How to Build an RC Oscillator Circuit

Oscillators use resistive and capacitive components to produce output waveforms with stable frequency. At very low frequencies, this oscillator provides additional advantages. A phase shift circuit can achieve 180 degrees of phase in an RC oscillator. The characteristics of the transistor allow for an additional 180 degrees phase, resulting in generation of oscillations. This article discussed RC oscillators in detail.

What is an RC Oscillator?

An RC oscillator uses linear electrical components to create a sine wave. At high frequencies, the oscillators work like tuned LC circuits, but at low frequencies, the capacitors and inductors in the electrical circuit would be quite large. This oscillator is preferable for low frequency-based applications. RC oscillator comprises an amplifier along with a feedback circuit. The feedback known as phase shift can be created using resistors and capacitors.

Working Principle

The RC oscillator circuit uses the RC network to provide the phase shift of the response signal it needs. These oscillators produce a clean sine wave for a wide variety of loads and have a high frequency.

The basic RC oscillator using a transistor is shown below. The transistor in this circuit is an active element of the amplification stage. The supply voltage V_cc and resistors R₁, R₂, RC and R_E define the DC operating point of the active region of the transistor.

C_E in the above circuit acts as a bypass capacitor. Here the three of the RC segments are equivalent and R’ = R – hie represents the final resistance of the section. The ‘hie’ represents resistance of the transistor’, so the overall network resistance of the circuit is ‘R’.

R₁ and R₂ resistors do not affect the circuit operation. The minimum value of impedance available from the R_E-C_E combination also has minimal effect on the AC operation.

The noise voltage causes the circuit to oscillate when power is applied. An amplifier with a small base current creates 180-degree phase shift currents in the transistor amplifier. This signal will be phase shifted 180 degrees again when it responds to the inputs of the amplifier. For unity gain, the oscillations will continue.

Using an analog AC circuit simplifies the circuit and gives the oscillation frequency:

If R_c/R <<1;

From the above equations, changing the capacitor and resistor values changes the oscillation frequency.

RC Oscillator with Operational Amplifier

The figure below shows an oscillator with an operational amplifier and three of the RC cascaded circuits used as feedback circuits.

Since this op-amp is inverting, its output signal is 180 degrees from the input signal at the inverting terminal. The RC feedback network adds 180 degrees of phase shift, causing oscillations.

Resistors such as R_f and R₁ can adjust the gain of an operational amplifier. Adjust the gain so that the gain of the feedback network and the gain of the op-amp are slightly greater than 1 to achieve the desired oscillations.

A circuit gain greater than 1 makes that circuit an oscillator if the op amp has a gain greater than 29. The oscillation frequency can be obtained using the following equation:

The oscillation condition can be ensured with A ≥ 29. The amplifier gain can be adjusted so that the oscillations occur within the circuit controlling R₁ and R_f.