Who says you can ignore RL?Can anyone explain that? I really get stuck.
Why we can ignore RL as it has a great impact in characteristic of the filter?
Any time you say that you can ignore something, you need to provide a justification for the claim (and I know it is exactly this justification that you are trying to get it, I'm just making a generic statement to establish a mental context).
That justification almost always ends up with qualifications attached, such as R5 can be ignored as long as R5 >> (R3||R2)
To identify these qualifiers, you start with the equation that counts -- the one that tells you the value of the quantity you are interested in -- and then manipulate it to see how sensitive it is to the parameter you are looking at.
In this case, the equation that counts is your transfer function, H(jw):
\(
H(j \omega ) \, = \, \frac{R_L}{R_S+R_L+R+j \omega R_L(R_S+R)C}
\)
What happens if we divide top and bottom by RL?
\(
H(j \omega ) \, = \, \frac{1}{ \( 1 + \frac{R_S+R}{R_L} \) +j \omega (R_S+R)C}
\)
Since RL now appears in only one place (the real part of the denominator), it becomes obvious what our qualification is. As long as RL is much larger than (Rs+R), the real term will be about 1.
The more formal way is to do a "sensitivity analysis" by taking the derivative of the transfer function with respect to RL and setting the magnitude of that less than some criterion Kmax. Then solve for RL and, in this case you would see that it has to be greater than (Rs+R), at least in the limit that Kmax gets arbitrarily small.