Operational amplifier as Differentiator
The operational amplifier as a differentiator has a capacitor at its input. This input capacitor blocks DC voltage and low frequencies.
Operational amplifier as Differentiator
The capacitor causes a phase shift between voltage and current of 90° (voltage lagging by 90°, i.e. -90°). Since the amplifier circuit additionally phase shifts the signal by 180°, the resulting phase shift at the output is +90°.