Even though we publish new blogs daily from your intrepid editor and a number of well-known writers, sometimes it's good to have a look back at our well-received articles from the recent past.
We touched on the basics of active filter designs once before when we looked at integrators in Op-Amp & Integrator Basics. This time, we go in deeper and look at active filters with more than one pole. Like the integrator, the active filter is constructed around an op-amp. The author of this article looks at the two common topologies: the Sallen-Key (a non-inverting filter); and the infinite-gain, multiple-feedback (an inverting filter):
The author also discusses sensitivity issues — how the filter behaves as the resistor and capacitor values deviate from the calculated values. All good stuff to know for your next filter design. Have a look here.
Hi Brad–you mention in the linked article that usually above a few MHz the filters are done with passives. I've seen plenty of that done, along with matching networks in RF systems. I was wondering what the highest frequency application you have seen using and op-amp based active filter is?
Also, we have talked elsewhere about active front ends for RF systems; these circuits seem like the original genesis of those devices. This is a bit off this topic but the holy grail in, say, a mobile phone would be an integrated active match for the antenna plus active filter, ideally able to switch among different bands and have very low insertion loss in-band. What limits come into play to prevent designing the perfect solution?
I would say you can easily build op-amp based filters at 100MHz, for just 2nd order low-pass even higher. For RF matching and low noise, inductors are needed! On-chip coils have bad Q, but with active LC filters you can handle even 2GHz and built calibrated narrow-band filters. Howver, the noise of that is not as good as it would be for a LC filter with high-Q elements. But it could be good enough for a post-LNA pre-Mixer RF filter, and not ultimate-performance RX systems.
Bye Stephan
I would expect at 100MHz the parasitics would start to be problematic – stray capacitance and inductance. And as you've noted, when you actually want an inductor, it's tough to make one that's hi-Q. But I'll say up front that this is not my area of expertise.
Both the architectures are great for implementing a 2nd order filters compared to other architectures like Tow Thomas Biquad…
The design equations for MFB are straight forward with a few assumptions while Sallen Key has few more set of equations to solve, which means the inter dependency is more… I feel MFB is better when we consider variation or sensitivity of Ao or Q of the circuit w.r.t. to the passive elements.
And we know that some versions of the active filter show sensitivity to minor component variations (per the author).
eafpres: I have seen these kind of second order filter implementations using opamps and passive networks using discrete components… But are these architectures used for on-chip implementations?
I feel many designers either use the RC filters or gm-C filters for on-chip implementations… Any details ??
If we go as per design equations then Sallen key might be stable architecture, but the positive loop is a concern, we have to ensure the negative feedback is always more than the positive feedback.
We dont have that kind of concern is MFB circuits, but MFB circuits have 2 loops. Deciding which loop is vulnerable, which loop to break for stability analysis is complex.
If you use some of the look-up tables for calculating values, you sould have a circuit that is stable (i.e., properly damped). If you design from scratch, certainly care must be taken – again, got to keep damping where it belongs/keep Q from getting to high. Otherwise, you end up with an oscillator.
“sometimes it's good to have a look back at our well-received articles from the recent past” — This is why I personally enjoy printing out some of my favourite articles. I'm not always meticulous about doing it because I'm pretty sure that there are a lot of articles out there that I just haven't gotten around to reading. But for the ones that have been helpful, there's folder of those somewhere for reference in my study!