A circuit was developed for a differential two-electrode biopotential amplifier. Current sources at the amplifier inputs were controlled by the common-mode voltage. This principle is well known in telephony for interfacing the telephone line with analogue-type phones. A low impedance of about 1 k(ohm) was obtained between each input and the common point of the circuit. The differential input impedance of 60 M(ohm) was obtained with the use of precision resistors. Considerable reduction in the common-mode voltages of more than 200 times resulted. The circuit can be useful for biosignal acquisition from subjects in areas of very high electromagnetic fields, where high common-mode voltages could saturate the input amplifier stages.