# Are RC /CR - Highpass filter the same?

I‘m quite new to electrical engineering and one of the tasks is to showcase and plot an CR high pass filter. I am familiar with RC HP filters but this is new to me. Just changing the order of the capacitor with resistor would make it an lowpass filter, right? I think there is more to it but I’m not sure. I‘ve found little to no information about CR-HP online.

• Edit your question and use the schematic tool to show us what you're referring to. Jul 18, 2023 at 14:44
• I‘ll give it a shot, but the promt is just „Showcase the Bode-Diagramm of CR-Highpass and adiditionanlly the effect of a ramping signal“ (It‘s german, I don‘t know how you would put it in english but i hope it‘s clear enough). Jul 18, 2023 at 14:51
• Is this homework? Jul 18, 2023 at 14:52
• What's not clear to me is whether you're using different words to describe the same thing, or if the components in the circuit you're describing are also being flipped around to match the words you're using. Jul 18, 2023 at 14:55
• Not really, its supposed to be a Project with a due date. Completing it is mandatory to attend the exams. Most of the questions are rather basic so i don‘t know if I‘m overthinking the problem or if it is the same as an regular RC-HP. Jul 18, 2023 at 14:56

In my opinion the best way is to jot down a the circuit with a configuration of interest and try it out. This would help you develop intuition and eventually you will be able to do it in your head. Here is the high pass configuration (CR). Basically, if you let: f-> infinity C->0 a short circuit. f-> 0 C->infinity an open circuit.

From this you can deduce that at low frequencies the capacitor will block your signal and at high frequency the capacitor will let your signal pass through

Here is the low pass configuration :

Basically, if you let: f-> infinity C->0 a short circuit. f-> 0 C->infinity an open circuit. In this configuration at low frequency the capacitor will "capture" your applied signal and at high frequencies it will pass your signal to the ground. Hence why it is a low pass configuration.

Here is a plot of the two responses

As you can see they have the same 3dB frequency but the signature of each response is different. One is a low-pass and the other a high pass.

• Thank you very much for showcasing this and taking the time to to go into so much detail. Jul 18, 2023 at 16:08
• You are welcome, this is something I used to struggle a lot with too! I have graduated 3 years ago, but I still have to jot down the circuit :D. I wish someone showed this technique earlier to me...
– DRF
Jul 18, 2023 at 16:13
• btw @LostMemories if you find my response to answer your question, you can like it and approve it as an answer for your question. :)
– DRF
Jul 18, 2023 at 18:38

The term "high-pass filter" and, the usage of a resistor and capacitor implies a 1st order filter with a resistor and capacitor that can be in only one configuration so, if you called it an RC high-pass filter or, you called it a CR high-pass filter it means the same.

Ditto an RC low-pass filter.

• So It‘s save to say im supposed to plot a regular RC-HP filter? The Task seems kinda abiguous since every student is assigned a random basic HP/LP filter consisting of 2 parts. (R,C,L) and considering the contents of the lecture i assume that 2nd order filters are out of the question anyways. Jul 18, 2023 at 15:19
• Only you know what you have been assigned. I'm just answering your question and not trying to guess what documents or instructions you have received. Jul 18, 2023 at 15:44

There is only one single circuit solution for a passive first-order highpass filter which contains one resistor R and one capacitor C. It does not matter at all if such a filter is called C-R-highpass or R-C-highpass.

Regarding the "ramp signal": You should remember the fact that a first-order highpass will act - more or less - as a (non-ideal) differentiating block.