Fig. 8 shows the circuit diagram of RC phase shift oscillator using transistor in CE configuration.
In this circuit, stabilized self-bias to the amplifier is provided by R1, R2, R3, R4 and Ce. Phase shift network (R-C network) provides a phase shift of 180o essential for positive feedback. The feedback signal is coupled through R‘ in series with the input resistance hie of the amplifier.
The value of R’ is such that R’+hie = R .
Working of phase shift oscillator
The base current of the amplifier is varied with the noise inherent in the transistor and hence oscillations are set up. The changing base current appears in the amplifier form in the collector circuit.
The output of the collector is fed to the phase shift network or R-C network, which produces a phase shift of 180o between the input and output voltages. The transistor in CE configuration also provides a phase shift of 180o between the input and output voltage.
Hence, a net phase shift of 360o or 0o is introduced between the input and output voltages, leading to positive feedback, which is the requirement for the sustained oscillations.
Analysis of RC phase shift oscillator
The equivalent circuit of phase shift oscillator ( shown in Fig.8.) is shown in Fig.9.
This simple equivalent circuit has been drawn by assuming that (1) hre and (2) hoe are very small.
Applying Kirchoff ‘s voltage law to the loop I, we get
Applying KVL to the loop II, we get
Applying KVL to the loop III, we get
Eliminating the values of I1 ,I2 and Ib in eqn. (1) to (3), we get
Comparing the coefficient of imaginary part of eqn. (4), we get
Which is the expression for the frequency of oscillation.
Now comparing coefficients of real part of eqn. (4), we get
which is the condition for the sustained oscillations.
Advantages of RC phase shift oscillator
- This type of oscillator provides good frequency stability.
- The circuit of this amplifier is simple.
- Since it consists of resistors and capacitors, so this type of oscillator is quit cheap.
- This oscillator has a wide range of frequencies.
- This oscillator is most suitable for low frequencies.
- The output of this oscillator is almost distortion free.
Disadvantages of RC phase shift oscillator
- Due to small feedback, the output is low.
- A high voltage battery is needed to develop large feedback voltage.