thank you for your reply,the input of this circuit is the coil L1 which induces a transient changing magnetic field , a transient current changing occurs in the coil and i want to amplify this signal, from my experiment,when there is no changing magnetic field the current across L1 is unvaried the ouput of the amplifier is zero when R3=R4 ,as long as i apply a changing magnetic field to L1 L1 will induce a current and the amplifier's output will change in terms of the current of L1 but i'd like to know their relationship