Passive Filters - Engineering

Q1:

An RC high-pass filter consists of a 0.2 F capacitor and a 220  resistor. The output is taken across the resistor. The circuit's critical frequency is

A 723 Hz

B 7,234 Hz

C 362 Hz

D 3,617 Hz

ANS:D - 3,617 Hz

Which formula is used here?

Please describe it.