Bandwidth limiting the lm3886

[Flemming J P]

Hi

I want to bandwidth limit my gain clones, using Cc cap between the 2 inputs on my NIGC.

In the datasheet they recormend using 50-500pf, but I can't find any formula telling what the result of the different values will be.

I have searched the net looking at standard opamps configuartions, but haven't been able to find any examples that uses this cap.

I know Joe Rasmussen uses it, but it is on a IGC (he is using 1,2nf).

Can anyone help me out?

Thanks and happy new year.

[Easyamp]

Use the supplied formmula for cf and rf2 to limit the GBP. Here is a very glazed over look at this:

http://tangentsoft.net/audio/hs-opamp.html

[Pedja]

Flemming,

Unless you have an additional pole around the opamp, that cap might easily make the amp oscillate rather than it will usefully band limit it.

There is one example of the (3 pole) filtered LM3886 on the Siegfried Linkwitz’s site:
http://www.linkwitzlab.com/images/graphics/3886amp.gif

If the simple 6dB LPF is your goal, the classic way to accomplish this is the usage of the series resistor and shunt cap (from the non-inverting input to ground), in which case you should left some headroom for possible non-zero source impedance (the amp’s input impedance could be neglected as it is sufficiently high).

Pedja

[Easyamp]

Thanks for the link, that is a much better way of limiting, thanx.