new (to me) all-pass circuit

It occurred to me that, in the conventional opamp all-pass 90 degree phase shifter circuit,

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a fast signal passes through all the opamp stages, so all the amps have to be fast. In this one,

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the fast signals just rip through the big caps in the slow stages. So in a high-order shifter we could use slow, cheap, low-noise amps in the first few stages and just use fast opamps towards the end. That will make less HF distortion too.

The down side is that the output impedance isn't low, like an opamp would be, so cascading stages gets a little more difficult, or more interesting depending on your attitude.

John Larkin         Highland Technology, Inc 

lunatic fringe electronics
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John Larkin
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If you split the capacitor into say 2/3rds on the left and 1/3rd on the right and picking an "approrpiately sized" resistor from the center tap to the inverting input it should improve the frequency response into a load, at the cost of it phase-shifting the opposite way.

e.g. with 0.68uF on the left 0.22 on the right, 47k resistor from the tap, into a 10k load, with LT1014 the output amplitude hasn't changed at all at 10 Hz vs down 100mV with the stock circuit

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For the just RC phase sequence filter the order of the R's and C's don't matter. I'm not sure if that's true for the all-pass version. But it might help you deal with the output impedance issue, when cascading.

George H.

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George Herold

On a sunny day (Sat, 22 Sep 2018 08:42:53 -0700) it happened John Larkin wrote in :

Thank you, did never read it.

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