Parallel RC low pass filter

Papabravo

Joined Feb 24, 2006
21,159
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?.
You wrote it as Vout/Iin. This cannot be a transfer function because it has units of Ω. The correct left hand side is Vout/Vin, and has been pointed out in the given diagram it is 1. If you include the source resistance, as is usual then you can get the more meaningful expression where the capacitor impedance gets smaller with an increase in frequency. It is quite apparent that you need to go back adn review basic concepts before trying to swim in waters where drowning is a distinct possibility.
 

LvW

Joined Jun 13, 2013
1,752
The transfer function is for voltages, not currents. The transfer function MUST be dimensionless.
Yes - in most cases, the transfer function is defined/given as a dimensionless expression (V/V).
However, I think it is not quite correct to say that it "MUST" be dimensionless.
When one of the terminal signals (input or output) is given as a current, the transfer function is given in V/I or I/V.
Examples:
* Operational Transconductance amplifier: out/in=I/V=1/Ω ,
* Transimpedance amplifier: out/in=V/I=Ω.
 

BobTPH

Joined Jun 5, 2013
8,813
There is no cutoff frequency because the circuit is not a filter, it passes all frequencies equally. Put in a series resistor and you will have a filter.

Bob
 

LvW

Joined Jun 13, 2013
1,752
Hi,
Shall I you use the active low-pass filter formula for parallel RC circuits, to calculate the cut of frequency?.
When the input of first (passive) RC-parallel-combination is connected to a signal current source , the given expression vor Vout/Iin is correct.
In this case, the cut-off frequency is wo=1/RC
 

Thread Starter

pinkyponky

Joined Nov 28, 2019
351
When the input of first (passive) RC-parallel-combination is connected to a signal current source , the given expression vor Vout/Iin is correct.
In this case, the cut-off frequency is wo=1/RC
Hi LvW,

See below equation:
1635864950166.png
If the above derivation is correct, then the same formula is for 'Active Low Pass Filter' and 'Parallel RC' circuit. Am I correct?.
 

BobTPH

Joined Jun 5, 2013
8,813
If you are filtering an AC current source, which I have never heard of. I suppose there could be such a thing, but I would have no idea how to design one.

Bob
 

LvW

Joined Jun 13, 2013
1,752
If you are filtering an AC current source, which I have never heard of. I suppose there could be such a thing, but I would have no idea how to design one.
Bob
Just one simple example: The OTA (operational transconductance amplifier) has a high-impedance output which for many applications can be regarded as a current source. The same is true when we use a second-generation current-conveyor (CCII) like the AD840.
Now - when an RC-parallel combination is connected at its output, you have a first-order lowpass - similar to the classical opamp circuit with this RC-combination in the feedback path.
 

BobTPH

Joined Jun 5, 2013
8,813
Okay, but I doubt that that is what the TS is trying to do.


In fact, he always seems to have trouble telling us what he is trying to do.

Bob
 

Thread Starter

pinkyponky

Joined Nov 28, 2019
351
How to calculate the cut of frequency of Parallel RC circuit for the circuit is shown below?.

1636128090263.png


I have shown the formula below for active RC filter, to calculate the cut of frequency for the below circuit. Please correct me If I am wrong?.

1636128400007.png
1636128654968.png
 

Thread Starter

pinkyponky

Joined Nov 28, 2019
351
Hi Everyone,

I have asked this question many days ago, as I know this is very basic question and the people working here in forum are brilliants. I don't know why I'm not getting the straight answer. Just Im asking formula. Please, anyone can answer to the below question.

What is the formula to calculate the cut of frequency of Parallel RC circuit for the circuit is shown below?.

View attachment 251978
 

BobTPH

Joined Jun 5, 2013
8,813
Look at the first circuit in your post above, and answer a question for me:

Can Vout be different from Vin in that circuit?

I have already given you the answer and explained why, so it should not be that difficult.

Bob
 
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