Build a precise dc floating-current source

D Ramírez, S Casans, C Reig, AE Navarro, and J Sánchez

EDN

Although well-known to active-filter theorists and designers, GICs (generalized impedance converters) may be less familiar to analog generalists. Comprising a one-port active circuit typically comprising low-cost 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 RLC-transfer function describes. In addition, the flexibility of a GIC's input-impedance equation permits the design of virtual impedances that don't exist as physical components – for example, frequency-dependent resistance (Reference 1). The GIC, which its developers introduced 30 years ago, has seen its greatest application in ac-circuit and active-filter applications.

Figure 1.	A classic generalized impedance converter provides a single-port impedance that appears at VIN. The schematic omits power connections for clarity.

Figure 1 shows a classic GIC circuit in which the input impedance, ZIN, depends on the nature of impedances Z₁ through Z₅. The following equation describes the circuit's input impedance:

For example, if Z₁, Z₂, Z₃, and Z₅ comprise resistors R₁, R₂, R₃, and R₅, and Z₄ comprises capacitor C₄, 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 Z₁ through Z₅ with pure resistances R₁ through R₅. Instead of an ac input-voltage source, connect a precision temperature- and time-stable dc reference voltage to the input port. A simple circuit analysis using ideal op amps for IC₁ and IC₂ shows that the reference input voltage, V_REF, appears across resistor R₅, and, as the following equation shows, a constant current, I_O, flows through R₅.

Figure 2.	Replacing all of a GIC’s impedances with resistors creates a constant-current source.

However, op amp C₂'s noninverting input diverts a small amount of current from the junction of R₄ and R₅, and I_O thus also flows through R₄. Selecting large values for R₁, R₂, and R₃ helps minimize current drawn from the reference voltage. For example, the circuit can supply 2 to 10 mA to R₄ and draw only a few tenths of a microampere from the reference source. Using tight-tolerance and low-drift components for V_REF and R₅ ensures the stability of I_O. Applications include providing constant-current drive for Wheatstone-bridge and platinum-element sensors (Reference 2). In addition, you can replace R₄ with a series of resistive sensors as in an Anderson loop (Reference 3).

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