Active filter question

[youyoung21147]

I'm currently designing my first active filter and I've seen on ESP website that a bandpass filter needs one section for the highpass and another one for the lowpass.

It means 4 opamps in series for a 24dB/oct bandpass filter ! Really not nice IMHO


So I'm wondering if it is possible to put the highpass and lowpass sections on the same opamp.

Is it possible ?

BTW : is it cleaner (sound meaning) to use a discrete transistor (eg. BC560C) or an opamp (eg NE5534) for active filtering ?


Thx in advance !

[adolphe]

There's several opamp packages available that have 2 or 4 opamps on a single chip. You could use one of those to save space.

-Scott

[joensd]

A bandpass of a 4th order is usually realized with two opamps.
What you do is basically cascade two 2nd order bandpasses.

Can´t seem to find a nice link that shows how it´s done.
Admittedly calculating filter components can get a bit complicated,
so maybe you should get yourself a filter program.
TI filter pro

It´s been a while but I installed the freeware filter program from Texas Instruments at one point and liked it very much.
I´d start from there.

Rod cascades highpass and lowpass to achieve bandpass which is too much parts indeed.

If you want to go discrete you can also make passive filters of max. 2nd order, buffer them and follow with another passive filter.
For a bandpass I´d use an opamp though.
discrete active XO

[youyoung21147]

Originally posted by adolphe
There's several opamp packages available that have 2 or 4 opamps on a single chip. You could use one of those to save space.

-Scott

Thanks for the suggestion, but I was speaking of sound issues : 4 components in series are never good. Place for me is never a problem

Rod cascades highpass and lowpass to achieve bandpass which is too much parts indeed.

That's why I want to put the components of the lowpass and highpass on the same opamp

Can´t seem to find a nice link that shows how it´s done.Admittedly calculating filter components can get a bit complicated,
so maybe you should get yourself a filter program.
TI filter pro

I have FilterPro but I prefer using this online design tool : http://www.analog.com/Analog_Root/st...er/filter.html

It proposes bandpass but strangely, it doesn't allow to adjust separately the two cutoff frequencies.
In fact, what I'd like to know is if the schematic of the bandpass filter shown can be used with the same components values as if I used two different sections.

Concerning discrete I was only considering it if it gave a better/more transparent sound than opamp. But for a 1st project, I'll stick with opamps

Thanks for the answers !