In this paper, we present an operational amplifier (op-amp) and a pulsewidth-modulation (PWM) controller fabricated with amorphous indium-gallium-zinc oxide thin-film transistors (a-InGaZnO TFTs), to build an adaptive dc-dc converter for display driving systems. The fabricated a-InGaZnO TFT presents electrical characteristics suitable for accomplishing an op-amp composed only with n-type TFTs to have an overall gain of 24.5 dB, a cutoff frequency of 6 kHz, and a unit gain frequency of 32 kHz operating at +/- 15 V. The fabricated op-amp is used to construct a comparator and an error amplifier on the glass, which are combined to realize a PWM controller. The resulting PWM controller can successfully control the output voltage of an external dc-dc converter that follows the input voltage to the error amplifier, and can operate at 100 Hz to control the duty ratio from 24.3% to 66.1% when the reference voltage varies from 2 to 6 V, respectively.