In this paper, we presented a novel operational amplifier (op-amp) only with solution-processed n-type amorphous indium-gallium-zinc oxide (a-IGZO) thin-film transistors (TFTs). The a-IGZO TFTs and common-source amplifier were fabricated on glass substrate through the solution process and confirmed to show stable electrical characteristics suitable for display driving circuits. Based on the experimental results, we designed an op-amp to have an overall gain of 30.5 dB, a cut-off frequency of 1.47 kHz, and a unit gain frequency of 6.65 kHz when supply voltage was +/- 15 V. Finally, by constituting the comparator, the proposed op-amp is expected to be used in power control and driving systems for display applications.