How to choose resistance and capacitance value in active bandpass filter?

Question

I am trying to make a bandpass filter around 300Hz using this circuit. I have calculated the values of the components using the equations shown in this website assuming low frequency cutoff 160Hz and high frequency cutoff 480Hz.

With the equations you get values of RC. So I thought I could select the resistance and capacitance as I wish as long as RC gets the right value.

But when I choose R1=1k, R2=330, C1=C2=1u and simulate it with Proteus the frequency response is nothing like a bandpass filter.

But when I choose R2=3300 and C2=0.1u frequency response is fine.

Why does one work and the other does not?

PS: I am using a LM358 operational amplifier.

Answer 1

the equations you are using for calculating the values of the components are established by considering that the OP-AMP is perfect.

Of course it's not the fact and by choosing R2=330 and C2=1uf you have a relative low impedence between the output of your amplifier and the (-) input. The LM358 cannot give enough current for this.

By using R2=3300 and C2=0.1u, the current needed is lower and then the LM358 is close to a perfect OP-AMP.