High pass filter and low pass filter question?

In summary, the conversation discusses finding the frequency for a low pass filter with a series circuit consisting of an ac power supply, 1150.0 ohm resistor, and 144.0 nF capacitor. The formula for gain is given as Vout / Vin = R / [R^2 + (1/ωC)^2]^1/2, however it is found to be incorrect. The correct gain expression for a high-pass filter is ωRC/√(1 + ω2R2C2). The conversation then shifts to calculating ω using ω=2πf, with the final calculated frequency being 3724 Hz. It is emphasized to not simply accept formulas, but to derive them
  • #1
Grendelle
2
0
You have a series circuit consisting of a ac power supply, a 1150.0 ohm resistor and a 144.0 nF capacitor. If the circuit is configured as a low pass filter, what frequency will cause the gain to be 0.25?
Formula
Gain = Vout / Vin = R / [R^2 + (1/ωC)^2]^1/2Solution
Plugging in R=1150 ohms
C=144*10^-9 F

to find a value for ω

And then using ω=2 π f
I got f= 0.05223Hz which is wrong.

Also what will be the formula for high pass filter?
Please help!
 
Physics news on Phys.org
  • #2
Show your calculations for finding ##\omega##.
 
  • #3
Grendelle;3785438[b said:
Formula[/b]
Gain = Vout / Vin = R / [R^2 + (1/ωC)^2]^1/2
Also what will be the formula for high pass filter?
Please help!

1. Formula for gain is wrong.

2. Gain expression for high-pass would be ωRC/√(1 + ω2R2C2)
 
  • #4
Calculations for ω

ω=2∏f

The formula by prof has given me is Gain = Vout / Vin = R / [R^2 + (1/ωC)^2]^1/2 for a low pass filter.

0.25 = 1150 / [1150^2 + (1/ω*144*10^-9)^2]^1/2

My bad. I had a calculation mistake previously. I did it again and this time I solved it to get ω=1.5592*10^3
Using ω=2∏f, f=248.15 Hz.

Which is still wrong... :(I'm confused as to what frequency they are asking me for. Is the this frequency just plain ω? Am I making a mistake by calculating f=ω/2∏?
 
  • #5
Grendelle said:
The formula by prof has given me is Gain = Vout / Vin = R / [R^2 + (1/ωC)^2]^1/2 for a low pass filter.
That is not a low pass filter. You seem to have dived into this problem without even sketching the circuit you are dealing with. Can you draw the circuit diagram for the low pass filter using the 2 specified components? Can you derive the transfer function Vout / Vin for it, rather than placing your faith in something without knowing whether it's the right formula or not?

I'm confused as to what frequency they are asking me for. Is the this frequency just plain ω? Am I making a mistake by calculating f=ω/2∏?
They are one and the same. But if in doubt, you can always write both.
 
  • #6
Grendelle said:
Also what will be the formula for high pass filter?
You are asking the wrong question. What you should be asking of yourself at this point is, "What is the circuit for my high pass filter?"

Then determine for yourself its transfer function, and sketch it. That way, you can't go wrong.
 
  • #7
Grendelle said:
Calculations for ω

ω=2∏f

The formula by prof has given me is Gain = Vout / Vin = R / [R^2 + (1/ωC)^2]^1/2 for a low pass filter.

0.25 = 1150 / [1150^2 + (1/ω*144*10^-9)^2]^1/2

My bad. I had a calculation mistake previously. I did it again and this time I solved it to get ω=1.5592*10^3
Using ω=2∏f, f=248.15 Hz.

Which is still wrong... :(


I'm confused as to what frequency they are asking me for. Is the this frequency just plain ω? Am I making a mistake by calculating f=ω/2∏?

No, you're right, f in Hz = ω/2π.

But the formula your prof gave you, if he really did, is still wrong.

Sketch the diagram for the low-pass circuit, write a summation of currents into the node at the capacitor. Then you can determine the right formula yourself. Don't just accept a formula from someone else, derive it for yourself, or you really won't learn what you need to.

BTW my answer is about 3724 Hz, is that what your prof got?
 

1. What is a high pass filter and how does it work?

A high pass filter is a type of electronic filter that allows high-frequency signals to pass through while attenuating or blocking low-frequency signals. It works by selectively transmitting signals with frequencies above a specified cutoff frequency, while blocking or reducing signals below that frequency.

2. What is a low pass filter and how does it work?

A low pass filter is a type of electronic filter that allows low-frequency signals to pass through while attenuating or blocking high-frequency signals. It works by selectively transmitting signals with frequencies below a specified cutoff frequency, while blocking or reducing signals above that frequency.

3. What are the applications of high pass and low pass filters?

High pass and low pass filters are commonly used in electronic circuits to control the frequency response of a system. They are also used in audio systems, signal processing, and communication systems to filter out unwanted frequencies and improve signal quality.

4. What is the difference between a high pass and low pass filter?

The main difference between a high pass and low pass filter is the frequency range that they allow to pass through. A high pass filter allows high-frequency signals to pass through, while a low pass filter allows low-frequency signals to pass through. Additionally, the cutoff frequency and roll-off rate may differ between the two types of filters.

5. Can high pass and low pass filters be combined?

Yes, high pass and low pass filters can be combined to create a bandpass filter. This type of filter allows signals within a specific frequency range to pass through while attenuating signals outside of that range. Bandpass filters are commonly used in audio and communication systems to isolate specific frequencies of interest.

Similar threads

  • Engineering and Comp Sci Homework Help
Replies
16
Views
907
  • Engineering and Comp Sci Homework Help
Replies
9
Views
2K
  • Engineering and Comp Sci Homework Help
Replies
11
Views
3K
  • Engineering and Comp Sci Homework Help
Replies
13
Views
4K
  • Engineering and Comp Sci Homework Help
Replies
9
Views
34K
  • Engineering and Comp Sci Homework Help
Replies
7
Views
2K
  • Electrical Engineering
Replies
15
Views
1K
  • Engineering and Comp Sci Homework Help
Replies
4
Views
1K
  • Engineering and Comp Sci Homework Help
Replies
3
Views
3K
  • Engineering and Comp Sci Homework Help
Replies
6
Views
2K
Back
Top