buffer stages performance

[emosms]

Hi,
I found these opamp buffer stages from the internet:

Apart from the phase flip (inverting-noninverting)
is there going to be any deterioration in quality for the inverting schemtaics?
The signal goes throug 1M resistor, then the amplification is set pretty high, is it worse to have it like this?`
Best Regards

 

[djoffe]

the inverting gain stage (gain=-1) should work ok, but it will be a bit noisy, given the 1 Meg input and feedback resistors.


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[nigelwright7557]

The 1 meg will pick up all sorts of frequencies that are floating around if the input is left unconnected.
The 1 meg also will give a phase shift in conjunction with the op-amp input capacitance. If this gets large you could end up with distortion or positive feedback.
The amplification is 1 in both cases.

 

[emosms]

thx!
both provide 1M input impedance, right

 

[rcw666]

Both have the same nominal input impedance but the inverting one puts one Mega Ohm of thermal noise directly in series with the signal and that is not good.

If you want an inverted signal a much better scheme is to use a second op amp connected in an inverting mode. With a 5534 type you can use a 1k series resistor.
This will give about as much thermal noise as a moving magnet cartridge gives, 60db. less than a one meg resistor in series.
rcw