Low-pass filters

By definition, a low-pass filter is a circuit offering easy passage to low-frequency signals and difficult passage to high-frequency signals. There are two basic kinds of circuits capable of accomplishing this objective, and many variations of each one: The inductive low-pass filter in Figure below and the capacitive low-pass filter in Figure below

Inductive low-pass filter

The inductor's impedance increases with increasing frequency. This high impedance in series tends to block high-frequency signals from getting to the load. This can be demonstrated with a SPICE analysis: (Figure below)

inductive lowpass filter             
v1 1 0 ac 1 sin
l1 1 2 3
rload 2 0 1k  
.ac lin 20 1 200
.plot ac v(2) 
.end  

The response of an inductive low-pass filter falls off with increasing frequency.

Capacitive low-pass filter.

The capacitor's impedance decreases with increasing frequency. This low impedance in parallel with the load resistance tends to short out high-frequency signals, dropping most of the voltage across series resistor R₁. (Figure below)

capacitive lowpass filter                                
v1 1 0 ac 1 sin
r1 1 2 500    
c1 2 0 7u     
rload 2 0 1k  
.ac lin 20 30 150     
.plot ac v(2) 
.end