What function is the op-amp doing in this configuration. - Page 1
Forum section: electronics / beginners

author: Helix Vlad
author_rank: Newbie
timestamp: January 13, 2015, 11:15:51 pm
content: The op-amps seems to have a few low pass and high pass filters, how are they filtering my signal, from their arrangement?I know this is the finest electronics forum around, I am anticipated to hear help from you guys, Thanks!

author: DmitryL
author_rank: Regular Contributor
timestamp: January 13, 2015, 11:22:24 pm
content: Err.. what is the question exactly ?Op amps in red circles make Sallen-key filters, seehttp://en.wikipedia.org/wiki/Sallen%E2%80%93Key_topologyFilter frequencies are written on the schematics along with filter type (low-pass, high-pass etc.)Whole circuit is supposed to blink LEDs when music plays, I suppose

author: Helix Vlad
author_rank: Newbie
timestamp: January 13, 2015, 11:26:23 pm
content: Indeed they do, The correct question would be how exactly is it filtering my signal within the quotes nodes.High five! same name.

author: DmitryL
author_rank: Regular Contributor
timestamp: January 13, 2015, 11:47:35 pm
content: The given filters filter the input signal exactly the way they are designed and accordingly to the laws of nature.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: January 14, 2015, 12:53:20 am
content: Looks like first order bandpass filters.

author: Paul Price
author_rank: Super Contributor
timestamp: January 14, 2015, 03:30:20 am
content: The op-amps are functioning as signal frequency detectors and activate LED's as indicators when certain input signal frequencies are input that match the band-pass frequency of the op-amp circuits.So, the op-amps are used to make a frequency detecting circuit, with a different frequency being detected by each op-amp.A common use of a circuit like this would be to detect certain frequencies to activate some control circuits.For instance, a dual-tone frequency signal generated by a telephone keypad or some other device output could be sent over a single wire to create a multi-frequency signal used to control one or more remote circuits by detecting pairs of frequencies.A single frequency input to the op-amps could be used to detect just one frequency of interest, so it would be possible to select the LED of interest to be lit, or music sounds could be input to activate the LEDs as a light show that responds to the different frequencies of various instruments.

author: c4757p
author_rank: Super Contributor
timestamp: January 14, 2015, 03:40:06 am
content: Quote from: Paul Price on January 14, 2015, 03:30:20 amNo, that's what the whole circuit does. He asked about theop amps, and made it quite clear that he knows the op amp is acting as a filter and wants to knowhow.

author: LukeW
author_rank: Frequent Contributor
timestamp: January 14, 2015, 07:48:27 am
content: The filters in that circuit, which are bandpass filters, are a topology known as a multiple feedback filter.http://www.analog.com/static/imported-files/tutorials/MT-218.pdfThere are several different topologies for opamp-based active filters, eg. Sallen-Key, multiple feedback and what not, and of course you can design for different filter types, i.e. HPF, LPF, BPF or notch filtering. But the basic principles are much the same.Basically an active filter like this feeds back an inverted signal in the unwanted frequency domain, so it cancels out the unwanted part of the signal.Here are various links and resources on how they work. But be aware that it can get a little bit math-heavy.http://www.electronics-tutorials.ws/filter/second-order-filters.htmlhttp://www.analog.com/static/imported-files/tutorials/MT-220.pdfhttp://www.ecircuitcenter.com/Circuits/MFB_bandpass/MFB_bandpass.htmhttp://www.electronics-tutorials.ws/filter/filter_7.htmlhttp://www.sensorsmag.com/sensors/electric-magnetic/an-introduction-analog-filters-1023

author: dannyf
author_rank: Super Contributor
timestamp: January 14, 2015, 01:35:08 pm
content: QuoteThink of those capacitors as resistors whose resistance varies with frequency. Then work out the gain / attenuation of the circuit with regard to the "resistance". Then you have the frequency response of the whole circuit.

author: Helix Vlad
author_rank: Newbie
timestamp: January 14, 2015, 06:20:09 pm
content: Thank you all for the wonderful help. This information will be of great aid to my project.

author: jlmoon
author_rank: Unknown
timestamp: January 14, 2015, 09:02:04 pm
content: I'm going to put this one in: How about variable (neg) feedback (or variable gain) based on the oh so popular formula for Xc... 1/2(pi)(f)(c)