Circuit Theory/Introduction to Filtering
Filter is a circuit constructed from Resistor and Capacitor or Inductor in order to pass certain range of frequencies . The range of frequencies that make the circuit stable
Let examine the following circuits
RC Circuit[edit]
A circuit with one resistor in series with the input and one capacitor parallel to the load

ω = 0 ω = Infinity
The RC circuit is more stable at frequencies from zero up to the response frequency : . This circuit is ideal for Low Pass Frequency Filter .
CR Circuit[edit]
A circuit with one capacitor in series with the input and one resistor parallel to the load

ω = 0 ω = ω = Infinity
The CR circuit is more stable at frequencies from the response frequency up to infinity. This circuit is ideal for High Pass Frequency Filter .
RL Circuit[edit]
A circuit with one resistor in serires with the input and one inductor parallel to the load

ω = 0 ω = Infinity
The RL circuit is more stable at frequencies from the response frequency up to infinity. This circuit is ideal for High Pass Frequency Filter .
LR Circuit[edit]
A circuit with one inductor in serires with the input and one resistor parallel to the load
ω = 0  ω = Infinity  

The LR circuit is more stable at frequencies from zero up to the response frequency . This circuit is ideal for Low Pass Frequency Filter .
In Conclusion, Resistor and Capacitor or Inductor can be used for constructing a Filter
 For Low Pass Filter use LR or RC
 For High Pass Filter use RL or CR
Conclusion[edit]
Filter Types  High Pass Filter  Low Pass Filter  Low Pass Filter  High Pass Filter 

Circuit  RL  LR  RC  CR 
ω_{ο}  
T  CR  CR  
Z  
Frequency Response 
ω = 0 V_{o} = 0
ω = ω_{ο} V_{o} = V_{i}
ω = 0 V_{o} = V_{i}

ω = 0 V_{o} = V_{i}
ω = ω_{ο} V_{o} = V_{i}
ω = 0 V_{o} = 0

ω = 0 V_{o} = V_{i}
ω = ω_{ο} V_{o} = V_{i}
ω = 0 V_{o} = 0

ω = 0 V_{o} = 0
ω = ω_{ο} V_{o} = V_{i}
ω = 0 V_{o} = V_{i}

Stability  Circuit is stable at Frequencies ω = ω_{ο}→Infinity  Circuit is stable at Frequencies ω = 0→ω_{ο}  Circuit is stable at Frequencies ω = 0→ω_{ο}  Circuit is stable at Frequencies ω = ω_{ο}→Infinity 