buffer stages performance

emosms · 2nd September 2012, 12:25 AM

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 · 2nd September 2012, 12:31 AM

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

Update My Dynaco

nigelwright7557 · 2nd September 2012, 01:16 AM

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 · 2nd September 2012, 09:52 AM

thx!
both provide 1M input impedance, right

rcw666 · 3rd September 2012, 03:08 AM

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

2nd September 2012, 01:16 AM	#3
nigelwright7557 diyAudio Member Join Date: Apr 2008 Location: Carlisle, England	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. __________________ http://www.murtonpikesystems.co.uk PCBCAD50 pcb design software.

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2nd September 2012, 12:31 AM	#2
djoffe diyAudio Member Join Date: Oct 2008	the inverting gain stage (gain=-1) should work ok, but it will be a bit noisy, given the 1 Meg input and feedback resistors. Update My Dynaco

2nd September 2012, 09:52 AM	#4
emosms diyAudio Member Join Date: Sep 2010	thx! both provide 1M input impedance, right

3rd September 2012, 03:08 AM	#5
rcw666 diyAudio Member Join Date: Aug 2012	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