# Electronic – 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.

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.