biasing single supply op amp - Page 1
Forum section: electronics / beginners

author: electrolust
author_rank: Unknown
timestamp: October 15, 2015, 08:48:32 pm
content: http://ocw.mit.edu/courses/media-arts-and-sciences/mas-836-sensor-technologies-for-interactive-environments-spring-2011/readings/MITMAS_836S11_read02_bias.pdfpage 6, inverting AC coupled configuration.That can't possibly be right can it? R3 and R4 set the common mode voltage, but without any bias on the signal, it can never swing positive and only very small negative (if at all).I'd simply ignore that document but it appears to be official course materials, not the ramblings of an undergrad.

author: panfilero
author_rank: Newbie
timestamp: October 15, 2015, 11:10:32 pm
content: Seems ok to me. R3 and R4 set the reference point for your input (in the basic non-inverting configuration this is ground as shown in page 1). So in this circuit you are just referenced around the voltage set by the R3 R4 divider instead of ground. It's the same thing.

author: Ribster
author_rank: Frequent Contributor
timestamp: October 15, 2015, 11:23:48 pm
content: Thanks for this!Nice little cheat sheet

author: Galaxyrise
author_rank: Frequent Contributor
timestamp: October 16, 2015, 01:07:06 am
content: Quote from: electrolust on October 15, 2015, 08:48:32 pmWhat do you think steady state looks like? ie, if the signal was dc for awhile, what voltage is the opamp outputting, what voltage is at the plate on the opamp side of the cap, what voltage is at the inverting input?From that steady state, what happens when the signal rises?

author: electrolust
author_rank: Unknown
timestamp: October 16, 2015, 05:00:15 am
content: Ah! Thanks Galaxyrise! Great questions that led me to (I think) the answer. You must be a teacher, if not you should be.The feedback loop keeps the V- input at the V+ (bias) input. This is then anti-modulated (if you will) by the signal!So the steady state output is Vbias. Which you can then "strip" by AC coupling the output. With a split supply it's the same thing: V+ goes to ground and the steady state output is at ground level. Here, "ground" is Vbias.Or, if you did add Vbias to the signal (after the cap of course), then the steady state output would be 0V and you do not have to AC couple the output. But you might be adding an RC filter to the input? I guess it depends on how the Vbias is generated and "injected".

author: Galaxyrise
author_rank: Frequent Contributor
timestamp: October 17, 2015, 05:26:32 pm
content: Quote from: electrolust on October 16, 2015, 05:00:15 amYep! exactly so.QuoteYou'd need a negative supply to the opamp for it to output 0V (and below), so you wouldn't bother with the extra bias and would just use 0 as V+ at that point. There are some other issues with that suggestion, too... I suggest getting something like LTSpice and experimenting with the circuit there. You do have to be careful trusting simulator results too much, but I think it can answer your questions about this circuit.AC coupling the output with a capacitor doesn't have the negative supply issue, but you are adding another RC block to consider when thinking about the frequency response. The R and C values matter even in the original circuit, for that matter.Another option is to buffer VBias with a unity gain opamp and then use it as "ground" in other parts of the circuit (or your headphone outputs, whatever.)