What are Passive Filters?

Filter is a four terminal device used in electronic circuits to isolate or separate signals of certain frequencies from a source before transfer of current to load. The filter passes on, or amplifies, desired frequencies while passing on to load, while attenuating or blocking unwanted signals beyond this range.

Impedance of a capacitor is a function of frequency. At high frequencies, capacitor acts as near short circuit, whereas it offers high impedance to low frequencies. Capacitor acts as an open circuit to DC voltage, and does not allow DC currents to pass through. At very high frequencies, its impedance is so low that it literally becomes a short-circuit.

This property of capacitor is used to selectively segregate (or filter out) unwanted frequencies from a circuit. Filter circuit, by definition, segregates undesired frequencies from input before passing on to output side. Filters are essential parts of all electronic circuits, and are frequently used for various functions in electrical and electronic circuits.

AC / DC rectifier filter

AC / DC Filters are extensively used in electronic and electrical circuits to obtain smooth DC voltage from a rectifier, as also pure AC voltages by filtering out unwanted frequencies and ripples.

An AC/DC filter may isolate an AC waveform, and pass DC current on the output side. This type of filter is commonly used with half wave or full wave rectifier, and in most cases, it could be a capacitor of sufficiently large value to get a DC output, which may comprise of AC component (ripples) to lesser or higher degree depending on design of rectifier and other components. There is a practical limit to which the value of capacitor can be increased, and capacitor value is selected to have ripple content within acceptable limits.

A capacitor is usually almost connected across the output of a half wave or full wave rectifier. This may be called a storage capacitor. As it stores energy during peak input voltage, and supplies it to load when input waveform level goes down, till it next cycle. This absorbs most ripples in DC voltage of rectifier. It must be understood that ripple can be minimized, but never fully eliminated.

If V_pp is acceptable ripple (peak to valley voltage difference), Capacitor value in Farads for full wave rectifier is given by C = I / (2 f V_pp), where I is load current, f is base ripple frequency (100 Hz for full wave 50 Hz supply). Considering a permissible V_pp of 1 Volt, and load current of 3 Amp., this works out to (3 / 2 x 100 x 1=) 0.15 Farad, or 15,000 μF. C will vary with load current design requirement.

Filter circuits are of several types, depending on frequencies passed or blocked by them. The main types of filter by their functions are as below.

1. Low Pass Filter- Passes low frequencies and attenuates high frequencies
2. High Pass Filter – Attenuates low frequencies and passes high frequencies
3. Band -Pass Filter – Passes frequencies within a specified frequency range, and attenuates those above and below this range
4. Band Stop Filter – Attenuates frequencies within a specified frequency range, and passes all frequencies above and below this range.

High Frequency Filters (Low Pass Filters)

In the RC circuit shown above, C is a short circuit for high frequencies, which are bypassed to ground through it, and voltage V2 is DC almost pure DC voltage, available to an equipment / circuit connected at output terminals. This is low pass capacitive filter, which filters out high frequencies.  RL filter circuit shown also is a low pass filter, as the inductor is a short circuit for DC, and attenuates higher frequencies.

Low Frequency filter (High Pass Filter)

Capacitor in RC filter above does not allow DC to pass through, and output V2 is AC, devoid of any DC signal, while signals of low frequencies may be present (which are not significantly attenuated by capacitor). The way a capacitor, or a combination of capacitors is connected in a circuit, decide the behavior of a circuit at different frequencies. This is low frequency filter. The LR circuit shown also is a high pass filter, and allows high frequencies to pass on to load, since inductor L offers lafrge impedance to high frequencies.

Filter circuits may utilize simple passive components, when they are called Passive Filters. They use combinations of resistive, inductive and capacitive elements to get desired characteristic outputs. The combinations could be RC, RL, LC, RLC in different configurations.  They are made as L filter, T filter, Pi filter, etc., depending on combination structure of circuit components.

There are number of types of passive filters with different functions and characteristics, and using RC, LC combinations. They may utilize one stage or multiple stages of filtering. Accordingly, they may be classified as first order, second order, third order filters etc.  the circuits shown above use only one stage of filtering, and are first order filters.

Filters can also be made using semiconductor components lie diodes, transistors or op-amps., and such filters are called Active Filters. Active filters can deal with very low frequencies and can even amplify the voltage, which cannot be done by passive circuits.

Passive filters

1. Use R, L, and C components in single or multiple stages.
2. Do not need external source to operate.
3. Simple and low cost to design
4. Any change in load will affect the output of passive filter.
5. Use of large inductor for low frequency operations makes the filter bulky.
6. Size larger than an active filter

Active filters

1. Use active components like op-amps and transistors
2. Include resistors and capacitors, but not inductors
3. Need external power source to operate.
4. They have high input impedance and low output impedance.
5. Can be cascaded in multiple stages without loss of gain. In fact, the gain may increase.
6. They are essentially meant for small current applications, since active filters cannot handle large currents.