driving voltage/current into op amp out put? - Page 1
Forum section: electronics / beginners

author: croyleje
author_rank: Regular Contributor
timestamp: May 29, 2012, 03:39:01 am
content: hello everyonei got a question about op amps i understand an op amp has basically an infinite input impedance but if nothing is being sent into the op amp other then its supply what happens when a voltage/current is driven back into the output? and also if there is a feedback connected to the output what happens? would it be a ground? etc?thanks Jason

author: Psi
author_rank: Super Contributor
timestamp: May 29, 2012, 03:53:23 am
content: Quote from: croyleje on May 29, 2012, 03:39:01 amHow do you mean? the output is an output, so nothing gets driven into it.

author: amspire
author_rank: Super Contributor
timestamp: May 29, 2012, 04:19:10 am
content: Quote from: croyleje on May 29, 2012, 03:39:01 amIf the non-inverting input is connected to ground (0V), and the inverting input is connected to the output via a resistor, then input becomes a virtual ground, as the opamp will change the output so that the current in the feedback resistor exactly matches any current you feed into the inverting input/feedback resistor junction. So whatever the current you put in that junction, the voltage stays at 0V, as long as the output does not reach a positive or negative limit of that opamp.So this input junction look like a 0 ohms resistance, even though no significant current is going into the input pin.By making the feedback resistor very big (like 10MOhm), you can use this input for a very low current ammeter circuit. 100nA into the junction will cause the output to go to -1V.Richard.

author: Mechatrommer
author_rank: Super Contributor
timestamp: May 29, 2012, 04:25:32 pm
content: QuoteFIGO... floating input... garbage output.

author: T4P
author_rank: Super Contributor
timestamp: May 29, 2012, 05:12:47 pm
content: ^ in relation toQuote from: Mechatrommer on May 29, 2012, 04:25:32 pmTS: FIGO it out yourselvesOKAY, seems like i was being stupid.The feedback from the output to the opposite polarity to the input that is connected, is indeed the feedback loop, it keeps the gain setWithout the feedback loop it goes into open loop gain "mode"with the gain set but when nothing is going into the opamp so nothing comes out ( apart from the quiescent current or thereof )Nothing goes in, nothing goes out, so nothing is going BACK into the feedback pin therefore nothing coming outopamps can simulate a load, therefore it can establish a constant current ... anyway look at a schematic for a LM324 ... what does it tell you ?One can simply control an opamp to sink a load but i guess they can't sink by themselves can they ?How do you manage to bypass a CE junction without the base being equally fed? You can't . Either you haven't been looking at a datasheet for an opamp or don't know transistors much ...And anyway you normally can't make a transistor conduct in reverse.(Sorry i might come across as a asshole, don't take my words too seriously, i make a bad teacher)

author: ejeffrey
author_rank: Super Contributor
timestamp: May 29, 2012, 05:29:12 pm
content: Quote from: DaveXRQ on May 29, 2012, 05:12:47 pmAll opamps can sink and source current. If they can't, they aren't opamps. They might be regulators or comparators, but they aren't opamps.QuoteMr Pot, meet Mr Kettle.QuoteI am not sure what this even means.QuoteI don't know how any of this is relevant.QuoteI don't mind assholes, but uninformed assholes are rather annoying.

author: Mechatrommer
author_rank: Super Contributor
timestamp: May 29, 2012, 05:31:46 pm
content: Quote from: DaveXRQ on May 29, 2012, 05:12:47 pmGISO!... garbage in, smoking out!i did that one time. exceeded the diff.V limit of the input. no smoke, just dead.

author: BravoV
author_rank: Super Contributor
timestamp: May 29, 2012, 06:16:02 pm
content: Quote from: ejeffrey on May 29, 2012, 05:29:12 pmEjeffrey, not blaming you quoting that crap, its very annoying when reading this forum without ever bumping with this kinda craps, its almost everywhere.Although he is already in my ignore list, it seems like he keeps posting garbage in every thread created in this forum, unbelievable.

author: T4P
author_rank: Super Contributor
timestamp: May 29, 2012, 06:17:52 pm
content: Quote from: Mechatrommer on May 29, 2012, 05:31:46 pmby how much did you exceed the diff.V limit ?Quote from: ejeffrey on May 29, 2012, 05:29:12 pm

author: Mechatrommer
author_rank: Super Contributor
timestamp: May 29, 2012, 08:34:56 pm
content: Quote from: DaveXRQ on May 29, 2012, 06:17:52 pmby much much more than whats stated in the datasheet i found out later. but then since OP didnt mention what type of opamp he's talking about, what config, circuit etc, then my guess the actual answer is... GIGO (you should've guess that earlier) the fact is, maybe i'm having trouble understanding him.

author: croyleje
author_rank: Regular Contributor
timestamp: May 29, 2012, 11:03:25 pm
content: thanks amspire and everyone else i figured it was something like that just couldnt get my head around it myself. thanks

author: vk6zgo
author_rank: Super Contributor
timestamp: May 30, 2012, 12:12:14 am
content: Quote from: croyleje on May 29, 2012, 03:39:01 amYes,you can do just that,with an ac signal.It is a legitimate way of determining the output impedance of an amplifier.free_electron has a good description in reply #30 of the following thread:Re: The 50ohm Termination: How, Why and When?