2-22
Phase-Shift Oscillators
The PHASE-SHIFT OSCILLATOR, shown in figure 2-17, is a sine-wave generator that uses a
resistive-capacitive (RC) network as its frequency-determining device.
As discussed earlier in the common-emitter amplifier configuration (figure 2-17), there is a
180-degree phase difference between the base and the collector signal. To obtain the regenerative
feedback in the phase-shift oscillator, you need a phase shift of 180 degrees between the output and the
input signal. An RC network consisting of three RC sections provides the proper feedback and phase
inversion to provide this regenerative feedback. Each section shifts the feedback signal 60 degrees in
phase.
Since the impedance of an RC network is capacitive, the current flowing through it leads the applied
voltage by a specific phase angle. The phase angle is determined by the amount of resistance and
capacitance of the RC section.
If the capacitance is a fixed value, a change in the resistance value will change the phase angle. If the
resistance could be changed to zero, we could get a maximum phase angle of 90 degrees. But since a
voltage cannot be developed across zero resistance, a 90-degree phase shift is not possible.
With a small value of resistance, however, the phase angle or phase shift is less than 90 degrees. In
the phase-shift oscillator, therefore, at least three RC sections are needed to give the required 180-degree
phase shift for regenerative feedback. The values of resistance and capacitance are generally chosen so
that each section provides about a 60-degree phase shift.
Resistors RB, RF, and RC provide base and collector bias. Capacitor C
E
bypasses ac variations around
the emitter resistor R
E. Capacitors C1, C2, and C3 and resistors R1, R2, and R
B
form the feedback and
phase-shifting network. Resistor R2 is variable for fine tuning to compensate for any small changes in
value of the other components of the phase-shifting network.
When power is applied to the circuit, oscillations are started by any random noise (random electrical
variations generated internally in electronic components). A change in the flow of base current results in
an amplified change in collector current which is phase-shifted the 180 degrees. When the signal is
returned to the base, it has been shifted 180 degrees by the action of the RC network, making the circuit
regenerative. View (A) of figure 2-18 shows the amount of phase shift produced by C1 and R1. View (B)
shows the amount of phase shift produced by C2 and R2 (signal received from C1 and R1), and view (C)
shows the complete phase shift as the signal leaves the RC network. With the correct amount of resistance
and capacitance in the phase-shifting network, the 180-degree phase shift occurs at only one frequency.
At any other than the desired frequency, the capacitive reactance increases or decreases and causes an
incorrect phase relationship (the feedback becomes degenerative). Thus, the oscillator works at only one
frequency. To find the resonant frequency (fr) of an RC phase shift oscillator, use the following formula:
where n is the number of RC sections.
