Finding other self-centering op amps like LM386 - Page 1
Forum section: electronics / beginners

author: ziplock9000
author_rank: Regular Contributor
timestamp: January 26, 2017, 12:59:57 pm
content: After having a run around with dual supply and single supply op amps not working easily with input signals that have -ve components, I stumbled upon the LM386.It has output biasing "Ground referenced input, Self-centering output quiescent voltage" which means that even though you're giving it a single power supply that is only GND-positive, it can amplify input signals that dip negative and reflect that in the output. Very useful indeed.It seems that the LM386 is optimised for audio range applications and I wanted to look for other self centering op amps with other characteristics. To cut a long story short, I want to make a general purpose amplifier project box that I can plug into other applications. I'm sure there's tons of issues with that (including frequency adaptive filtering, shielding, connection capacitance) that are outside of the scope of this question.So how would I find a list of other op amps that are self-centering like the LM386 that are in DIP packages?I've tried Google and it's not helped. Farnell's (Element14) site doesn't seem to have filters for that.Currently I'm manually looking at each datasheet, but surely there's a better way? (BTW, I've not found a single other self-centering OP AMP)Thanks.EDIT: I incorrectly reffered to the LM386 as an op amp, not a power amp. Discussion has somewhat evolved since the original question.

author: Ian.M
author_rank: Super Contributor
timestamp: January 26, 2017, 01:23:50 pm
content: LM386 is *NOT* an OPAMP, its a low power audio power amp.You can find some single supply OPAMPs that have an input common mode range that extends below ground, but biassing them so the output doesn't 'rail' while keeping them DC coupled will be problematic

author: ziplock9000
author_rank: Regular Contributor
timestamp: January 26, 2017, 01:56:01 pm
content: Quote from: Ian.M on January 26, 2017, 01:23:50 pmI completely missed that distinction thanks.You've got me thinking however.For my project it looks like I need to go back to the drawing board and decide if I want to use an op amp or power amplifier, and if I need one or two stages of each.I want something that can not only take uV source voltages and amplify them to analog equivalent of logic level to be used with things like ADCs. But also that can take logic level to something that can power modest speakers. I'm assuming that a single IC can't do that, although I've seen op-amps with combined power amps I think?.Would this be considered a pre-amps / amp combo?

author: Ian.M
author_rank: Super Contributor
timestamp: January 26, 2017, 02:23:46 pm
content: It appears that you have two completely different needs:1. monitoring approximately line level audio signals on a speaker.2. amplifying and signal conditioning low level signals for data acquisition.The LM386 is reasonably well suited to (1), in a quiet room or to headphones. However you will find its distortion rises rapidly above 200mW output power so its unsuitable in a nosier environment. You'd hook it up with AC coupling on the input, probably low-pass filtered to keep RF out then straight to a volume control pot and into its input. Optionally, you'd add switchable gain so it could be used with low level signals, and make up a RF probe for prodding around in AM radio circuits.For (2) you want an instrumentation amplifier with adjustable output clamping. You'd probably want switchable fixed gains + a variable gain option, and it would probably be useful to be able to introduce a DC offset. I would suggest specifying it for up to 10V output.

author: ziplock9000
author_rank: Regular Contributor
timestamp: January 26, 2017, 02:31:11 pm
content: Thanks. You've given me a good start for further investigation. On a side note, isn't an LM386 considered old these days and there's much better performing and still low cost alternatives?

author: Ian.M
author_rank: Super Contributor
timestamp: January 26, 2017, 03:04:05 pm
content: Yes, the LM386 is old and crappy. However you wont find much else currently available in DIP that doesn't need special heatsinking. e.g if you search for mono class AB amps in DIP packages, Digikey only stocks the LM380N and LM384N, both of which are fixed x50 gain so if you want a sensitive low-level input for signal tracing work, you'd need to stick an OPAMP in front of them. There are one or two nice class D chips, but you don't really want a monitor amp that radiates RFI, possibly upsetting the circuit you are testing.

author: Zero999
author_rank: Super Contributor
timestamp: January 26, 2017, 11:11:33 pm
content: If you just want an op-amp which can work with both inputs slightly below the negative rail then there are ICs which will do that.The old CA3140 is specified down to 0.5V below the negative rail.https://www.intersil.com/content/dam/Intersil/documents/ca31/ca3140-a.pdfEven though it's not specified on the datasheets, I believe the old LM358/324 will work with the inputs at 0.3V below the negative rail and the TS272 may too but you should test them to be sure.