op amps ... again?

[ilbs]

Hello,

I thought I had it down packed, but no, this combination of voltages doesn't work with any of the 4 calculative attempts 

Its 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 in
attachment ... 

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   

[c4757p]

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.

[ilbs]

Posted by: c4757p
« on: Today at 10:53:11 AM » Insert Quote
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.

okay.... if you say so!!! I will take your word for it.

So there are certain situations that are not doable !!

thanks

[c4757p]

So there are certain situations that are not doable !!

Easy 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.

[w2aew]

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.

[c4757p]

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.

Or just scrap the 5V input and do it between V+ and ground!

[w2aew]

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.

Or just scrap the 5V input and do it between V+ and ground!

I was assuming that the OP had a particular reason for needing the +5v as the other input...

[ilbs]

okay okay I got it guys..... I see the dilemma 

I'm just experimenting with values and I realize now that
at best Vra can be 3.5Vdc making my inv input 5.5Vdc....

and therefore I can't squeeze more lemon juice
out o a dried out lemon.... the lemon being the 5vdc....

so okay I got it thanks for your hep 

[c4757p]

and therefore I can't squeeze more lemon juice
out o a dried out lemon.... the lemon being the 5vdc....

Maybe you could squeeze your extra 2V out of the lemon...

[ilbs]

 

[w2aew]

okay okay I got it guys..... I see the dilemma 

I'm just experimenting with values and I realize now that
at best Vra can be 3.5Vdc making my inv input 5.5Vdc....

and therefore I can't squeeze more lemon juice
out o a dried out lemon.... the lemon being the 5vdc....

so okay I got it thanks for your hep 

Hmm, ok.  My suggestion didn't suit your needs? Worked on my bench...
https://www.eevblog.com/forum/beginners/op-amps-again/msg303228/#msg303228