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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING Economical direct amplification receiver. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / radio reception The unrelenting interest of novice radio amateurs in building simple direct gain receivers led the author to take up the development of another economical medium-wave receiver that works on low-impedance headphones. Naturally, the design also used its previous developments, in particular, a sensitive amplitude detector described in "Radio", 1994, No. 7, p. 10. It turned out that this detector allows you to quite simply introduce an automatic gain control (AGC) system into the radio frequency amplifier (URCH) of the receiver, and it only works with sufficiently strong signals, that is, it turned out to be "ARC with a delay." Reception is carried out on a magnetic antenna WA1 (see figure). The input circuit is formed by a coil and and a variable capacitor (KPI) C1. Since the URF receiver uses bipolar transistors that noticeably load the input circuit, a rarely used scheme for serial connection to the input circuit of the first stage, made on a transistor VT1 according to a common base (OB) circuit, is used. She also allowed to abandon the communication coil.
Let's take a closer look at the operation of the input circuit. As you know, the input resistance of the cascade with OB is small and amounts to tens, a maximum of hundreds of ohms, increasing with decreasing current through the transistor. By including this resistance r in series with the input circuit, we obtain its quality factor, approximately equal to X / r, where X is the reactance of the coil or circuit capacitor (they are equal at the resonant frequency). We neglect the own active resistance of the coil, since with high-quality manufacturing it is much less than r. When the circuit is tuned up in frequency, the reactance increases linearly and the quality factor Q increases in proportion to the frequency f. At the same time, the loop bandwidth is f/Q. Therefore, it must remain constant during range tuning, which eliminates the main disadvantage of direct amplification receivers - a very narrow bandwidth at the low-frequency end of the range and an unnecessarily wide one at the high-frequency one. Let's make an estimate. At a frequency of 500 kHz, the KPI capacitance is maximum (180 pF) and its reactance is 1.7 kOhm. Taking the input impedance of the stage together with the resistor R1 r - 50 Ohm connected in parallel, we get Q = 35 and a bandwidth of 15 kHz. At the high-frequency end of the range, the frequency triples (1500 kHz), the reactance - up to 5 kOhm, and the quality factor - up to 100. In this case, the bandwidth remains the same (15 kHz). For this to be true in reality, the intrinsic (constructive) quality factor of the circuit, which is practically determined by the quality factor of the coil, must be high, at least 250. resistor R1 with a capacitor, which is selected during adjustment. True, this is due to some loss of sensitivity at the high-frequency edge of the range. The URF of the receiver is two-stage, made on transistors VT1, VT2 of different structures, with a direct connection between the cascades by direct current. The main voltage amplification is provided by the first stage, the second one is switched on by an emitter follower and amplifies only the signal current. From the URF output, the signal is fed to an amplitude detector assembled on a VT3 transistor and VD1, VD2 diodes. In the absence of a signal, the voltage at the collector of transistor VT3 is approximately 1 V. at the base - 0.5 V. Diode VD1 is opened by a small base current, i.e., the operating point is in the section of the characteristic with maximum curvature corresponding to the threshold voltage of silicon semiconductor devices of about 0.5 V. Negative half-waves of the input signal cannot close the transistor because this is prevented by the increasing current through the diode VD 1. Positive half-waves open the transistor, and the voltage on its collector drops. The diode closes, and negative half-waves of the detected signal are emitted on the collector of the transistor. Through the diode VD2, the filter capacitor C5 is discharged by these half-waves, and the detected voltage appears at the output of the detector. Depending on the signal amplitude, this voltage decreases from 1.5 V (in the absence of a signal) to -0.5 V (maximum signal). From the detector output, the bias voltage is supplied through the VD3R4 circuit to the RF transistors. The VD3 diode "eats" about 0.5 V, therefore, at the maximum signal, the bias current decreases to almost 0 and the RF transistors close. This is how the AGC system works, which made it possible to abandon the use of a volume control in the receiver. Capacitors C2 and C3 filter the AGC voltage, closing the audio frequencies and passing only the DC component to the base. The necessary capacitance is provided by an oxide capacitor C3, but since it can have a noticeable resistance at high frequencies, a ceramic capacitor C2 is also needed. Both capacitors can be replaced with one ceramic capacitance of 0,15 ... 