The use of the gyrator in resonant amplifiers and generators. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Radio amateur designer

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When developing low-frequency resonant amplifiers and harmonic oscillation generators, designers usually try to do without labor-intensive inductors. Most often in these cases, they use the Wien bridge, which allows you to build a quasi-resonant device using only frequency-dependent RC circuits. However, along with such an indisputable advantage as simplicity, constructions based on the Wien bridge, unfortunately, have a significant drawback. They are extremely sensitive to the slightest imbalance in the parameters of the bridge elements. To circumvent this shortcoming, the author of the published article proposes to use an LC circuit based on an artificial inductor implemented using an electronic device, called a gyrator in radio engineering, instead of the Wien bridge. Although the circuits of resonant amplifiers and harmonic oscillators in this case are more complicated, they allow you to get more stable results.

The use of a gyrator in amateur radio designs, the scheme of which is given in [1], is very convenient. Unfortunately, in the original source this device is described only in general terms and many of its positive properties are not disclosed at all. There are no examples of the practical use of the gyrator.

The schematic diagram of the gyrator is shown in fig. one.

The theoretical analysis of its work shows that with ideal operational amplifiers (op-amps) the input impedance of the gyrator Zin is purely inductive. Moreover, the value of the inductance is determined by the following relationship:

Zin \u1d Lin \u2d R4 * R1 * R3 * CXNUMX / RXNUMX, where R is Ohm; C - nF; L - Mr.

However, since the gain of real op-amps is not infinite, and their gain decreases with increasing frequency, losses appear in the inductance created by the gyrator and its quality factor decreases. If we take R1=R2=R, R3=R4=r and wRC1=1, the quality factor can be calculated by the formula: Q=K0/(2+2K0f/fv), where Ko is the gain of the op-amp; f and fv - operating frequency and frequency at which the gain of the op-amp decreases by 1,41 times. Since K0 is usually very high, very high quality factors can be obtained at low frequencies.

If a capacitor is connected to such an artificial inductor, then the oscillatory circuit formed by them can be used in resonant amplifiers and generators of harmonic oscillations. A diagram of one of the amplifiers with a parallel oscillatory circuit is shown in fig. 2.

At low frequencies, when K0f/fv << 1 (and only this case will be considered further), the resonant frequency of such a circuit f0=(R3/R1*C1*R2*R4*C2)1/2 /(2*PI ). quality factor Q=R0(R3*C1/R1*R2*R4*C2)1/2, bandwidth df=1/2PI*R0*C1. The gain of the entire amplifying path Km=2.

As follows from the relationship, in order to determine the resonant frequency, in addition to single and double variable capacitors, it can be tuned with single and double variable resistors. The use of double elements makes it possible to obtain a much wider range of tuning, and the use of single elements is more convenient constructively. A large tuning range can be obtained if the functions of the frequency tuning body are performed by a variable resistor included instead of fixed resistors R3 and R4. However, in this case, the output signal should be removed from the slider of this resistor, otherwise the voltage gain will depend on the tuning frequency.

In the amplifier, the circuit of which is shown in Fig. 3, a series resonant circuit is used.

In this case, the gain increases sharply at the resonant frequency. Instead of two, it becomes equal to Km=2Q. The quality factor will be determined by the ratio:

Q = (R1*R2*R4*C2/R3*С1)^1/2/R0.

The gain of the amplifier will not depend on frequency if a dual variable capacitor is used to tune it, but the bandwidth will change.

On the basis of a resonant amplifier with a parallel circuit (Fig. 2), a notch amplifier can be easily built (Fig. 4). Since in a resonant amplifier at a resonant frequency, the signal at the inverting input of the op amp DA1 is equal to the input signal, it is enough to subtract the second signal from the first signal to get no output. The subtraction operation is performed by the op-amp DA3. It will no longer be possible to provide a zero signal difference at other frequencies.

To convert a resonant amplifier into a generator of harmonic oscillations, it is necessary to compensate for energy losses in the oscillatory circuit [2]. In generators, the circuits of which are shown in fig. 5 and 6, compensation is achieved by introducing an adjustable negative resistance into the circuit. In the generator (Fig. 5), its functions are performed by a voltage divider, consisting of a constant resistor R6 and a semiconductor thermistor R5. With an increase in the amplitude of the generated voltage, the temperature of the thermistor will increase and its resistance will begin to fall. As a result, the negative resistance introduced by him into the oscillatory circuit will decrease and thus stabilize the voltage generated by the generator. By selecting the resistance of the resistor R6, you can achieve the maximum stabilizing effect of the thermistor.

As the latter, it is best to use devices designed to stabilize the operating mode of harmonic oscillation generators with a Wien bridge, for example, the PTM2 / 0.5 thermistor indicated in the diagram. If such a thermistor cannot be obtained, then the thermistors used in power meters can be used, or the generator can be made according to the circuit shown in Fig. 6. In this generator, the stabilization function is performed by a subminiature incandescent signal lamp SMN. Such lamps were widely used in old computers. Stabilization of the generator's operating mode can be achieved only when the filament of the lamp is heated red-hot. However, a conventional op-amp cannot provide such a current, so a current amplifier based on a KT603B transistor had to be introduced into the generator.

The devices for stabilizing the generated voltage considered here are quite effective. Suffice it to say that when the variable resistor changed the generation frequency by a factor of five, the value of the generated voltage changed by no more than 1%. The coefficient of non-linear distortion in the audio frequency range did not exceed 0,1% and increased at lower and higher frequencies. In the first case, due to insufficient thermal inertia of the thermistor or light bulb, and in the second, due to a decrease in the quality factor of the circuit with a gyrator as an artificial inductance .

Literature

1. Horowitz P., Hill W. The art of circuitry. T. 1. - M.: Mir, 1993, p. 297.
2. Petin G.P. Transistor amplifiers, generators and stabilizers. - M.: Energy, 1978.

Author: G. Petin, Rostov-on-Don

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