Phase Shift Compensation

Author: Leonard Krugman

Fig. 6-13. (B) Base-controlled phase compensated oscillator.

The reduction of value of the in-phase feedback signal requires either an increase in feedback E_F or a form of phase compensation to decrease the angle φ. In the base-controlled oscillator, some phase shift compensation is provided by shunting either or both the emitter and collector electrodes to ground through a small capacitor (3 or 4 pF). This simple modification usually doubles the upper frequency limit of a transistor.

One method of increasing the available feedback is to connect a resistor from the emitter to a tap point on the base tank coil. This provides regenerative voltage feedback to supplement the inherent current feedback of the circuit. The value of the resistor R_F is critical. The upper limit of the oscillator frequency drops as R_F is either increased or decreased from its critical value. For this reason, the feedback resistance is best determined on an experimental basis. Figure 6-13 (B) illustrates a basic oscillator incorporating these two methods of phase shift control and compensation.