The invention relates to an instrumental amplifier for non-invasive measurement of bio-signals of low level on the skin surface. According to the invention, the bio-signal amplifier consists of a differential input pre-amplifier carried out with two operational amplifiers () to whom a bio-signal sampled by means of some measuring electrodes () and a reference electrode () is applied, the so-obtained common mode signal, separated by means of an operational amplifier (), in voltage repeater mounting, is applied to another operational amplifier (), namely at the negative input of two integration circuits made with two other operational amplifiers () which are connected in the negative reaction loops of the first two operational amplifiers () which form the differential input pre-amplifier. The operational amplifier () adds to the negative value of the common mode signal, the reference voltage (), which is applied to the bio-signal amplifier from outside. The differential signal from the differential pre-amplifier output is applied to the input of an instrumental transconductance amplifier () whose output current is applied to an input of an adder () and the output current of a second instrumental transconductance amplifier () is applied at another input of the adder (), the output current of the adder () being further applied to a transimpedance amplifier made with an operational amplifier (), the output voltage of the transimpedance amplifier, which represents the output signal of the bio-signal amplifying circuit, is applied, by means of a resistive voltage divider, to the input of the transconductance amplifier (). The reference signal is added to the output signal of the bio-signal amplifier, by applying it to the negative input of the amplifier ().Invenţia se referă la un amplificator instrumental pentru măsurarea neinvazivă, de pe suprafaţa pielii, a biosemnalelor de nivel scăzut. Amplificatorul de biosemnale, conform invenţiei, este alcătuit dintr-un preamplific
