Parallel RC low pass filter

Thread Starter

pinkyponky

Joined Nov 28, 2019
235
Hi,

I would like to design a parallel RC low pass filter. I found lot of online calculators to calculate the cut of frequency of the series RC low pass filter, but I couldn't found any help regarding the parallel RC filter. So, please could you provide me that what is the formula to calculate the cut of frequency?.
 

LvW

Joined Jun 13, 2013
1,353
Hi,

I would like to design a parallel RC low pass filter. I found lot of online calculators to calculate the cut of frequency of the series RC low pass filter, but I couldn't found any help regarding the parallel RC filter. So, please could you provide me that what is the formula to calculate the cut of frequency?.
In contrast to the principle of cascade synthesis, in parallel structures the transfer function to be realized is not decomposed into a product of partial functions, but into a sum of partial functions whose output signals are added (partial fraction decomposition method).
The method is usually used only for functions of maximal fourth order. This design method is only relatively rarely considered in books and filter programs.
If compared with the classical filter structures (cascade of 2nd order blocks), this parallel design strategy offers some advantages, in particular, for transfer functions with transmission zeros (Chebyshev invers, elliptical filters).

* Example: A Chebyshev lowpass of 3rd order can be realized as a parallel combination of a first-order lowpass and a second-order (inverting) bandpass.
 

LvW

Joined Jun 13, 2013
1,353
May be I'm wrong, Is it called as parallel RC filter?. Please correct me.
The name is correct: Parallel filter topology.
Here are two filter programs which allow to design parallel structures:
(1) "Filter free 2007", (2) "Filter Wiz Pro".
 
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Thread Starter

pinkyponky

Joined Nov 28, 2019
235
The name is correct: Parallel filter topology.
Here are two filter programs which allow to design parallel structures:
(1) "Filter free 2007", (2) "Filter Wiz Pro".
Hi LvW,

Can you also share one example circuit with the mathematical calculations?.
 

LvW

Joined Jun 13, 2013
1,353
The above file shows
(1) The function to be realized (3rd-order, Chebyshev inverse)
(2) The decomposition into two parts (partial fraction method)
(3) Rewriting the function - keeping the denominator of the original function.
(4) If both expressions (1 and 3) are to be identical, the numerators must be identical
(5) Comparing the coefficients (and setting ao,2=0) we arrive at a0,1=1 and a1,2=-1/wpQp
(6) As we can see - we now have the sum (of a 1st-order lowpass and a 2nd-order bandpass (with a negative sign).
 
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LvW

Joined Jun 13, 2013
1,353
First order only..
I guess from my answers you have learned that parallel filters - by definition (!) - are higher-order filters.
For a first-order RC-filter there are only two realization alternatives: (a) passive RC or (b) active RC (with opamp).
 

Thread Starter

pinkyponky

Joined Nov 28, 2019
235
I guess from my answers you have learned that parallel filters - by definition (!) - are higher-order filters.
For a first-order RC-filter there are only two realization alternatives: (a) passive RC or (b) active RC (with opamp).
Can you show me both diagrams of passive RC and Active RC?. What are the formulas for those?. It is confusing for me..
 

Thread Starter

pinkyponky

Joined Nov 28, 2019
235
Hi Bob,

Please don't be angry. I'm fully stress. Please understand the situation. I know some times Im asking too silly question. These questions are coming due to stress. I hope you understand.

I have found the difference between them.

1635777384856.png 1635777398034.png

1635777424601.png 1635777442596.png

Am I right?.
 

BobTPH

Joined Jun 5, 2013
4,244
Finally, 16 posts in, we get a circuit. You have wasted the time of forum members who either ask you for more info or answer the wrong question. The has been your pattern in all of your postings. If you want to be taken seriously, next time show us exactly what you are talking about by posting the exact circuit in your first post.

Bob
 

Thread Starter

pinkyponky

Joined Nov 28, 2019
235
Hi Bob,

If I have circuits definitely I will post it.

Please can you tell that how to calculate the cut of frequency of Parallel RC circuit?. Please could you help with that.
 

Papabravo

Joined Feb 24, 2006
17,545
Hi Bob,

Please don't be angry. I'm fully stress. Please understand the situation. I know some times Im asking too silly question. These questions are coming due to stress. I hope you understand.

I have found the difference between them.

View attachment 251607 View attachment 251608

View attachment 251609 View attachment 251610

Am I right?.
For the passive circuit, the expression for the parallel impedance is correct. The transfer function is for voltages, not currents. The transfer function MUST be dimensionless. You would normally include a source resistance in this analysis. The corner frequency will be the frequency where the magnitude of the transfer function is -3 dB.
 

Thread Starter

pinkyponky

Joined Nov 28, 2019
235
For the passive circuit, the expression for the parallel impedance is correct. The transfer function is for voltages, not currents. The transfer function MUST be dimensionless. You would normally include a source resistance in this analysis. The corner frequency will be the frequency where the magnitude of the transfer function is -3 dB.
Hi Papa,

Please can you provide me the correct transfer function if the above function is wrong. And also, please can you do the mathematical calculations to find out cut/corner frequency?.
 

BobTPH

Joined Jun 5, 2013
4,244
The transfer function for your first curcuit is:

Vout = Vin

The output voltage is the same as a the input voltage, as is obvious since Vout and Vin are taken across the same two nodes.

Bob
 
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