[sdiy] input buffer question

[Neil Johnson]

Hi,

Scott Nordlund wrote: 
> 
> Clamping diodes may also offer the benefit of preventing the dreaded
> "phase inversion" distortion that some op amps are susceptible to
> in this configuration.  Not that it's likely to be a big issue if you're
> powering it from +/- 15V.

Actually, quite the opposite.  Phase inversion/reversal is (copied from my website):

"Phase reversal happens when the input signals exceed the common-mode input range and negative feedback turns into positive feedback. The result is that the output of the op-amp latches on to one of the rails and stays there until power is removed. "

For the TL07x family the typical common mode input range is -12V to +15V.  For example, V- at -15V and V+ to +15V (or vice-versa) will more than likely put it into the phase reversal state.

If you want to avoid phase reversal you could, for example, put anti-parallel diodes across the inputs.  In that case I would also suggest putting a feedback resistor in, even though in the unity-gain buffer case it doesn't have any effect on the ideal gain calculation.

Neil
--
http://www.njohnson.co.uk