A major problem encountered by filter designers is that caused by the need for inductance at low frequencies. We will now consider a concept which will enable the designer to dispense with inductors for certain applications by using what is referred to in the literature as the passive network simulation method. Central to the basis of the design is a device referred to as a gyrator whose principle of operation was outlined by Tellegan. It will be seen that we fortunately possess an op-amp based gyrator circuit and several such devices are manufactured using IC technology. A distinct advantage of the active simulation approach is that the circuits produced have low sensitivities which are basically those of the passive counterparts. We shall see that a distinct disadvantage of producing synthetic inductance, as it is called, is that being an active device, one of its terminals must be connected to earth. Furthermore, if matched op-amps are used, then the effects of non-ideal amplifier parameters on the inductance realisation are minimised.
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