<P> A phase - shift oscillator is a linear electronic oscillator circuit that produces a sine wave output . It consists of an inverting amplifier element such as a transistor or op amp with its output fed back to its input through a phase - shift network consisting of resistors and capacitors in a ladder network . The feedback network' shifts' the phase of the amplifier output by 180 degrees at the oscillation frequency to give positive feedback . Phase - shift oscillators are often used at audio frequency as audio oscillators . </P> <P> The filter produces a phase shift that increases with frequency . It must have a maximum phase shift of more than 180 degrees at high frequencies so the phase shift at the desired oscillation frequency can be 180 degrees . The most common phase - shift network cascades three identical resistor - capacitor stages that produce a phase shift of zero at low frequencies and 270 ° at high frequencies . </P> <P> The implementation using NPN type bipolar transistor is based on example from popular electronics hobby magazine . The circuit generates a sinusoidal wave at its output . The RC phase - shift network is used to form a positive feedback loop . Resistor Rb provides base bias current . Resistor Rc is the collector load resistor for the collector current . Resistor Rs isolates the circuit from the external load . The circuit is taken from </P> <P> The circuit implements the RC phase shift oscillator with FET . Note that the topology used for positive feedback is voltage series feedback . </P>

For a system to work as oscillator the total phase shift of the loop gain must be equal to