Gain Boosted Amp Layout

[sharkies]

Do you have any suggestions on how to layout an Gain boosted Amp?
I'm not so sure if I should place the two differential gain enhancing amplifiers in a stacked fashion between the PMOS and the NMOS tail current source of the main amp. Or I could place the gain enhancing amp on oneside of the main amp and the other one on the otherside? Stacking will give a more symmetric form.

any experience?

 

[erikl]

You can build both layout types rather symmetrically (by repetition, not mirroring).

I think the required aspect ratio and/or simplicity of routing (lower parasitics) should make the decision.

 

[sharkies]

I'm not sure what you mean here.
If I stack them, then the layout get vertically long.
But the two boost amps are different (one is PMOS folded, other is NMOS folded) and if I place them at the sides of the main opamp the symmetry is not maintained. What do you mean by repetition?

Thanks again~

 

[erikl]

If I stack them, then the layout get vertically long.

Of course.

But the two boost amps are different (one is PMOS folded, other is NMOS folded)

Didn't think of this, sorry.

... and if I place them at the sides of the main opamp the symmetry is not maintained. What do you mean by repetition?

Repetition is only possible for equal blocks. This means copying - not mirroring! - the block from the left side to the right side (or vice versa).

I think absolute symmetry is not possible and not so important in your case. Take the alignment
- which better suits the required aspect ratio
- or
- if more important because of frequency consideration - which uses shorter connections, i.e. smaller parasitic caps.