VacuumVoodoo
Well-known member
Hi,
it' about time i contributed with more than a comment now and then....so here is my first "real" contribution:
A standard 2nd order active filter with Sallen-Key topology has a fixed gain but you can change cut-off frequency easily. Things get screwy when you want to change the passband gain.
Here is the idea which I originally developed for current feedback opamp, it works just as well with our normal opamps.
In principle: since the feedback loop around the opamp forces the inverting input to assume same potential as the noninverting input then the signal path between these two inputs can be looked upon as a virtual voltage follower. So we build the filter around this virtual voltage follower leaving the rest of the opamp to provide variable gain in the passband by varying the feedback resisitor's (R2) value. You can use either a logpot or a switched resistor ladder.
A much clearer PDF with both HP and LP filter curves is here:
http://anacon-tech.com/IPdocs/VARIGAIN_HP_LP_FILTER.pdf
Simulation using Microcap8 and 5532 opamp model:
This might save you one opamp in a signal chain since the filter provides gain too. Hope you can find use for this.
I used it originally directly on the input stage in a RIAA preamp to provide effective HP at 30Hz.
Alex
it' about time i contributed with more than a comment now and then....so here is my first "real" contribution:
A standard 2nd order active filter with Sallen-Key topology has a fixed gain but you can change cut-off frequency easily. Things get screwy when you want to change the passband gain.
Here is the idea which I originally developed for current feedback opamp, it works just as well with our normal opamps.
In principle: since the feedback loop around the opamp forces the inverting input to assume same potential as the noninverting input then the signal path between these two inputs can be looked upon as a virtual voltage follower. So we build the filter around this virtual voltage follower leaving the rest of the opamp to provide variable gain in the passband by varying the feedback resisitor's (R2) value. You can use either a logpot or a switched resistor ladder.
A much clearer PDF with both HP and LP filter curves is here:
http://anacon-tech.com/IPdocs/VARIGAIN_HP_LP_FILTER.pdf
Simulation using Microcap8 and 5532 opamp model:
This might save you one opamp in a signal chain since the filter provides gain too. Hope you can find use for this.
I used it originally directly on the input stage in a RIAA preamp to provide effective HP at 30Hz.
Alex