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OpAmp OPA835 single rail design

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raff5184:
yes that's what the single stage looks like, thanks for confirming. Yes the gain problem is clear, I know I can't reach the 60dB. I just want to understand the circuit and the OpAmp now.

The two opamps are simply in series, not in the same loop.

Zero999:
When you said in series, did you mean like this?

SiliconWizard:
If the input signal is AC-coupled anyway, you may want to AC-couple the second stage as well, thus avoiding for it to amplify the output offset of the first stage (in case it could be significant).

Zero999:

--- Quote from: SiliconWizard on September 19, 2019, 08:31:43 pm ---If the input signal is AC-coupled anyway, you may want to AC-couple the second stage as well, thus avoiding for it to amplify the output offset of the first stage (in case it could be significant).

--- End quote ---
Well no because:
1) You'd need an extra resistor between the non-inverting input and Vref, to bias the op-amp.
2) AC coupling the second stage is unnecessary, as the second stage has a unity gain as DC, so it won't amplify the offset voltage.
3) DC coupling the two stages gives the lowest part count.

SiliconWizard:

--- Quote from: Zero999 on September 19, 2019, 10:07:09 pm ---Well no because:
1) You'd need an extra resistor between the non-inverting input and Vref, to bias the op-amp.
--- End quote ---

Well, of course. You'd need to duplicate the first stage, biasing the input properly. Didn't mean to simply add a series capacitor.


--- Quote from: Zero999 on September 19, 2019, 10:07:09 pm ---2) AC coupling the second stage is unnecessary, as the second stage has a unity gain as DC, so it won't amplify the offset voltage.
--- End quote ---

Right, thanks to the capacitor between R_G and ground. I had another configuration in mind. So nevermind, with your schematic, it won't *amplify* the output offset of the first stage, as this is a gain of 1 at DC.

To nitpick though (even though again I actually had another topology in mind), there is still a unity gain at DC, and not zero gain. So the output offset of the first stage will still add up to the output offset of the second stage. Probably not a big deal here. But that's still not quite like AC-coupling between the first and second stage. :P


--- Quote from: Zero999 on September 19, 2019, 10:07:09 pm ---3) DC coupling the two stages gives the lowest part count.

--- End quote ---

That's for sure.

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