buffer architectures without dc level shift

[Arielle]

Hi guys,
it's me again... with another question: I use resistive feedback to have a variable gain. Therefore I need a buffer at the output of my opamp. Until now I have an ideal buffer but the problem is that the real buffer should not have a dc level shift (like source follower) and it should be possible to drive at least 200kOhms resistances (rather less). Does anyone have a good architecture in mind?
Is it even possible to get such a thing?
Arielle

 

[crutschow]

Most op amps can drive a load as low as 2k ohms. Why do you need a buffer amp for 200k ohms? :-?

If you put the buffer inside the op amp feedback loop, the offset will be essentially cancelled by the feedback loop gain.

 

[FvM]

It should be said that the question is apparently related to single stage amplifiers as in your previous post, otherwise it's hardly understandable.

Analog design text books are full of circuit structures for many purposes, including buffers.

 

[Arielle]

Oh my ***... everytime I post sth. I feel super stupid here and by now I truly hate electronics... I just have to stick to it a couple of months more before it will be over forever... but nevermind it: I would immediately add a second stage to my single stage if I'd understand why such a thing should be able to drive 2kOhms load... do you have any example, a screenshot, anything? I have no idea how I could put your suggestion into praxis.
And as to your post FvM: I have truly tried many buffers from books (voltage follower, flipped voltage follower, super follower and more but I dont know their names... I estimate it should be about 10 circuits at least that I tried.) Oh... I also used the book from Baker, Razavi, Allen & Holberg, Sedra & Smith. It's not like I didn't try you know, it's just not working so I wanted to ask experts for help...

 

[KerimF]

In this case, seeing your not-working circuit as a schematic would give the experts a greater chance to analyse better your problem.

 

[Arielle]

Yeah, right... tomorrow morning when I have access to the program again, you can have a look at all the things I tried. Not a problem for me, just a lot of work for whoever wants to help me.

 

[Arielle]

So... in the attachment you can find the things I tried. The one buffer that works best but consumens 4 times the current of my opamp is actually connected to my opamp right now.
Perhaps one of you can judge better than me which buffer has a chance to work best for my specifications:
I want no more than 2µA current consumption, 600mV input and output swing (1.8V supply) the resisitors that have to be driven are 200kOhms or less... yeah, that's it... hope someone can help... by the way, I basically stole all structures from papers even if I didn't understand the working principle...