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I have come across about active all pass filters which provides phase shift with unity gain.

I dont know this is the common practice in part of circuit design. For a fixed or variable unity gain phase shifting application is this the common way? Is it the only way?

enter image description here

Is the above topology common way to go or is there a more ad hoc IC for the purpose of phase shifting? I know it could be a very large topic but any big picture would make things clear.

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  • \$\begingroup\$ In what frequency band? Is current buffering needed? How much output current is needed? \$\endgroup\$
    – The Photon
    Commented Mar 26, 2019 at 18:39
  • \$\begingroup\$ Sorry don’t know what to say this was to find an overview methods. But Im guessing this topology wont work for very low frequencies less than 10Hz or so. For very. Low freq I guess one uses digital techniques. I must say the freq Im asking about greater than 10Hz smaller than 100MegHz. Output current can be low since it can be buffered by a transistor circuit. \$\endgroup\$
    – floppy380
    Commented Mar 26, 2019 at 18:53

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

schematic

simulate this circuit – 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 & 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.

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This is an inverting (180 deg) unity gain Op Amp at DC and unity gain non-inverting at high frequency (Av+ = 2 + Av- = -1 equals +1= non-inverting)

The intermediate leading phase shift is the RC breakpoint with -90 deg. The expected phase shift from DC to max is -180 deg to -90 deg to 0 Deg, which occurs mostly over +/- 1 frequency decade centred around the breakpoint of RC. Actual phase shift +/-1 decade is -169 deg and -11 deg and is linear over +/- 1 octave.

Inverting low frequency

enter image description here

Swapping the RC pair, results again in a phase leading shift rising with f from 0 to +90 to + 180 deg phase shift, centred at the breakpoint for RC. Also known as, (aka) the non-inverting phase shifter.

Non-inverting low frequency

enter image description here ( the above shows a 1KHz breakpoint at +90 deg going from 0 to 180 deg.)


Practical large values for RC= 160s using 10M and 16uF for e-caps result in a breakpoint of 1 millihertz.

The upper-frequency range begins to lag about 1 decade below the GBW product.


More commonly in most SMPS voltage feedback circuits and phase error in PLL`s you will need a phase lead compensation circuit to improve step response (improved stability to prevent 2nd order ringing and overshoot) by adding 40 degrees of phase lead compensation to a negative (inverting) feedback loop.

It may only have 2 R for DC ratio and 1 C in the external circuit with an internal Op Amp. Yet here, shown with 3 R`s to limit the DC gain(x10) and the high frequency gain (x1). There are many variations of this.

enter image description here

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