We propose a Π-shaped circuit model for active metamaterials using periodic circuit structures with series inductors and shunt capacitors (LC) and series capacitors and shunt inductors (CL). Using the circuit model, we develop a general theory for active metamaterials, revealing the role of active components and demonstrating the principle for metamaterials to be active. Byinserting negative and positive resistances in the LC and CL structures, we first attain the active superlens using three slabs of effective media made by such metamaterial circuit structures andshow that the amplitude of voltage will be augmented on two interfaces after adding the active/lossy components. Then we realize the active amplification of evanescent waves and show that thetransmission coefficient will be larger than 0 dB near the resonant frequency, which evidently illustrates the active feature of the medium. Both of these two applications have to satisfy theanti-matching conditions to fulfill the resonance and tunneling effect.