Receiving antenna amplifier. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Antenna amplifiers

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As you know, it is desirable to use an antenna amplifier for receiving loop antennas ("flags"). In its manufacture, there are no problems with the noise level and amplification. This is easy to do. But such antennas require a very high common-mode attenuation coefficient (CMRR or, in English, CMRR - from the Common-Mode Rejection Ratio) from the amplifier. Otherwise, such interference can completely "spoil" the parameters of the antenna, which often happens in practice and serves as the basis for the opinion that such antennas work "so-so".

The easiest way to achieve the goal is to make the amplifier differential with a large CMRR. And you need just such an amplifier. The use of a balancing transformer with an unbalanced amplifier will not give a good result. Even the best such transformers (we are talking about high-impedance transformers) have a common-mode rejection ratio at frequencies of 1,8 and 3,5 MHz (and receiving antennas are needed mainly on amateur low-frequency bands) rarely exceeds 40 dB. And this is not enough - in real conditions, according to the author, a minimum of 50 ... 60 dB of attenuation of the common-mode component is required.

Such suppression can be provided by differential amplifiers. The easiest way to assemble them on integrated circuits. The idea to make a differential amplifier based on discrete elements is broken by the practical impossibility of selecting components with an accuracy of 0,1 ... 0,3%.

The usual implementation of the differential stage on an operational amplifier gives such suppression, but has the disadvantage that the input impedances of its inputs are different. From this, the antenna loses symmetry.

A completely satisfying solution is the use of a dedicated AD8129 differential amplifier. At frequencies below 4 MHz, it has a CMRR of 80 (!) dB, in addition, this microcircuit has two differential inputs with equal and very high (more than 4 MΩ) impedance. A separate plus is that the differential inputs are not used to set the gain, i.e. they do not need to be loaded with anything additional.

The circuit diagram of the amplifier is shown in fig. 1. When using an amplifier with a loop antenna, do not install VD1-VD4 varicaps and elements of their control circuit (R1, C1, R5, C9), and when using a ferrite magnetic antenna, do not install resistor R2.

Rice. 1. Schematic diagram of the amplifier (click to enlarge)

The voltage gain (in this case, it is approximately equal to 30) is set by the ratio of the resistance of the resistors R7 / R6. These resistors do not affect the input impedance of the working inputs (pins 1 and 8 of the DA1 chip).

This chip requires a bipolar power supply. Please note that there are two different "grounds" in the device, and they are not directly connected to each other. One of them is the common wire of the amplifier, and the other is the braid of the coaxial cable connecting the amplifier to the receiver (transceiver). Circuits L1C2C4 and L2C3C5 additionally filter the power. The voltage at the midpoint ("amplifier ground") sets the stabilizer DA2. Power is supplied to the amplifier via a coaxial cable. For additional protection against "dirt" that can be induced on the cable sheath, an isolation transformer T2 is installed. It is wound in two wires on a ferrite low-frequency magnetic core so that the inductance of its windings is not less than 1 mH.

The output of the amplifier through the resistor R8 is connected to an isolating RF transformer T1, with a small turn-to-turn capacitance and a ratio of the number of turns of the windings of 1:1. This transformer is needed for common mode decoupling between the common wire of the amplifier and the braid of the coaxial cable. Resistor R8 sets the output impedance of the amplifier (the DA1 chip itself has a low output impedance).

Diodes VD7 and VD8 (any high-frequency silicon) protect the input circuits of the receiver. The fact is that the DA1 chip can produce an output signal with an amplitude of up to 5 V, which is not acceptable for all receivers. Capacitor C7 is separating.

Elements L3, C10 share in the "neck" the power supply of the amplifier and the input of the receiver.

As already mentioned, pins 1 and 8 of the DA1 chip are high-resistance differential inputs. They need to solve three problems.

First, "bind" them in direct current to the common wire of the amplifier. This is done by resistors R3, R4. Their resistance is not very important (except in the case of working with a ferrite magnetic antenna, see below) - from 100 kΩ to 1 MΩ, but their identity is very important. These resistors must be selected using a digital multimeter with a difference of no more than 0,1% (even less is better). Otherwise, they "skew" the input of the amplifier with a corresponding reduction in CMRR.

Secondly, it is necessary to protect the inputs when the transmitter is operating. A pair of RF diodes VD5, VD6 copes with this.

Thirdly, connect the antenna and the elements it needs. It depends on which antenna will be used.

If it is a frame, such as a "flag", it is connected directly to the inputs. Additionally, a resistor R2 is installed with a resistance equal to the output resistance of the frame (usually several hundred ohms).

If this is a ferrite magnetic antenna, R2 is not needed, but the tuning varicaps VD1 -VD4 and the control circuit for them from the "shek" (R1R5C1C9) are installed. In addition, when working with a ferrite magnetic antenna (MA), you need to think about the resistance of resistors R3 and R4. They determine the quality factor of the antenna circuit (of course, in addition to the quality factor of the antenna coil itself). Depending on the inductance, quality factor MA and the desired bandwidth (without tuning), you should choose the values ​​​​of the resistors R3, R4.

On fig. 2 shows the spectrum in the 100 kHz band at the output of the described amplifier with a resistance of these resistors of 390 kOhm and a connected ferrite magnetic antenna wound on a rod with a diameter of 8 mm and a length of 100 mm with a magnetic permeability of 400. Reception occurs on a range of 160 meters. The antenna is located indoors, therefore, in addition to useful signals, a lot of interference is also visible.

Rice. 2. Spectrum in the 100 kHz band at the output of the amplifier

At the output, the level of airborne noise at the MA resonance frequency is 93 dBm (the vertical scale in the figure is in dBm), i.e. 5 μV, which approximately corresponds to the noise level of a full-size antenna. If you need to change the gain, this is done by selecting resistors R7 / R6. The AD8129 can provide up to 100x amplification on the low-frequency HF bands.

The use of an amplifier allows you to place the antenna away from local sources of interference and thereby improve the quality of reception.

Author: Igor Goncharenko (DL2KQ)

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