op amps ... again? - Page 1
Forum section: electronics / beginners

author: ilbs
author_rank: Contributor
timestamp: October 06, 2013, 11:48:59 pm
content: Hello,I thought I had it down packed, but no, this combination of voltages doesn't work with any of the 4 calculative attemptsIts an op amp with a 9vdc to 12Vdc ramp at its inverting input and a steady 5 Vdc at its non inverting input. The output was selected as going from 5.5 down to 3.5 Vdc.In my last post, a fellow warned me to try several values for the gain and I tried doing so and which have always resulted in weird anomalies like the one shown inattachment ...Once I even got a -1.5Vdc as Vgnd... which would be impossible to attain with the value of V2 shown in the attachment!I tried gains like 2.0, 0.5 ect...So confused!Can anyone help please

author: c4757p
author_rank: Super Contributor
timestamp: October 06, 2013, 11:53:11 pm
content: Simple... 5V isn't enough. The virtual ground point is going to be somewhere between the input and output ranges. Right in the middle of 9-12V and 5.5-3.5V is around 7V.

author: ilbs
author_rank: Contributor
timestamp: October 07, 2013, 01:31:42 am
content: Quoteokay.... if you say so!!! I will take your word for it.So there are certain situations that are not doable !!thanks

author: c4757p
author_rank: Super Contributor
timestamp: October 07, 2013, 01:42:41 am
content: Quote from: ilbs on October 07, 2013, 01:31:42 amEasy enough to imagine for yourself . You can feed 5V into a 5000 gain amplifier all you want - you're not getting 25 kilovolts. If you don't have it you can't get it - and the same applies for trying to get 7V out of a voltage divider when you've only put in 5V.If the power supply rails are stable enough, just use them. Assuming they're close to +-12V or +-15V, you should have plenty of voltage there to pick off whatever you want with a voltage divider.

author: w2aew
author_rank: Super Contributor
timestamp: October 07, 2013, 02:32:45 am
content: Ok, so you want a convert a ramp that goes from 9 to 12v into a ramp that goes from 5.5v down to 3.5v, right?You're taking a 3v swing and dropping to a 2v swing, and you're inverting, so you want a gain of negative 2/3rds. So, something like 2.2k from output to inverting input, and 3.3k from inverting input to the 9-12v ramp. Be careful of your choice for op amp, some might not be stable with a gain of less than unity.Now, some simple algebra says that the voltage at the non-inverting input that gives you the output swing you want is 6.9v. So, make a resistor divider between your 5v input and some convenient voltage source (positive supply rail?) to give yourself a 6.9vdc input on the non-inverting input.Done.

author: c4757p
author_rank: Super Contributor
timestamp: October 07, 2013, 02:37:19 am
content: Quote from: w2aew on October 07, 2013, 02:32:45 amOr just scrap the 5V input and do it between V+ and ground!

author: w2aew
author_rank: Super Contributor
timestamp: October 07, 2013, 02:43:18 am
content: Quote from: c4757p on October 07, 2013, 02:37:19 amI was assuming that the OP had a particular reason for needing the +5v as the other input...

author: ilbs
author_rank: Contributor
timestamp: October 07, 2013, 02:52:00 am
content: okay okay I got it guys..... I see the dilemmaI'm just experimenting with values and I realize now thatat best Vra can be 3.5Vdc making my inv input 5.5Vdc....and therefore I can't squeeze more lemon juiceout o a dried out lemon.... the lemon being the 5vdc....so okay I got it thanks for your hep

author: c4757p
author_rank: Super Contributor
timestamp: October 07, 2013, 02:54:16 am
content: Quote from: ilbs on October 07, 2013, 02:52:00 amMaybe you could squeeze your extra 2V out of the lemon...

author: w2aew
author_rank: Super Contributor
timestamp: October 07, 2013, 03:36:18 am
content: Quote from: ilbs on October 07, 2013, 02:52:00 amHmm, ok. My suggestion didn't suit your needs? Worked on my bench...https://www.eevblog.com/forum/beginners/op-amps-again/msg303228/#msg303228