0,68 microfarads. In addition to reducing the gain, another favorable phenomenon occurs in this device: the input impedance of the first stage of the URF increases with strong signals, since it closes and the emitter current of the transistor VT1 decreases. This reduces the quality factor of the input circuit and expands its bandwidth, which is useful when receiving local stations - the reproduction of higher frequencies of the audio spectrum is improved. Now consider the issue of signal levels in various places of the radio frequency path of the receiver. A not too powerful medium wave radio station creates a field strength of about 10 mV / m at a distance of several hundred kilometers. The effective height of the magnetic antenna is about 0.01 m. As a result, a signal voltage of approximately 100 μV acts in the input circuit. It is it that will be applied to the emitter of the first URF transistor (the voltage on the coil L1 or on the capacitor C1 is Q times greater, but this fact is not used in this development). The voltage gain of the first transistor is about 100, and the second is close to unity. This means that the detector will receive a signal voltage of about 10 mV, which is quite enough for its normal operation. The amplitude of the detected AF signal in this case reaches tenths of a volt. For the operation of low-resistance phones, this voltage is quite enough, but the detector output current must be significantly increased. For this reason, the AF amplifier is made according to the scheme of a composite emitter follower on transistors VT3, VT4 of different structures. The required bias current is obtained not from the power source, but from the output of the detector, where there is a stable voltage of 1.5 V that does not depend much on the degree of battery discharge, which decreases slightly with increasing signal level. This purpose is served by the R7C6 chain. Resistor R7 affects the initial current of the AF amplifier transistors, and the capacitor C6 ensures the unhindered passage of the AF signals. In order for the receiver to not deteriorate when using highly discharged galvanic cells with increased internal resistance, the power supply is shunted by capacitors C7 and C8. The first provides low impedance at radio frequencies, and the second at audio frequencies. Headphones are plugged into connector X1. A little about the details. It is better to wind the magnetic antenna on a large core magnetic circuit, for example, with a diameter of 10 and a length of 200 mm, made of 400NN or 600NN ferrite. Coil L1 in this case contains 75 turns of LESHO wire (Litz wire) 21x0,07. The wire is wound coil to coil, in one layer on a waxed paper frame. You can use a ready-made medium-wave magnetic antenna from outdated transistor receivers. Usually it also has a coupling coil, which is best removed or connected in series with the circuit so that it does not create parasitic resonances at high frequencies, thereby opening the way for interference. KPE C1 with a solid dielectric is used from a children's amateur radio set. With equal success, any KPI from transistor receivers will do. If there is a KPI block, then it is advisable to connect both of its sections in parallel to expand the tuning range of the KPI with an air dielectric, no worse, but it is much larger. The transistors of the series indicated in the diagram can be with any letter indices. Diodes VD1-VD3 - any silicon, low-power high-frequency or pulse, for example, the KD520 - KD522 series. Resistors and capacitors - any type. Ceramic capacitors C2, C4, C6, C7 and C9 can have a capacitance from 0,01 to 0,15 microfarads, an oxide capacitor C3 - from 0,15 to 2 microfarads, C8 - from 20 microfarads and higher. Low-resistance headphones - TM-2, TM-4 or from imported players. In the latter version, a pair of stereo phones can be connected in parallel by connecting the corresponding contacts on the connector, or better, in series to increase their resistance, which allows you to "save" the UZCH current at equal volume. In this case, however, you will have to switch the outputs of one of the phones like this. so that they work in tandem. The receiver is mounted on a printed circuit board, on a perforated getinax plate or on a thick cardboard with holes for the leads of the parts. It is advisable not to place the detector parts in the immediate vicinity of the magnetic antenna and KPI in order to avoid parasitic couplings and self-excitation of the URF. The board will be placed in any case that is suitable in size. Setting up the receiver begins with setting the quiescent current of the UZCH (2 2 5 mA) with connected phones by selecting the resistor R7. The current is measured with a milliammeter connected in parallel to the open contacts of the switch SA1. At the time of measurement of the URF, it is advisable to "de-energize" by turning on the wire jumper between the base of the transistor VT1 and the common wire. Then the jumper is disconnected and the current of the URC is determined by increasing the current consumed (approximately 0,7 mA). More precisely, the URC mode is set by selecting the resistor R4, measuring the voltage at the emitter of the transistor VT2 - it should be about half the supply voltage. The last operation is to set the boundaries of the received range by selecting the number of turns and the position of the L1 coil on the magnetic antenna rod. It is convenient to navigate by the powerful Mayak radio station at a frequency of 549 kHz - it should be received at a KPI capacity close to maximum. A properly assembled and tuned receiver is quite economical, consuming about 3 mA of current from a battery of two "finger" cells (type 316 or AA) connected in series. Within the Moscow region, it provided reliable reception of almost all central radio stations broadcasting in the MW range. Author: V.Polyakov, Moscow
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