Encyclopedia of radio electronics and electrical engineering / radio reception

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As you know, simple direct gain receivers have low sensitivity and selectivity, especially in the MW range. You can increase them by applying quality factor multipliers.

The disadvantages of simple receivers are primarily due to the fact that reception is carried out, as a rule, on a magnetic antenna, which is also the only selective element of the direct amplification receiver. When winding with a single-core wire, the quality factor of the magnetic antenna is low, especially if the first stage of the radio frequency amplifier (URCH) is assembled on a bipolar transistor. Because of this, the bandwidth at the upper end of the MW range is 40 kHz or more.

One way to improve the sensitivity and selectivity of a simple direct gain receiver is to use a quality factor multiplier (Q-multiplier).

It will be possible to implement it by introducing positive feedback in the first stage of the RFP, i.e., by refining it, as proposed in [1]. You can also introduce a separate switchable device, described in [2], or an element with a negative differential resistance.

A more stable operation of the Q multiplier can be obtained by using an analogue of a tunnel diode as an element with a negative differential resistance, for example, a "lambda diode" [3, 4] on two field-effect transistors (Fig. 1). Here, the analog is connected in series with a variable resistor R1 and a coil L3. The resistor smoothly changes the total value of the negative resistance introduced into the circuit, and hence its quality factor.

Fig. 1

DC mode provides an integrated voltage regulator DA1. Voltage is supplied to the device by the switch SA1, combined with a resistor.

Tests of the device showed that in the high-frequency section of the MW range (1,2 ... 1,6 MHz), the multiplier allows you to increase the signal at the input of the URF by 10 ... 15 dB (3 ... 5 times) and narrow the bandwidth

several times - from 50 ... 60 to 10 kHz. True, due to the introduction of additional capacitance into the receiver circuit, simultaneously with a change in the quality factor, its tuning frequency also changed - about 10 kHz down in frequency.

The L3 coil contained 15...25% of the turns of the L1 coil and was placed near its grounded end. The transistors were selected with close values ​​of the initial current and cutoff voltage.

The resistor, transistors and capacitor must be placed in the receiver housing in close proximity to the magnetic antenna, the installation should be carried out by the hinged method with conductors of minimum length, the variable resistor housing should be connected to a common wire. It is permissible to use transistors with a lower initial current (letter indices A-D), but then it will be necessary to reduce the voltage applied to the analogue of the lambda diode so that it corresponds approximately to the middle of the section with negative differential resistance [4].

Setting up the device comes down to selecting the number of turns of the L3 coil in such a way that in the upper position of the resistor engine according to the scheme, generation occurs at the tuning frequency of the circuit, and disappears with a slight downward shift. You can check this with any receiver with a CB range.

In such a design, it is permissible to use an analog of a tunnel diode on bipolar transistors (Fig. 2). It works similarly, but due to the fact that the slope of the section with negative differential resistance is higher here, the variable resistor R5 (it can be combined with switch SA1) is applied with less resistance. The L3 coil contains fewer turns compared to the previous version of the multiplier - 5% of the number of turns of the L1 coil.

Fig. 2

The described design is reliable in operation and allows you to increase the signal level at the URF input up to 20 dB (10 times). But the bandwidth is reduced to 6 kHz, and the tuning frequency is shifted down by about 3%. The latter is due to the large intrinsic capacitances of the transistors. To reduce this effect, it is necessary to use transistors with a lower capacitance.

Literature

1. Nechaev I. Direct amplification receiver with variable bandwidth. - Radio, 1990, No. 2. p. 78, 79.
2. Abroad. A simple Q factor. - Radio, 1968, No. 3, p. 60.
3. Nechaev I. Lambda diode and its possibilities. - Radio, 1984, No. 2, p. 54.
4. Nechaev I. Lambda diode in amateur radio designs. - Radio, 1996, No. 5, p. 35-37.

Author: I. Nechaev, Kursk

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