# Fundamental Physics/Electronics/Electronics Filter/Low pass filter

## Low pass filter

Low pass filter is an electronics device that has a constant voltage over low frequency . Low pass filter can be constructed from LR and RC as shown .

## R-C Low pass filter has a transfer function

$H(j\omega )={\frac {v_{o}}{v_{i}}}={\frac {1/j\omega c}{R+1/j\omega c}}={\frac {1}{1+j\omega T}}$ Time constant

$T=RC$ Frequency response of Low pass filter

$\omega =0.v_{o}=v_{i}$ $\omega =\omega _{o}.v_{o}={\frac {v_{i}}{2}}$ $\omega =00.v_{o}=0$ Cut off frequency, $\omega _{o}$ , frequency at which $v_{o}={\frac {1}{2}}v_{i}$ $\omega _{o}={\frac {1}{T}}$ ## L-R

Low pass filter has a transfer function

$H(j\omega )={\frac {v_{o}}{v_{i}}}={\frac {R}{R+j\omega L}}={\frac {1}{1+j\omega T}}$ Time constant

$T={\frac {L}{R}}$ Frequency response of Low pass filter

$\omega =0.v_{o}=v_{i}$ $\omega =\omega _{o}.v_{o}={\frac {v_{i}}{2}}$ $\omega =00.v_{o}=0$ Cut off frequency, $\omega _{o}$ , frequency at which $v_{o}={\frac {1}{2}}v_{i}$ $\omega _{o}={\frac {1}{T}}$ 