D Ramírez, S Casans, C Reig, AE Navarro, and J Sánchez EDN Although wellknown to activefilter theorists and designers, GICs (generalized impedance converters) may be less familiar to analog generalists. Comprising a oneport active circuit typically comprising lowcost operational amplifiers, resistors, and capacitors, a GIC transforms capacitive reactance into inductive reactance and thus can substitute for an inductor in a filter that an RLCtransfer function describes. In addition, the flexibility of a GIC's inputimpedance equation permits the design of virtual impedances that don't exist as physical components – for example, frequencydependent resistance (Reference 1). The GIC, which its developers introduced 30 years ago, has seen its greatest application in accircuit and activefilter applications.
Figure 1 shows a classic GIC circuit in which the input impedance, ZIN, depends on the nature of impedances Z_{1} through Z_{5}. The following equation describes the circuit's input impedance: For example, if Z_{1}, Z_{2}, Z_{3}, and Z_{5} comprise resistors R_{1}, R_{2}, R_{3}, and R_{5}, and Z_{4} comprises capacitor C_{4}, then the input impedance, Z_{IN}, appears as a virtual inductor of value L_{IN}: Figure 2 shows the GIC circuit in its dc configuration. When you consider the GIC circuit in a purely dc environment, you can envision new applications. For example, you could replace impedances Z_{1} through Z_{5} with pure resistances R_{1} through R_{5}. Instead of an ac inputvoltage source, connect a precision temperature and timestable dc reference voltage to the input port. A simple circuit analysis using ideal op amps for IC_{1} and IC_{2} shows that the reference input voltage, V_{REF}, appears across resistor R_{5}, and, as the following equation shows, a constant current, I_{O}, flows through R_{5}.
However, op amp C_{2}'s noninverting input diverts a small amount of current from the junction of R_{4} and R_{5}, and I_{O} thus also flows through R_{4}. Selecting large values for R_{1}, R_{2}, and R_{3} helps minimize current drawn from the reference voltage. For example, the circuit can supply 2 to 10 mA to R_{4} and draw only a few tenths of a microampere from the reference source. Using tighttolerance and lowdrift components for V_{REF} and R_{5} ensures the stability of I_{O}. Applications include providing constantcurrent drive for Wheatstonebridge and platinumelement sensors (Reference 2). In addition, you can replace R_{4} with a series of resistive sensors as in an Anderson loop (Reference 3). References
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