Simple BP Filter, Cap Formula.

[chemelec]

Does anyone have a formula to determine C1, C2 and C3 for this circuit?
I thought I had it, but can't find it.

 

[k7elp60]

As I recall a BP(band pass)filter consists of a low pass filter and I high pass filter.
I think R4,C3 and R6,C4 is the high pass. I am not sure what the low pass components are.
I also recall that for both the filters the -3db point is where the reactance of the resistor and the capacitor is equal. So choose the 3db frequency, choose a resistor, then calculate the capacitor using the reactance formula.

 

[Russlk]

That is not very simple, what is the bandpass?

 

[chemelec]

The Bandpass Frequency can be Changed to almost anything in the audio range.
This Circuit works fine and I can easily put in various caps and measure the BP Frequency. But it would be nice to have a Formula for it.

I Can't seem to come up with one that works.

In this Circuit, C1, C2 and C3 are All Equil in Value.
Typical Values range from about 500 pF to .2 uF.

The Pot is set to a point Just Below Self Oscillation, With No Input Signal.

 

[Russlk]

I use this circuit. To change the center frequency, change the two capacitors.

 

[Russlk]

The equation for this circuit is known:

 

[chemelec]

Yes I have used that circuit, Many times before. But in my present application I want to use this transistor Circuit.