RC low-pass filter passive vs active (OpAmp) - Page 1
Forum section: electronics / beginners

author: Vulpecula
author_rank: Newbie
timestamp: November 27, 2021, 03:50:09 pm
content: Hey there!I have a question regarding RC low-pass filters. Actually I'd like to know the difference of what is going on when having a passive RC LPF versus an active RC LPF with an OpAmp. I know how to measure the input and output signal at increasing sinusoidal frequencies of a passive LPF and how to determine the cutoff frequency from that and so on... But I do not fully understand or know what I'd have to expect when the LPF is implemented as an active filter by using an OpAmp, especially when connecting a load to the filter. Maybe someone is able to share some wisdom with me.ThanksVulpecula

author: Vulpecula
author_rank: Newbie
timestamp: November 27, 2021, 04:15:12 pm
content: To be a bit more specific: I'm especially wondering what to expect in terms of cutoff frequency. (E.g.: Looking at a passive RC LPF, I know that the cutoff frequency rises once I apply a load to that LPF.)If my tiny understanding of OpAmps is correct, the cutoff frequency is not supposed to shift (or shift as much) as I would see in a passive circuit. (Which is one of the reasons why you would want to use an active filter in the first place, I assume.)

author: rstofer
author_rank: Super Contributor
timestamp: November 27, 2021, 04:21:58 pm
content: https://www.electronics-tutorials.ws/filter/filter_5.htmlThe difference is amplification in the case where an active gain stage follows a passive LPF. This allows the output impedance of the LPF to be very low across the entire frequency range which might be useful for driving following stages.QuoteThe ideal op amp doesn't apply any load to the LPF section. In the real world it is still a very small number.

author: Kleinstein
author_rank: Super Contributor
timestamp: November 27, 2021, 04:28:18 pm
content: Passive RC filters are made of 1st order stages and they thus have a rather gradual cross over. It gets even worse if 1 st order stages are coupled without an extra buffer in between or massive higher impedance for the 2nd stage. Load to a passive fitler effect the frequency response. The load is part of the filter.Active filters usually are not much effected by the load, as the load is driven by the amplifier and thus ideally has no effect. It could still be a problem when highly capacitive so the OP does not like the load.The active fitler stages are usually 2nd order. Every linear filter can be separted in a chain of 2nd order stages and if odd order 1 first order stage. However one can not seprate every 2nd order element in 2 1st order ones. The combination of a passive 1 st order stage and than a 2nd order active is quite common.

author: Terry Bites
author_rank: Super Contributor
timestamp: November 27, 2021, 04:35:52 pm
content: Do you mean the difference between a buffered RC and an unbuffered RC?If the load is a very high resistance compared to the Series R, not much.if R series is in the same region as the load then you have a resistive divider and the RC time constant will change. Along with passband attenuation that you dont usually want.CR is now the effective thevenin Reistance*Chttps://www.allaboutcircuits.com/textbook/alternating-current/chpt-8/low-pass-filters/A simple RC is a first order filter and that may be ok for your application. If you are using an opamp you may as well go for a second order active filter with better control of parameters.

author: tooki
author_rank: Super Contributor
timestamp: November 29, 2021, 09:15:19 pm
content: Quote from: Vulpecula on November 27, 2021, 03:50:09 pmExcept for the load, this website makes it easy to calculate the results, with complete simulations:http://sim.okawa-denshi.jp/en/Fkeisan.htmWhat’s stopping you from using LTspice or Micro-Cap to simulate your filter designs, including the loads?