Phase Shift Audio Frequency Oscillators

Phase Shift Audio Frequency Oscillators Points : phase shift audio frequency oscillators In the phase shift oscillator an external resistor capacitor (R-C) network feeds a portion of the ac output of an amplifier back to the amplifier input, lithe amplifier has an internal phase shift of 180° degree, and the phase shift network provides a further phase shift of 180 degree, the signal feedback to the input can be amplified to reproduce the output. The circuit is then generating its own input signal and a state of oscillations is sustained. An R-C phase shift oscillator with phase lead network in the feedback path is illustrated in fig.

An R-C network consisting of three equal value resistors and three equal value capacitors is connected between the amplifier output and the input terminals. Each R-C section provides a phase shift of 60 degree. Thus a total phase shift of 3 x 60 = 180° degree is provided by the R-C feedback network. The output of this network is now in the same phase as the originally assumed input to the amplifier, as shown in the fig. If the condition Aβ = 1 is satisfied, oscillations will be maintained.

The frequency of the oscillator output depends upon the values of capacitors C and resistors R used in the phase shift network. Using basic RC circuit analysis technique. it will be shown that the network phase shift is 180° degree when

The frequency can be adjusted over a wide range if variable capacitors are used. As well as phase shifting, the R -C net work attenuates the amplifier output. Network analysis shows that, when the necessary 180° degree phase shift is obtained, this network attenuates the output voltage by a factor of 1/29. This means that the amplifier must have a voltage gain of 29 or more.

When the amplifier voltage gain is 29 and feedback factor of R-C network, β = 1/29 then the loop gain is β A = 1/29 x 290= 1, the amplifier phase shift of 180° combined with the network phase shift of + 180° degree gives a loop phase shift of zero.