I'm looking for a smart solution to obtain a different gain for a not
inverting OpAmp The goal is have the same gain at the beginning and at the
end of the ramp source 0-5V DC signal (NO FREQUENCY) and a different gain in the middle of the ramp.

As example (where Vi= DC ramp signal, A=OpAmp gain)

Vi=1 A=2, Vi=2 A=1,5 Vi=3 A=1 Vi=4 A=1,5 Vi=5 A=2

In the attached img in red the source ramp, Vu1 and Vu2 the possible results
wanted

Thanks for any help

Do you realise you have "triple" posted!..


>> Vi=1 A=2, Vi=2 A=1,5 Vi=3 A=1 Vi=4 A=1,5 Vi=5 A=2

Do you want the Gain to change abruptly at these points or to change slowly as you have drawn?

Please don't make duplicate posts, they're very annoying and don't help you or us to help you. Creating duplicate threads does not increase your chance of a useful response, it only reduces it as people get confused and can't always see what others have suggest in the other thread.

Why do you want to do this?

What exactly are you trying to do?

The chances are you're doing it the wrong way.

hi brian,

If he wants abrupt Gain transitions I was thinking along the lines of using a dual comparator window detector
that would run in 'parallel' to the main amp.

If the comp window was setup at the transition voltages it could switch in resistors on the input of the main amp,
using FET's..

He may want a gradual change in Gain, he hasnt replied.

Regards

Eric,

Yes that is a bit of a grey area. The graph seems to show a gradual transition though so I'm tending to sway towards that for the moment. You do have a good idea though and it may well prove to be far simpler than mine. I always think the KISS way is best (Keep It Simple, Stupid).

I was thinking of using a colpitts oscillator with varicaps which would alter the output frequency in response to a change in voltage. The output of the oscillator could then be fed into an Op-Amp filter with cut off points set where he wants the gain to fall off.

Brian

As with many posts, it would be far more helpful if you said why you need this, and EXACTLY what you're trying to do.

Originally Posted by ThermalRunaway

Irrelevant in this particular example as sufficient explanation of the problem was given (all be it some minor details unclear)

It is relevant.

It's highly likely that there is a far better way of doing what he wants.

All too often people approach things from completely the wrong direction. Then (after further intergeneration and 50 odd replies) they tell us exactly what the circuit for so we point them in the right direction and something that appeared quite complected turns out to be pretty simple.

Read his post!

I did, if he worded it properly then maybe I might understand what exactly.

If he told us exactly why he wants to do this then we could point him in the right direction.

Such a negative response. He didn't actually offer any of his own suggestions, so I'm not sure where you get that from. But he painted the basics of the problem quite clearly from where I'm sitting.

How was my response negative?

The only negitive comment my post contained was a reprimand for posting the same thread three sodding times so he bloodywell deserved the negative comment!

Oh I didn't notice his reply!

Originally Posted by pagini

I wolud avoid to solve it with very complex circuit or with a microPIC

Do you mean you don't want to use a PIC?

Surely wouldn't that be the best solution though?

The only other thing I can think of that would be able to give such an odd response is an op-amp with the feedback resistors connected via analoge switchs which are connected to some comparators which would be horrible.

hi Pagini,

Are we 'hitting the can'?

Need more input!