Sinusoidal generator on the LM386 chip. Scheme, description

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Sinusoidal generator on the LM386 chip. Encyclopedia of radio electronics and electrical engineering

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Encyclopedia of radio electronics and electrical engineering / Measuring technology

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Sinusoidal voltage generators are widely used to test and adjust various electronic equipment. The article proposes the design of a simple generator 34 based on the Wien bridge and the LM386 UZCH microcircuit.

The Wien bridge is often used to build RC oscillators for the AF and higher frequency ranges. It is convenient to use an op amp as an amplifying element in such generators (Fig. 1). The frequency-setting elements R1, C1, R2, C2 are included in the positive feedback circuit of the OS, while the following conditions must be met: R1 = R2 and C1 = C2. The generated frequency can be determined by the formula f = 1/(27piRC), where R = R1 = R2, C = C1 = C2.

Sinusoidal generator on the LM386 chip
Fig. 1

The negative feedback circuit includes elements for stabilizing the output voltage - thermistor RK1 with direct heating and positive TCR, as well as resistor R3. With an increase in the output voltage, the current through the thermistor RK1 increases, it heats up and its resistance increases, which leads to a decrease in the gain and stabilization of the output voltage. Sometimes non-linear elements are used to stabilize the voltage - diodes and zener diodes, which are switched on instead of the resistor R3, and the thermistor is replaced by a resistor.

By changing the values of the frequency-setting elements, it is possible to change the generation frequency over a wide range. To make such an oscillator tunable, it is necessary to use a double variable capacitor or a double variable resistor.

Rice. 2 (click to enlarge)

The scheme of the tunable generator is shown in fig. 2, it is assembled on UZCH DA1. The entire operating frequency range of 30 Hz ... 30 kHz is divided into two subranges: 0,03 ... 1 and 1 ... 30 kHz. The desired switch SA1 is selected, and the frequency is smoothly changed with a double variable resistor R2. An EL1 incandescent lamp is used as a thermistor. LED HL2 indicates the presence of supply voltage, HL1 - output. Diodes VD1, VD2 protect the ultrasonic frequency converter from voltage that can be supplied to its output from the load side. In total, the generator has three outputs: two with a fixed voltage. "Output 1" - 3 V (peak value), "Output 2" - the voltage is ten times less, and one, "Output 0", with smooth adjustment (3 ... 8 V) with a variable resistor R50. A load with a resistance of at least 50 ohms can be connected to the output of the device, and the output current of the op-amp should not exceed XNUMX mA.

The generator uses fixed resistors S2-23, MLT, variable - SPO, SP4-1, dual - SPZ-ZZ-ZO, SPZ-33-23, tuning - SP5-2. Oxide capacitors are imported, the rest are K73-16, and C1, C2, C4 and C7 should be selected with a tolerance of at least 5%. Incandescent lamp - SMN6.3-20 (voltage - 6,3 V, rated current - 20 mA). The switch - any small-sized on two provisions and two directions.

The adjustment comes down to setting the trimmer resistor R5 of the above voltage at "Output 1". It is desirable to do this on the first subrange in the middle position of the variable resistor R2 slider.

The device can be assembled on a dielectric board using hardwiring and housed in a suitable plastic case. Variable resistors, LEDs and a subrange switch are installed on the front panel. The scale of the generator is calibrated by the frequency meter.

Author: P. Petrov, Sofia, Bulgaria; Publication: radioradar.net

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There is no data on the level of non-linear distortion of the generator, the operational amplifier used is not broadband, which does not allow us to hope for low non-linear distortion ...

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