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 