The classic three- or two-op-amp instrumentation-amplifier circuits are standard ways to amplify a small-amplitude differential signal contaminated with high common-mode noise. In some applications, the signal source is floating with high-series-output impedance and thus requires an appropriate high-input-impedance amplifier. This Design Idea proposes an alternative approach using a simplified amplifier circuit (Figure 1). The basic principle is to sense the current in the amplifier input B (I_B) and inject a current of the same value in the amplifier input A (I_A), by combining a virtual-ground transimpedance amplifier (A₁) with a voltage-controlled current source (G₁). Thus, G₁ balances the common- mode interference currents. In addition, the voltage at input B is at virtual-ground potential.

A practical circuit is a two-electrode biosignal amplifier for electrocardiogram signals (Figure 2). IC_2B is the transimpedance amplifier. The feedback capacitor, C_FB, ensures circuit stability. The INA134 difference amplifier, IC₁, and the op amp, IC_2A, make up a high-quality, bidirectional voltage-controlled current source. You could use many similar ICs, such as INA132, 133, 152, 154, 105, or AMP03, for IC₁. The remaining part of the circuit comprises two conventional noninverting stages. The proposed circuit can be useful in many two-wire or two-electrode applications, in which you need to maintain high amplifier input-impedance values.