RC Phase Shift Oscillator Using Transistor (BJT): Circuit & Working

Description: This section introduces a transistor oscillator circuit known as the RC Phase Shift Oscillator. An oscillator is an electronic circuit that functions as a sine wave generator, requiring only a DC power supply. It is commonly used in variable frequency signal generators, making it a fundamental electronic circuit. Previous discussions have covered the Op-Amp RC Phase Shift Oscillator. Various types of oscillators exist, categorized by their performance and components, such as the Wien Bridge Oscillator, Hartley Oscillator, and Colpitts Oscillator. This study focuses on obtaining an RC Phase Shift Oscillator using a silicon transistor. In an oscillator, the frequency generation is determined by the circuit elements employed. A basic oscillator circuit comprises several components. Utilizing a common emitter amplifier with a resistive collector load results in a 180-degree phase shift between the base and collector voltages, along with signal amplification. The phase shift network consists of three sections formed by resistive-capacitor combinations, each introducing a 60-degree phase shift at the resonant frequency. The phase shift oscillator is a specific type of audio frequency oscillator, with the output signal taken across a 1 µF capacitor and ground terminal, as depicted in the circuit schematic. When all resistors (R) and capacitors (C) in the phase shift network are equal, the frequency of oscillation for the RC oscillator can be calculated. The number of RC stages enhances frequency stability, with the total phase shift provided by the feedback network needing to reach 180 degrees for sustained oscillations. When utilizing N RC stages, each RC section contributes a phase shift of 180/N degrees. Cascading two RC sections results in poor frequency stability, while three sections yield a higher phase change rate, improving stability. However, four RC sections provide an even better phase change rate, leading to the most stable oscillator configuration. The drawback of four RC segments is increased cost and circuit complexity. Therefore, phase shift oscillators typically employ three RC sections, with each section delivering a 60-degree phase shift. This configuration is often used in high-precision applications where cost considerations are secondary to accuracy.

The RC Phase Shift Oscillator is a fundamental circuit in electronics that utilizes a transistor to generate a sinusoidal output. The circuit typically consists of a common emitter amplifier, which provides the necessary gain and phase shift for oscillation. The resistive-capacitor (RC) network plays a crucial role in determining the frequency of oscillation. Each of the three RC sections contributes a 60-degree phase shift at the desired frequency, summing to a total of 180 degrees when combined with the 180-degree phase shift from the amplifier, thereby fulfilling the Barkhausen criterion for sustained oscillations.

The output of the oscillator is usually taken from the junction between the last RC stage and the ground, where a capacitor (often 1 µF) is connected to filter the output signal. The frequency of oscillation can be calculated using the formula:

\[ f = \frac{1}{2\pi R C \sqrt{6}} \]

where R is the resistance and C is the capacitance of each RC stage. The stability of the oscillator is influenced by the values of R and C, as well as the quality of the components used.

In practical applications, ensuring that the resistors and capacitors are precisely matched is critical for achieving the desired frequency and minimizing drift. The RC Phase Shift Oscillator is widely employed in audio applications, signal generation, and various forms of communication systems, where precise frequency control is essential. Its simplicity and effectiveness make it a preferred choice for many electronic designs.In this section we would like to introduce you a transistor oscillator circuit, called as RC Phase shift Oscillator. First of all we need to know what is an oscillator. An oscillator is an electronic circuit which acts as a sine wave generator. The only requirement of an oscillator is a DC power supply source. It is widely used in frequency variab le signal generators, so it is a common basic electronic circuit. We had already discussed about Op amp RC phase shift oscillator in earlier posts. Different types of oscillators are available which are based on their performance and components used, like Wein bridge oscillator, Hartly Colpitts oscillator etc. Here we are going to study how we can obtain RC Phase shift Oscillator using a silicon transistor. In an oscillator the frequency generating circuit is decided by the circuit elements used. A basic oscillator circuit contains the following parts If we use a common emitter amplifier with a resistive collector load, there will be a 180 phase shift between the voltages at base and collector.

It will also amplify the signal. Three sections of phase shift networks are used which is constituted by resistive-capacitor combination. In that each section introduces 60 phase shift at resonant frequency. Phase shift oscillator is a particular type of audio frequency oscillator. Output signal is obtained across 1 µF capacitor and ground terminal as shown in circuit schematic. If all the resistors, R and the capacitors, C in the phase shift network are equal in value, then the frequency of oscillations produced by the RC oscillator is given as: Number of RC stages will improve the frequency permanence.

The total phase shift established by the feedback network must be 180 degrees for sustained oscillations. If we are using N` RC stages, each RC section provides 180/N degree phase shift. When 2 RC sections are cascaded, the frequency stability is poor. For 3 sections cascaded, the phase change rate is high so we get improved frequency stability. However for 4 RC sections there is a good phase change rate resulting in the most stable oscillator configuration.

But 4 RC segments enhance cost and makes circuit complexity. Hence phase shift oscillators make use of 3 RC sections and in that each section gives a phase shift of 60 degree. The latter 3 RC networks are generally used in high precision applications where cost is not much considered and only accuracy plays a major role.