Re: To maintain CMRR (common mode rejection ratio) of op amp

"Ancient_Hacker" <grg2@xxxxxxxxxxx> wrote in message
news:1164201871.086535.225040@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx

[snip]

Also any attempt at improving CMMR is going to require all those
resistors hanging on the op-amp inputs to be of very high precision and
stability. For 60dB CMMR you'll need resistors of considerably better
than 0.1% precision and stability. That's assuming the transistors do
anything.


[snip]

That's where the long-tailed pair can come into its own, with most of the
common-mode voltage supported across the transistors' c-e.

Say you have a tail resistor R, with bias voltage Vt across it and two
matched collector resistors to ground of 2R, then you have a common-mode
output voltage of Vt to deal with in a subsequent diff amp, which could be
much less than the common-mode at the supply. Errors then reduce to
transistor pair offset performance and mismatch between the two collector
resistors as a function of Vt, not Vcm.

Unfortunately, I'm blocked from accessing the link to the circuit here at
work, so I'm unable to add anything further in that respect.

--
T

If it's not broken, don't fix it.



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