RC phase shift oscillator are commonly used for generating audio frequencies, that is 1Hz to 10MHz.
RC phase shift oscillator consist of an inverting amplifier followed by a feed back network with phase shift of 180 degree. The 180 degree phase shift at the feedback can be obtained by the combination of resistors and capacitors. The transistor amplifier itself produces a phase shift of 180 degree. So the RC phase shift oscillator produces a sine wave output signal using regenerative feedback from RC combination. This regenerative feedback from RC network is due to the ability of the capacitor to store an electric charge
The R and C of the phase shift network must be selected so as to produce a phase shift of 180 degree at the desired frequency of oscillation. The generalised equation for the frequency of oscillation produced by a RC phase shift oscillator is given by
Where N is the number of RC stages.
Figure shows the circuit of a RC phase shift oscillator. The circuit consist of a amplifier using NPN transistor followed by a 3-section RC phase shift network. The resistor R1 and R2 form a voltage divider that provides dc emitter base bias. RE and CE provides the temperature stability and prevents AC signal degeneration. The output of oscillator is coupled to the load through capacitor Cc.
OPERATION OF RC phase shift oscillators
The circuit is set in to oscillations by minor variation in the power supply. This variation at the base current is amplified by the amplifier. The output of the amplifier is 180 degree phase shifted and is applied to the RC feedback network. The RC network produces a phase shift of 180 degree. Hence the circuit provides a phase shift of 360 degree or 0 degree which is essential for sustained oscillations or to satisfy Barkhausen Criteria.
Because of 180 degree phase shift obtained in the phase shift network, the network attenuates the output voltage by a factor of 1/29. This means that amplifier must have a voltage gain of 29 or more.
Then the feedback factor of RC network
Then loop gain
BA=1/29 × 29 =1
This means that it satisfy Barkhausen Criteria.
ADVANTAGES OF RC phase shift oscillators
- It is simple and cheap circuit because it contains only resistors and capacitors
- The sinusoidal output is quite distortion free
- These are suitable for very low frequencies about 1Hz
- It has good frequency stability
- Negative feedback and stabilization arrangements are not required
DISADVANTAGES OF RC phase shift oscillators
- It is difficult for the circuit to start oscillations as the feedback is very small
- It needs high feedback voltage
- Output is small because of smaller feedback
- Frequency stability is not so good
APPLICATIONS OF RC phase shift oscillators
RC phase shift oscillator is used for generating signals power a wide frequency ra nge. The frequency can be varied from few Hz to 200 Hz by employing one set of resistor with three capacitor ganged together to vary over a capacitance range in the 1:10 ratio. Similarly the frequency ranges of 200Hz to 2KHz, 2KHz to 20KHz, and 20KHz to 200KHz can be obtained by using other set of resistors.
Created by : Anju Radhakrishnan
Image courtesy : Google image