A simple amplifier for a free-energy powered receiver. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Beginner radio amateur

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Sometimes a simple outdoor antenna with a length of 10 m or more allows you to get enough signal energy received from powerful radio stations to operate the loudspeaker. In this case, it is possible to amplify the audio signal using the uselessly wasted energy of the AM radio carrier.

An amplifier realizing this possibility was described in [1]. However, it contains four transistors and three rather bulky low-frequency transformers, which in some cases can make it difficult to manufacture. Experimenting with similar devices, the author managed to develop a simpler single-transistor amplifier, only slightly inferior in efficiency to the one mentioned.

The scheme of the detector receiver with the proposed amplifier is shown in the figure.

The oscillatory circuit is formed by the capacitance of the antenna and the inductance of the coil L1. It tunes in resonance to the frequency of the received radio station. Capacitor C1 matches the input impedance of the detector, made on germanium diodes VD1, VD2, with the resistance of the circuit, and capacitor C2 smoothes the ripple of the detected voltage. Actually the receiver on it also comes to an end. His work is described in more detail in [2].

The output voltage of the receiver, whose oscillogram is also shown in the figure, has a certain constant level U0, on which 3-hour oscillations with amplitude Um are superimposed. The Um/U0 ratio is exactly equal to the signal modulation factor m. As you know, the DC power is found as U02/R, and the AC power as Um2/2R.

It is easy to find that the 3H oscillation power is only m2 /2 of the direct current power in the detector load. By the way, this exactly corresponds to the ratio of sideband power to carrier power in the AM signal spectrum. The modulation coefficient rarely reaches unity and, due to quiet sounds and pauses, is much smaller. Even with an average modulation coefficient of 30% (m = 0,3), normalized in GOST, the power of the detected 3H signal is only 4,5% of the power of the detected carrier. From this it is clear that by using the direct current power in the detector load to power the amplifier, one can significantly increase the power of the 3H oscillations.

The amplifier itself is assembled on a transistor VT1 and a diode VD3. Strange as it may seem, this is a push-pull amplifier operating in class AB mode. Since the principle of its operation is original and unusual, we will consider it in more detail.

In the carrier mode, when there is no modulation, a high-capacity oxide capacitor C4 (accumulative) is charged through the diode VD3 and the primary winding of the transformer T1 almost to the voltage U0. In this case, only a small quiescent current flows through the transistor, determined by the large resistance of the bias resistor R1 in the base circuit. The quiescent current, of course, consumes the charge accumulated in the capacitor C4, but this charge is continuously replenished through the diode VD3.

With a positive modulation half-wave, the voltage at the output of the detector increases. This change is transmitted through the capacitor C3 to the base of the transistor, which closes. At the same time, the VD3 diode is fully opened and the voltage of the positive modulation half-wave is applied to the primary winding of the output transformer T1. At the same time, the charge of the capacitor C4 is intensively replenished.

During the negative half-wave of modulation, the voltage at the output of the detector is lower than on the capacitor C4, and the diode VD3 closes. Entering through the capacitor C3 to the base of the transistor, the negative half-wave 3H opens it and the charge accumulated by the capacitor C4 flows through the transistor and the primary winding of the transformer T1. Since, with respect to the output signal of the detector, the transistor is switched on by an emitter follower, the distortion of the 3H waveform is small.

From the description of the principle of operation of the amplifier, it is easy to see that the detector of the receiver, and, consequently, its oscillatory circuit, is heavily loaded only with positive half-waves of the 3H signal, when the amplitude of the RF oscillations is large. With negative half-waves, the load on the circuit is significantly reduced. This leads to "demodulation" of AM oscillations in the circuit, which favorably affects the operation of the detector itself and, to some extent, increases the selectivity of the receiver. A stronger connection between the detector and the circuit becomes optimal than in [2] - this is achieved by simply increasing the capacitance of the tuning capacitor C1.

Two words about details. The coil of the DV or SV range of any magnetic antenna will successfully serve as a contour coil. If the inductance of one coil is not enough to tune into the longest wavelength stations, two are switched on in series. Tune the receiver at the station by moving the ferrite rod of the antenna. Since it is used only for tuning, and not for receiving, the rod can also be short - 40 ... 60 mm.

The tuning capacitor C1 is KPK-2, but since its capacitance is not too critical, a constant capacitor can also be successfully used by selecting its capacitance according to the maximum reception volume (remembering to adjust the circuit when changing the capacitor). The rest of the parts can be of any type, denominations of 1,5 ... 2 times in any direction have little effect on the operation of the receiver. Diodes - any low-power high-frequency germanium, for example, D18, it is also desirable to use a germanium transistor of the MP16, MP39-MP42 series with any letter index. The output transformer is suitable from any broadcast loudspeaker, preferably for a 15-volt (city) network. Small-sized power transformers 220/9 ... 12 V and TVZ and TVK transformers from old TVs are also suitable.

For those who want to repeat the described design, the following tips will be useful. First of all, install an outdoor antenna and arrange a ground or counterweight (you can use water pipes or heating pipes, but not gas pipes). Do not connect to the mains in any case, firstly, for safety reasons, and secondly, because of the high level of interference in it.

After assembling the receiver (left side of the diagram) and connecting high-impedance headphones or a broadcast loudspeaker with a transformer to its output, find powerful stations on the air. Only if the voltage U0, measured by a high-ohm voltmeter on capacitor C2 (with phones or loudspeaker turned off) reaches at least 1 V, it makes sense to experiment with an amplifier.

Setting up the amplifier comes down to selecting the resistor R1 for maximum volume with good sound quality. If we already optimize the amplifier completely, it is useful to choose the transformation ratio of the output transformer by changing the number of turns of the secondary winding. This is laborious work, so it is advisable to immediately wind more turns of the secondary winding by making several taps. The installation provides loud and high-quality sound with several dynamic heads (you can even have different types) installed in an open box of a large volume. In this case, you can choose the optimal load resistance for the amplifier without rewinding the transformer, but turning on the heads in series or in parallel.

Literature

1. Polyakov V. Loud-speaking receiver with a bridge amplifier and "free energy" power supply. - Radio, 2001, No. 12, p. 12, 13.
2. Polyakov V. Improvement of the detector receiver. - Radio, 2001, No. 1, p. 52, 53.

Author: V.Polyakov, Moscow

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