Practical Realization of a Gyrator Circuit and RC-Gyrator Filters

A gyrator circuit containing only three transistors has been designed by providing an active feedback path such that the circuit behaves like a two-way feedback system with transfer immittance parameters that are equal in magnitude and opposite in phase. At the same time, very low input and output immittances are obtained by this arrangement so that the need for additional negative resistance circuits or negative impedance converters (NIC) to neutralize the residual input and output impedances is obviated. Experimental data exhibit evidence of the excellent performance of the circuit as a gyrator, impedance inverter, and isolator over wide ranges of load and, frequencies that are well below the microwave region. From a theoretical analysis of the circuit, design equations are derived and their application demonstrated by actual design of the gyrator circuit to realize specified open-circuit (o.c.) impedance parameters of the two-port network. These equations show how the network parameters can be individually varied; desired values of the gyration resistance are easily obtained by adjusting single passive elements. The gyrator is then cascaded in between two RC two-port networks to realize complex conjugate poles in such a way that the sensitivity of the poles due to variation in the gyration resistance is minimized. Experimental study of the frequency response of the filter and the sensitivity of the poles when the gyration resistance is varied gives very good results in agreement with theory.