# How does the resistance-capacitance oscillator shift voltage by 180 degrees?

I'm trying to wrap my head around how the resistance-capacitance oscillator shifts voltage by 180 degrees. As far as understand, a capacitor shifts phase for current, not voltage.

What am I missing here?

• ... that current develops a voltage across the following R? Jan 5 '21 at 22:48
• What if the capacitor shifts current as you say, and resistor converts that current to voltage, so you get voltage shift? Jan 5 '21 at 22:50
• You might refer to this answer for some thoughts that may help.
– jonk
Jan 6 '21 at 4:14
• Simple answer: A 3rd-order highpass provides (for very large frequencies) a phase shift between input and output of 270deg. Hence, there must be a frequency in between which causes a phase shift of 180 deg.
– LvW
Jan 6 '21 at 10:02

You are correct that a capacitor "shifts phase for current, not voltage" or more accurately, the current in a capacitor is out of phase with the voltage. If the capcitor were ideal, the phase difference would be $$\90^{\circ}\$$.
If a voltage sine wave is applied to a capacitor and resistor connected in series, and an output voltage is measured across the resistor, there is a frequency dependant phase shift between the voltage across the resistor as compared to the voltage across the both components together. That phase shift, (for ideal components), lies between $$\0^{\circ}\$$ and $$\90^{\circ}\$$.
If the phase shift networks were identical, and separated by amplifiying or buffering circuitry, at the frequency where each network shifts the phase by $$\60^{\circ}\$$, their combined effect will be to shift the phase by $$\180^{\circ}\$$, providing an appropriate phase shift for an oscillator.