made on the elements R3, C1, L1. Resistor R3 determines the matching of the amplifying module in the connecting joints in the meter and lower part of the decimeter range.

In the upper part of the operating range, where the gain falls off by 2-4 dB, the inductance L1 weakens the effect of this feedback, equalizing the amplitude-frequency characteristic (AFC). Capacitor C1 provides decoupling of the feedback circuit from the power supply circuit and at the same time forms a low-frequency cutoff of the transfer characteristic of the device. Circuit R4, C3 is an element of series current feedback that determines the main parameters of the cascade in a small-signal mode: resistor R4 sets the nominal gain of the cascade, and setting C3 controls the rise in the frequency response in the upper part of the operating range. The specified parameters of the dynamic range are provided by the choice of the type of transistor and its mode of operation. In the presented circuit, the DC cascade operation mode is set by R4 together with the elements of the base divider R1 and R2. Capacitor C2 shunts R1 and provides asymmetric connection of Tr1 to the module circuit. The amplifying module, implemented on a third-generation medium-power transistor, provides a gain of 15 dB in the frequency band of 40-800 MHz, the noise figure of the device does not exceed 3,5 dB, and the dynamic range for television signals is 75 dB. Reducing the noise figure and realizing a greater linearity of the device is possible when complex active elements with cascode switching are used in the circuit or when switching to two-transistor cascades.

Two schematic diagrams, which are a two-stage aperiodic amplifier based on microwave bipolar transistors, connected according to the OE circuit, are shown in Fig. 2.

The amplifier in fig. 2a contains two broadband amplification stages on transistors VT1 and VT2. The signal from the antenna itself through a matching transformer (not shown in the diagram) and capacitor C1 enters the base of the transistor VT1, which is connected according to the OE circuit.

Rice. 2a. Two Stage Amplifier

The operating point of the transistor is set by the bias voltage determined by the resistor R1. The negative voltage feedback (NFB) acting in this case linearizes the characteristic of the first stage, stabilizes the position of the operating point, but reduces its gain. There is no frequency correction in the first stage. The second stage is also made on a transistor according to the scheme with OE and OOS for voltage through resistors R2 and R3, but it also has a current OOS through resistor R4 in the emitter circuit, which stabilizes the mode of transistor VT2. To avoid a large gain loss, resistor R4 is shunted in alternating current by capacitor C3, the capacitance of which is chosen relatively small (10 pF). As a result, at the lower frequencies of the range, the capacitance of the capacitor C3 turns out to be significant and the resulting AC feedback reduces the gain, thereby correcting the frequency response of the amplifier. The disadvantages of such an amplifier circuit include passive losses in the output circuit on the resistor R5, which is connected so that both the constant supply voltage and the signal voltage drop across it.

The amplifier in Fig. 2b, which also has two cascades assembled according to the OE scheme. It differs from the previous amplifier in better decoupling of the supply circuits through L1 C6, R5 C4 L-shaped filters and increased gain due to the presence of capacitor C5 in the OOS circuit (R3 C5 R6) of the second stage and the transition capacitor C7 at the output.

Rice. 2b. Two Stage Amplifier

In cascades on transistors connected according to the OE circuit, the influence of internal connections and capacitances of transistor junctions is the greatest. It manifests itself in the limitation of the bandwidth and the tendency of the amplifier to self-excitation, the probability of which is greater, the higher the gain. To evaluate it, the concept of the stability threshold is known - the limiting value of the gain, above which the amplifier turns into a generator. As measures to improve stability, it is possible to propose the inclusion of transistors in a cascode circuit with OE-OB.

The cascode connection of transistors VT1 and VT2 (Fig. 3) makes it possible to realize good unidirectionality and obtain a large bandwidth of the amplifying module. This makes it possible to abandon the use of signal feedback, stabilizing and correcting the amplitude-frequency characteristic, as well as the input and output impedances of the link. Here, the transmission coefficient and the connecting parameters of the circuit are set in mode. To reduce the influence of parasitic inductances of the common terminals, which reduce the gain of the cascade at high frequencies, the emitter terminals of the input transistors are connected directly to the case, and the operation mode is stabilized by a fixed base current. The high-frequency cutoff is controlled by the inductance L1 included in the collector circuit of the terminal transistor.

Rice. 3. VT Cascode Amplifier

Range adjustment and stabilization of the output impedance of the module are performed by resistive-capacitive circuits. The cascode scheme, when implementing the optimal operating mode of the transistors, makes it possible to obtain reduced intermodulation distortion.

In the presence of an MV-UHF antenna, which is structurally made in the form of two electrically unconnected antennas, it is possible to use an amplifying module that amplifies the signals from each of them, summarizes and transmits to the TV receiver via one cable. The amplifier is powered by the same cable. A schematic diagram of such an amplifying module is shown in Figure 4. It contains two independent amplification channels. The signal from the MV antenna is fed to the contacts XT1, XT2, to which the input stage of the MV channel is connected, assembled on transistors VT1, VT2 according to the differential amplifier circuit. This allows you to get good matching with high-impedance antennas, as well as suppress common mode noise.

Rice. 4. Amplifier with separate inputs MV and DTSV

Coils L1, L2 are installed at the input of the cascade, which eliminate the accumulation of static electricity charges on some antennas, as well as diodes VD1 - VD4, which protect the amplifier from lightning discharges. An additional amplifying stage is assembled on the VT5 transistor. The channel transfer coefficient is 15…20 dB. MV signals pass to the cable through a L6 C19 L7 low-pass filter with a cutoff frequency of 250 MHz. Through the same filter and inductor L5, the channel receives the supply voltage from the drop cable. In addition, the filter does not pass LDC signals.

The UHF amplification channel consists of two identical amplifying stages connected in series. The first of them is assembled on transistors VT3, VT4 according to a galvanic-coupled circuit, due to which there is an automatic exit to the specified operating mode and its maintenance when the temperature and supply voltage change. At the input of the cascade, a C1 L3 C2 high-pass filter with a cutoff frequency of 450 MHz is installed, which suppresses low-frequency signals and noise. A similar high-pass filter C21 L9 C22 at the output of the second stage passes the UHF signals and does not pass the VHF signals. Consequently, the filters at the outputs of the channels decouple them mutually. Coil L4 provides coordination between the cascades of the UHF channel and correction of the total frequency response. The total channel gain is 32…36 dB. The UHF channel is fed through the L8 inductor from the drop cable. The amplifying module is powered by 12 V at a current of at least 70 mA.

It is important to note that modules with a cascade-chain structure usually provide a greater linearity of the transfer characteristic, which is associated, first of all, with the possibility of separate tuning of the cascades (optimization of the transfer characteristic, matching modes and dynamic range parameters), in which the overload thresholds increase relay-race and proportionally increase in the transfer coefficient.

A comparative analysis of technical solutions and functional and energy characteristics of the modules shows that it is expedient to choose schemes with chain-connected cascades with combined frequency-dependent feedbacks as basic structures when designing amplifying modules for active broadband antennas. Moreover, in the first stage, the feedback depth is selected based on the required value of the noise figure and the stability of the connecting impedance. The mode of operation and the type of transistor of the output stage are mainly determined by the required load capacity of the module.

Publication: library.espec.ws

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