A passive switched capacitor filter is a specialized electronic filter that uses only capacitors and switches, typically implemented with transistors. Unlike active filters, it contains no active components such as operational amplifiers. The key innovation is that it simulates resistor behavior using capacitors and a switching frequency, allowing for precise filter design without physical resistors.
The core principle of switched capacitor filters is that a capacitor with periodic switching can simulate the behavior of a resistor. The equivalent resistance is given by the formula R equals one over f s times C, where f s is the switching frequency and C is the capacitor value. This allows precise control of resistance values through frequency and capacitor ratios, making it ideal for integrated circuit applications.
Passive switched capacitor filters have distinct characteristics. Being passive, they provide no active amplification and typically introduce signal attenuation or insertion loss. They consume no power for amplification and have simple circuit topology. These filters operate in discrete time, processing sampled data signals with clock-dependent operation. The frequency response depends on capacitor ratios and the switching clock frequency, making them highly controllable and suitable for integrated circuit implementation.
A basic switched capacitor filter implementation uses a two-phase operation. During phase one, the input is sampled onto the first capacitor. During phase two, the stored charge is transferred to the second capacitor through controlled switches. This creates the filtering action without physical resistors. The implementation offers precise frequency control, eliminates the need for physical resistors, and is highly suitable for integrated circuit fabrication with excellent temperature stability.
To summarize what we have learned: Passive switched capacitor filters are electronic circuits that use only capacitors and switches to create filtering effects. They simulate resistor behavior through precise switching frequency control and operate in discrete time with sampled data. These filters provide excellent frequency control without requiring physical resistors, making them ideal for integrated circuit applications with superior temperature stability and manufacturing precision.