Phase shifting with unity gain
The other slight variation of unity-gain phase shifter is:







simulate this circuit &ndash; Schematic created using CircuitLab

To keep things simple, R1xC1 sets the phase, requiring a well-behaved source resistance of V1 (assumed here to be insignificantly small). And the op-amp is chosen not to add much phase shift too. That is, its gain-bandwidth product is much higher than the useful bandwidth of the network.

These networks are often ganged in a series string to provide a wider band phasing network.

Phase &amp; amplitude error tolerance is often very tight, so good temperature coefficient of components is important, as well as component tolerance. Even fractions of a degree  phase error and fractions of a dB gain variation can compromise a 90-degree phasing network used in sideband cancellation of a single-sideband phasing network.
Digital methods are an attractive alternative.