Can you use an active LP filter with gain=1 as a buffer also? - Page 1
Forum section: electronics / beginners

author: logancane
author_rank: Regular Contributor
timestamp: August 01, 2021, 08:54:12 am
content: I guess the question really boils down to whether an active filter circuit has a low output impedance. In my design I am using an opamp to create a low pass filter with a gain of 1. Can I also use this as the buffer amp, or do I need another opamp after it to specifically be used as a buffer? There doesn't appear to be an easy answer on Google.Thanks in advance

author: Kleinstein
author_rank: Super Contributor
timestamp: August 01, 2021, 08:57:46 am
content: The usual active fitlers (Sallen-Key and multiple feedback) have a low output impedance, however that are also others that need an extra buffer. If directly from an OPs ouput there is usually no need for an extra buffer.

author: logancane
author_rank: Regular Contributor
timestamp: August 01, 2021, 09:05:40 am
content: Excellent. Thanks for the prompt reply.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 01, 2021, 02:35:57 pm
content: Note that the input impedance matters as well, so you may need a buffer there instead.Tim

author: mawyatt
author_rank: Super Contributor
timestamp: August 01, 2021, 03:13:30 pm
content: This is important point about input impedance as "seen" from the filter input, often overlooked and one cause (there are many) of performance deviations from ideal. The SK types have a very complex and frequency dependent input impedance over the intended frequency range. To reduce the effect of the input impedance, the driving source impedance needs to be low relative to the input impedance over the frequency range.Best,

author: TimFox
author_rank: Super Contributor
timestamp: August 01, 2021, 03:42:06 pm
content: The input impedance comments above are very important.As a simple check for output impedance: if the filter output comes directly from the op-amp output, it is almost certainly low-impedance until you approach the op-amp's frequency limitations.

author: mawyatt
author_rank: Super Contributor
timestamp: August 01, 2021, 03:54:46 pm
content: Most Op-Amps used in closed loop form have an inductive looking output, meaning the output impedance rises with frequency. This can cause all sorts of problems with active filters when the reactive looking output impedance becomes significant relative to the next stage effective input impedance. This also degrades the low pass SK stop band rejection because the series C feeds directly to the Op-Amp output, and why we often used an heavily biased emitter follower (inside the closed loop) at the Op-Amp output, or just forced the Op-Amp output device into Class A operation away from Class A/B, by using a resistor to the negative supply to sink a few millamps.Best,

author: TimFox
author_rank: Super Contributor
timestamp: August 01, 2021, 04:09:28 pm
content: The same effect occurs with fixed-feedback buffer amplifiers, but the frequency-dependent feedback in an active filter can exacerbate the effect.

author: mawyatt
author_rank: Super Contributor
timestamp: August 01, 2021, 04:24:04 pm
content: The Class A trick works in just about any application IF you can afford the extra supply current & power.Best,

author: TimFox
author_rank: Super Contributor
timestamp: August 01, 2021, 05:22:50 pm
content: This trick is well-known in audio circles to avoid crossover distortion.Inductive output impedance: this follows directly from the fact that the closed-loop output impedance must increase with frequency, as the loop gain decreases with the open-loop gain of the op-amp. It is perhaps better known in linear voltage regulators, since they often see large capacitive loads.