# Purpose of resistor across op-amp inputs

The following (from TI app note slaa414a) is an active bias-T that sums the AC and DC inputs to drive a load for the purpose of testing PSRR. What does the 100 Ohm resistor across the inputs do? Both inputs have DC paths without it.

• This is a current feedback amplifier, not a conventional op amp. – user69795 Oct 27 at 23:49
• I suspect the resistor doesn't do anything, that it was accidentally carried over from a prior design. Possibly a differential signal termination. – 65Roadster Oct 28 at 18:01
• Max differential input voltage is listed as +- 4V. May also be related opamp behavior when powered but not connected to a DC input source (positive input floating). – sstobbe Nov 2 at 1:10

The 100Ω shunts some of the feedback signal away from the inverting input and decreases the feedback factor (commonly labeled $$\\beta\$$ in texts on loop gain analysis). Decreasing the feedback decreases the loop gain ($$\A_{ol}\beta\$$) and the loop gain becomes unity at a lower frequency.
Since the open loop gain ($$\A_{ol}\$$) of the op amp has more phase lag at higher frequencies, it is desirable to make the loop gain unity at a lower frequency to decrease the amount of phase lag from the op amp. This improves phase margin and stability.