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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 EF 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 RF is critical. The upper limit of the oscillator frequency drops as RF 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.

Last Update: 2010-11-17