Op Amp choice for Oscillator - Page 1
Forum section: electronics / projects

author: DrMag
author_rank: Regular Contributor
timestamp: January 26, 2015, 03:03:21 am
content: I've been experimenting with op amp oscillators, and would like some input on choosing an op amp for a project.The circuit design is a standard Bubba Oscillator, configured to output an audio tone. (One quad op-amp for oscillator, followed by a single op-amp buffer before being amplified and put to a speaker.) Getting an op-amp to oscillate is easy enough; what I'd like to do is have the output be as near perfect a sine wave as possible. The circuit should be able to run off of a single, 3.3 V supply.I've been comparing these amplifiers (all from TI, since I'm used to looking at their datasheets):LM324TLV2374OPA344TLV2474I'm not opposed to SM, but one reason I've looked specifically at these is the availability of DIP in both the quad and single amp configurations. These are listed in order of increasing cost.Here are my questions:What are the most important characteristics I should look for in choosing an amp to get the best quality output I can?Will the op amp quality make as much a difference as using precise passives in the filter stages?Is there a better choice than these TI offerings that comes to mind? (I'm comfortable switching to SMD for the final design.)

author: atferrari
author_rank: Frequent Contributor
timestamp: January 26, 2015, 01:17:30 pm
content: I would go for the highest slew rate and lowest noise.

author: macboy
author_rank: Super Contributor
timestamp: January 26, 2015, 04:17:46 pm
content: For a low distortion and low noise audio oscillator, you need a low distortion, low noise audio op-amp. Since you are looking at TI already, look at LM49710 or LM49720 (single/dual). TI makes many very good audio op-amps, but these, acquired from National, are the best. Some of their other good ones were acquired from Burr-Brown.Edit:I missed the part about running from single 3.3V supply. Look at LM49721 instead.

author: Paul Price
author_rank: Super Contributor
timestamp: January 26, 2015, 04:18:54 pm
content: The choice of op-amp need only be based on the maximum frequency of oscillation which is limited by slew rate.Since you are using a speaker, the frequency would be in the audio range and any of these op-amps might be overkill and any of them would work fine at audio frequencies with low distortion.In fact you can simplify things and just use a single transistor or a single op-amp state(see attached).Use a a single op-amp buffer and a transistor oscillator or a dual op-amp and you will have both an output buffer and low-distortion amplifier at audio frequencies. A minimum cost dual op-amp LM358 might even do the job well at audio frequencies with a little tweeking of the + input bias voltage to center the output swing, but should be close to what I show, but a cheap low voltage rail to rail op-amp would be another choice. In sinewave oscillators, the main factor for lowest distortion at audio frequencies would be having passive components that match the theoretical values calculated for a given oscillation frequency. A good matching of all three RC phase-shift components will give you the required 60-deg phase shifts x 3 for a clean sinewave output.Since your ear is going to be the judge of distortion, the simpler solution would likely give you what you want to hear with fewest PCB area used, least components and cost. Since the output swing of the oscillator would be in the volts range, a low-noise op-amp is not at all needed.The op-amp oscillator shown below exhibits .4% distortion, you just can't hear distortion this low.

author: void_error
author_rank: Frequent Contributor
timestamp: January 26, 2015, 06:53:46 pm
content: I've built something like this last summer and it seemed pretty good. It's a quadrature oscillator. I was running mine off 5V using an LM324, with another opamp of the four acting as a virtual ground buffer set at VCC/2. Some tweaking will be required as the LM324 (LM358 and the lot) doesn't swing close enough to VCCwhich limits the amplitude of the sinewave. I suspect a rail-to-rail input/output opamp should work well at 3.3V.More of the oscillator stuff where the above came from -here.

author: f5r5e5d
author_rank: Frequent Contributor
timestamp: January 26, 2015, 07:52:02 pm
content: easy enough to get gain and phase shift at audio with most op ampsbut a good sine wave oscillator requires a amplitude stabilizing method - the light bulb filament of the Wien Bridge or various other rectifier and gain control auxiliary feedback loopshttp://en.wikipedia.org/wiki/Wien_bridge_oscillatoryou can't make a low distortion oscillator without a gain control element in the loopif you just want audio sine right now - make a .wav file and play on any digital audio device from Sansa Clip, though smart phones to desktop PC or even burn a disc for a CD/DVD player

author: dannyf
author_rank: Super Contributor
timestamp: January 26, 2015, 08:11:24 pm
content: QuoteThose are audio frequency oscillators - even the lowest performance opamps are more than enough for this.

author: macboy
author_rank: Super Contributor
timestamp: January 26, 2015, 09:28:33 pm
content: Quote from: dannyf on January 26, 2015, 08:11:24 pmHe said he wanted as "near perfect since as possible". You won't get that with any old bottom of the bin op-amp. A top-shelf device like the LM49721 that I mentioned can form the basis of an oscillator with THD in single-digit ppm (-110 to -120 dB). With a very good design and construction, 0.1 ppm is possible (-140 dB).

author: f5r5e5d
author_rank: Frequent Contributor
timestamp: January 26, 2015, 11:53:29 pm
content: yes there are the audio oscillator fanatic equivalents to Volt or Time Nutzno one should ask on a engineering site for "the best possible"but you in fact do get -100 to -110 dB distortion numbers from flagship audio DAC app note circuits with NE5532/4, or soundcards with middling NJM/JRC output op ampsspecial op amps aren't your 1st limit when "starting to play"search for low distortion oscillator with any of: Cordell, Bateman, Williams, Eagar, Groner for the audio oscillator nutz level circuits

author: DrMag
author_rank: Regular Contributor
timestamp: January 27, 2015, 12:52:41 am
content: Thanks, everyone. This has been very helpful. Looking through things, I'm leaving towards the OPA4322, which looks to be a good compromise on features and cost for what I'm thinking.QuoteWe could quibble about my word choice, or we could agree that, "Hey, I've made an oscillator! I wonder how I could make it better?" is a reasonable impetus for asking others with experience for their input. =)