# high pass active filter

Discussion in 'Homework Help' started by skaterboy, Apr 12, 2010.

Apr 12, 2010
4
0
2. ### Audioguru New Member

Dec 20, 2007
9,411
896
The gain is 1 like a piece of wire at high frequencies because the opamp is a follower.
Its Rin doesn't matter because it must be fed from a very low impedance like the output of an opamp.

3. ### skaterboy Thread Starter New Member

Apr 12, 2010
4
0
Thanks alot.
But do you know how to find V- and V+ ?

And what is the formula for the Gain? I need to show formulas.

4. ### Audioguru New Member

Dec 20, 2007
9,411
896
if it is the dual polarity power supply then select the voltages to match your opamp and to match your desired max output level.

Don't you know the calculation of gain in a non-inverting opamp circuit?
It is 1 + (Rf/Rg). This simple circuit has the opamp as a follower without Rf and without Rg so its gain is 1 at high frequencies.

skaterboy likes this.
5. ### skaterboy Thread Starter New Member

Apr 12, 2010
4
0
Isn´t R1 = Rg ?

Am I supposed to take the C1 and C2 out and just short circuit it instead?

Is there no current flow in R2?

Last edited: Apr 12, 2010
6. ### The Electrician AAC Fanatic!

Oct 9, 2007
2,284
328
Are you supposed to be able to derive the formula, or just find it?

7. ### Audioguru New Member

Dec 20, 2007
9,411
896
The output of the opamp is directly connected to the (-) input so the opamp has a gain of 1 like a piece of wire but its input impedance is very high and its output impedance is very low.
R1 applies a little positive feedback at the cutoff frequency of the filter so that the response is sharp and is not droopy.
C2 and R2 make the second RC of the second-order Butterworth filter.

8. ### skaterboy Thread Starter New Member

Apr 12, 2010
4
0
Sorry i don´t understand those complicated explanations. I need only to know simply how to find the gain...

Apr 14, 2005
7,050
657
10. ### Audioguru New Member

Dec 20, 2007
9,411
896
The "geek talk" is not complicated. It is the basics.
I explained why the gain is only 1.