Audio Oscillator Circuit

INTRODUCTION

Oscillator is a circuit that produces a continuous signal or waveform on its output with only the dc supply voltage as an input. Depending on the type of oscillator, the output voltage can be either sinusoidal or non- sinusoidal.

Oscillators are often characterized by the freq of their output signal: A low-freq oscillator (LFO) is an electronic oscillator that generating freq below 20 Hz. To distinguish it from an audio freq oscillator this term is used in the field of audio synthesizers An audio oscillator produces frequencies , about 16 Hz to 20 kHz in the audio range. An RF oscillator produces signals of frequencies of about 100 kHz to 100 GHz in the radio freq (RF) range..

There are three basic types of oscillators namely

The general graph of y = sin (t - c) is obtained by the operation of shifting the graph of y = sin t by horizontally with the c units. If c is considered as positive, the graph is shifted c units from right and if c is considered as the negative, the graph is shifted c units from left. The number c is called as the phase shift. fig. 1.2 Phase Shift Graph

The sine function will complete one cycle when its argument varies from 0 to 2π. The function will start out when the argument is equal to 0. We can produce a total phase shift in the circuit of 180o at the chosen freq by connecting together three RC networks in series and this forms the bases of a “PSO” also known as a RC Oscillator circuit.

fig. 1.3 Vector Diagram

To overcome the circuit losses, the gain of the amplifier must be sufficient. The resistor-capacitor combination in the RC Oscillator circuit also acts as an attenuator and produces attenuation of -1/29th ( Vo/Vi = β ) per stage. Therefore, in our three stage RC network above the amplifier, gain must be greater than 29.

fig. 1.4 Phase Shift and

External resistors or capacitors can be connected to the op-amp in a number of ways to form basic “Building Block” circuits. Basic building blocks can be Inverting, Non-Inverting, , Summing , Differential, Voltage Follower , Integrator and Differentiator type amplifiers.

Op-amp IC’s are available which are suitable for every possible application. Op-amps are available in IC packages of single, dual or quad op-amps within one single device. The most commonly available and used of all operational amplifiers in basic electronic kits and projects is the industry standard μA-741 IC.

fig.1.5 741 IC

PRINCIPLE

Op-Amp acts as an inverting amplifier. A PSO works on the principle that a fraction of the output single-stage amplifier is passed through a phase-shift network before feeding back to input.

fig. 2.1 Three-Stage Feedback Network

It not only amplifies the input signal but also shifts its phase by 180º. However, to produce oscillations we must have positive feedback of sufficient