Weird opamp results [solved] - Page 1
Forum section: electronics / beginners

author: xchip
author_rank: Regular Contributor
timestamp: June 23, 2013, 01:37:48 pm
content: Hi guys,I put together the circuit you can see in the picThe gain should be -100k/1k = -100.Since the input is connected to ground I'd expect to see 0 volts at the output, unfortunately I get a dc offset of about +2 volts.Any idea why is that? I am using an LM324N with a single supply of 5 volts, and I create virtual ground using a voltage divider.Thanks!

author: SeanB
author_rank: Super Contributor
timestamp: June 23, 2013, 03:26:25 pm
content: A combination of offset voltage, input bias current and too high a gain expected from the stage. You need to use either lower value resistors ( 10k and 100R will do as a try) and a better opamp with lower bias current. Dave did a nice set of videos about this in the last month, worth a watch.

author: xchip
author_rank: Regular Contributor
timestamp: June 23, 2013, 05:14:45 pm
content: Quote from: SeanB on June 23, 2013, 03:26:25 pmThe typical one Vos in this op amp is 5mV, with a gain of 1000 I should expect at the output an error of 5mV*1000 = 5 V, is this the reason?

author: SeanB
author_rank: Super Contributor
timestamp: June 23, 2013, 05:45:28 pm
content: Remember this is a bipolar amp with lateral PNP input transistors, they have a bias current that needs to be supplied as well. Bias current may not be the same for each input ( there is a input bias offset current as well as the voltage) and running it off 5V will probably saturate internal transistor nodes, and will give an incorrect output. 5V is a little on the low side, use a higher voltage ( like 15V) or a different opamp that is rail to rail both on input and output.

author: Rufus
author_rank: Super Contributor
timestamp: June 23, 2013, 05:57:49 pm
content: Quote from: xchip on June 23, 2013, 01:37:48 pmSometimes wonder why Dave bothers to make his vblogs.......

author: SeanB
author_rank: Super Contributor
timestamp: June 23, 2013, 06:09:11 pm
content: Quote from: Rufus on June 23, 2013, 05:57:49 pmHorses and watwer come to mind, but all of us have at some point been beginners.I went to write my learners license this week, even though I have been driving for a long time, I want to get a heavy duty drivers license to drive over a 3.5 ton vehicle. Had to refamiliarise myself with the rules, and the signs and such. Lucky I know a guy who will let me drive his truck with him along to practise, so I can pass the practical side and get a new card.

author: xchip
author_rank: Regular Contributor
timestamp: June 23, 2013, 07:03:40 pm
content: Guys,The Input Offset Voltage of this op amp is 2mV, with a gain of 1000 that gives me an offset at the output of 2V exactly. And yes, Dave talks about this in one of his videosI want to use the op amp to amplify some signal from a coil and I really need a way to kill Vos effect (adding a cap at Vout is not a solution)

author: fcb
author_rank: Super Contributor
timestamp: June 23, 2013, 07:50:24 pm
content: Where does the OP state they want a gain of 1000?My reading of the drawing is they will end up with 100 (-100 to be correct).And with a 5mV Vos they shouldn't end up with more than 500mV Vos error. Also, the bias current is in the nA region so shouldn't contribute anything significant.Probably suggest they look more carefully at the virtual ground scheme they are using - please post a complete schematic xchip.Another thing dawned on me. Let's speculate they are using main rails of 0v and 5v and have created a 2.5v virtual ground, to which the opamp INV and transducer a connected to. Let's say the have a 5mV Vos and a gain of -100 then I would expect the output to idle at between 2.0v and 3.0v depending on the polarity of the Vos.Perhaps they are measuring the output of the OPAMP w.r.t. system 0v and not the virtual ground??

author: Andy Watson
author_rank: Super Contributor
timestamp: June 23, 2013, 08:03:14 pm
content: Quote from: fcb on June 23, 2013, 07:50:24 pm+1There is a slightly fuller circuit in the top corner of the latest picture. The ground appears to be defined by two resistors labelled only as R. On the breadboard these appear to have a value of 10k.

author: xchip
author_rank: Regular Contributor
timestamp: June 23, 2013, 10:03:47 pm
content: Quote from: Andy Watson on June 23, 2013, 08:03:14 pmYou nailed it guys, it was the virtual ground, the voltage divider was not a good one...Now I set the gain to 150, I added a buffer op amp and now I'm getting 0.14V ~ 0V without it was getting 1.16V which is very wrong.Awesomeness, this problem had been worrying me for a long time and finally it's solvedwohoo!! Thanks everyone!