# First order and second order circuits

#### Abhinavrajan

Joined Aug 7, 2016
83
In Electronics, I have heard the terms "First order circuit" and "second order circuit" used with filter circuits.
What do they actually mean?

I have heard these terms in mathematics.
But what do these terms tell in electronics (please explain with a less math) ?

What can I infer from these terms ?

Thank you!

#### ninjaman

Joined May 18, 2013
339
Hello,

I did some stuff with third order filters in audio filters. I had to make two third order filters, one high pass and the other low pass. Look at filter topology
https://en.wikipedia.org/wiki/Electronic_filter_topology
or ladder topology as I think it is mainly known.
I think, the higher the order of filter the faster the drop off at the -3db point. Meaning that where the filter stops working, it stops working quicker in a higher order filter. I think the drop off is measured in either octaves or decibels.

• Abhinavrajan

#### Papabravo

Joined Feb 24, 2006
18,467
In electronics the word "order" refers to the differential equation that can be used to describe the circuit. In simpler terms it is the number of "reactive" elements. Resistors are not reactive elements since their impedance does not depend on frequency. For example, a circuit with a single inductor, or a single capacitor along with a resistor would be a first order circuit. A circuit with an inductor, a capacitor, and a resistor would be a second order circuit.

Filters don't "stop" working at the corner frequency. In fact they do their best work in the stopband.

#### crutschow

Joined Mar 14, 2008
29,844
In electronic filters, the order determines the rate of rolloff after the passband.
The rolloff rate after the -3dB corner frequency is 6dB/octave (20dB/decade) of frequency per degree of order.
Thus a 1st order filter rolls off at 6dB/octave, a 2nd order rolls off at 12dB/octave (40dB/decade), etc.

An octave is change in frequency of a factor of two and a decade is a change in frequency of a factor of ten.

Last edited:
• Abhinavrajan