We demonstrate an ink-jet printed ambipolar transistor and inverter based on a semiconducting carbon nanotube (CNT) network as a channel by employing a solution-based chemical doping technique with an amine-rich polyethyleneimine (PEI) polymer. The PEI polymer has been reported as an efficient electron dopant and thus contributes to enhancing n-type conduction in CNT transistors. However, because of the presence of ambient oxygen and moisture and the hygroscopicity of the PEI polymer, their p-type conduction did not seem to be effectively reduced, resulting in rather ambipolar conduction. Therefore, we utilize a simple solution-based doping technique to convert p-type semiconducting CNT transistors into ambipolar transistors and fabricate the ambipolar CNT transistor by combining a cost-effective ink-jet printing technique and a simple spin-coating method. Finally, the electrical performance of the logic inverter consisting of identical two ambipolar CNT transistors is also evaluated and optimized by adjusting the concentration of PEI polymer. Published by AIP Publishing.