Introduction to the simulated inductor circuit and working principle for obtaining large inductance

Function of the circuit

Inductors above several 100MH are heavy and large in size, which are not suitable for current use requirements. Except for special purposes, low-frequency LC filters can basically be replaced with active filters. This circuit uses a positive feedback circuit to invert the frequency-impedance characteristics of the capacitor C to form an equivalent inductor, and one end of the coil L is grounded. If the capacitor is connected in series, an LCR series resonant circuit can be realized, which can be used for graphic equalizers.

[page]Circuit working principle

When the open-loop gain of the OP amplifier is very high, the equivalent inductance L in the circuit diagram can be expressed as L=C1.R1.R2, the equivalent series resistance RO=R2, and the equivalent parallel resistance RF=R1. Usually, the smaller the RS, the better the performance of the coil L, so a resistance of less than several 100 ohms should be selected, and the larger the parallel resistance, the better. If R1>R2, a resistance of several 100K~100M can be obtained.

In this circuit, to achieve an inductance of 10H, C1=0.1UF, R1+VR1=100K, and R2=1K are taken respectively.

Because the performance of the simulated inductor L depends on the open-loop characteristics of the OP amplifier used, it can only be used in the low-frequency range. The following uses the 50HZ notch filter in Figure A for reference.

In Figure A, when the resistance between the non-inverting input terminal of the OP amplifier and the ground is 10K, the output is equal to zero. Because the LCR series resonant circuit is connected here, a large attenuation can be obtained at the resonant frequency FO=1/2π√LC. If the input signal frequency deviates from FO, the resonant impedance changes from emotional to capacitive, and the amplification factor is close to 1.

In order to make FO = 50HZ, the capacitor CO should be:

The notch temperature of the FO frequency can be adjusted by using the variable resistor VR2. In order to make L of the circuit in the dotted line diagram variable, VR1 in the circuit diagram can be used to adjust it, and the notch frequency will change accordingly.

The Q value of the resonant circuit is calculated using the following formula:

If used to filter out power supply AC noise, the Q value should be 5~10. In order to make the value within the square root 25~100, CO=10C1 and R1=100R2.

Adjustment

Because the error of capacitor C1 will also cause the error of inductor L, VR1 must be used to adjust the resonant frequency. Input a 50HZ sine wave from the input end, and adjust VR1 and VR2 alternately to make the output level at 50HZ the lowest. If adjusted well, 60~70DB attenuation can be obtained.