Phase Shifters

Author: J.B. Hoag

In Section 4.5 we learned that the current through a resistance reaches its peak at the same instant that the voltage across it reaches its maximum value, i.e., the current and voltage are "in phase" with each other. We also learned that the current through a condenser leads the impressed voltage by a quarter-period or 90°, while that through an inductance lags behind the applied voltage by 90°.

Fig. 19 H. An RC phase shifter

In Fig. 19 H, the current from the generator divides along the two paths r and RC. In path r, the voltage and current are in phase. In path RC, the current leads the generator voltage by something between 0° and 90°. Thus the voltage at point 4 reaches its peak value ahead of that at point 2. If R is large, this "phase difference" between 4 and 2 will be small, whereas if C is large it will be appreciable.

A phase shifter using an inductance and a resistance is shown in Fig. 19 I.

Fig. 19 I. An LR phase shifter

By changing L and/or R, the phase difference between the current and voltage through the load can be changed.

Fig. 19 J. A bridge-type phase shifter

Figure 19 J shows a bridge-type phase shifter where adjustments of the resistances and condensers permit of changing the output phase from 0° to 180° with respect to the input. Reversing the leads of either the input or the output gives phase changes from 180° to 360°. Figure 19 K shows practical circuits for the phase control of gas-filled tubes.

Fig. 19 K. Circuits for the phase control of thyratrons