Oscillator & its types

Electronic circuit that produces a repetitive electronic signal is called an Oscillator. An oscillator produces signals with constant amplitude and frequency. They are used in many applications like radio and television transmitters to generate career signals, to generate clock signals in sequential circuits etc. Inverters that generate low frequency and high-power signals from DC supply can also be referred to as oscillators.

Operation of an Oscillator

Operation of an oscillator can be explained by considering an inductor connected to a charged capacitor. Both capacitor and inductor can store energy. A capacitor stores energy in the form of an electrostatic energy, while an inductor as a magnetic field.

In the above circuit, capacitor is initially charged using an external battery. When the switch is operated, inductor is introduced into the circuit and the connection to the battery is opened.Capacitor will start to discharge through the inductor. As it does, energy stored in the capacitor is transferred into the inductor. Inductor will store this energy in the form of magnetic field. Once the capacitor discharges, the entire energy which was stored in the capacitor will now be stored in the inductor. Inductor will now charge up the capacitor. Once the inductor's field collapses, the capacitor is again completely recharged. This entire process is repeated to produces continuous oscillating signal across the inductor and capacitor. Oscillations will continue until the circuit runs out of energy due to the resistance in the wire. Frequency of the oscillation depends on the magnitude of the inductance and the capacitance. 

 

There are two main types of electronic oscillators: the harmonic oscillator and the relaxation oscillator.

(i). Harmonic Oscillator

Harmonic oscillators are also called as linear oscillators. They produce a sinusoidal output. Some of the commonly used harmonic oscillators are discussed here.

(a).Tuned Circuit Oscillators

Tuned circuit oscillators or resonant circuit, also called as LC circuit are mainly used in receivers to regenerate the received signal without noise. LC circuits are used either for generating signals at a particular frequency, or picking out a signal at a particular frequency from a more complex signal. Hartley and Colpitts oscillators are the most common examples of tuned oscillators.

Thousands of sine waves from different radio stations hit the antenna of the receiver. The sine wave that matches the resonant frequency of the receiver circuit will get amplified and all the other frequencies will be ignored. Either the capacitor or the inductor in the resonator circuit is adjustable such that the resonant frequency can be varied. This process is called tuning and is used in radios to detect a particular channel from the incoming complex wave.

(b). RC Oscillators

 RC oscillator uses resistors and capacitors to generate low or audio-frequency signals. RC phase-shift and Wien-bridge oscillators come under this category.

(1). RC phase shift Oscillator

One of the simplest implementations for this type of oscillator uses an operational amplifier. Figure shows Op-Amp realization of an RC phase shift oscillator. It produces a sine wave output at low frequency. Output of the Op-Amp is fed back to its input through a phase-shift network consisting of resistors and capacitors. Feedback network 'shifts' the phase of the amplifier output by 180 degrees. Feedback is given to the negative terminal of the Op-Amp therefore it shifts an additional 180 degrees. Effectively therefore is no phase shift between input and output which is an important criteria for a circuit to work as an oscillator.

 (2). Wien Bridge oscillator

In Wien-bridge oscillator, the output of the operational amplifier is fed back to both the inputs of the amplifier. One part of the feedback signal is connected to the inverting input through the resistor divider, which determines the voltage gain. And the other part is fed back to the non-inverting input terminal through the RC Wien Bridge network. Wien-bridge ensures that the required phase produces the necessary stable oscillations. A Wien-bridge oscillator is used in electronic circuits to generate sine wave at low frequency.

(c). Crystal Oscillators

Crystal oscillators use piezoelectric crystals to generate highly stabilized output signal. Stability, small size and low cost make them superior over other oscillator circuits. It works on the principle of piezoelectric effect. When voltage is applied across a piezoelectric crystal, it vibrates at the frequency of the applied voltage. Conversely, if we apply a mechanical force to vibrate the crystal, it generates an AC voltage of the same frequency. Quartz, Rochelle salts, tourmaline are the most common crystals that show piezoelectric effect. Rochelle salt vibrates more but breaks easily; Tourmaline shows least activity but is stronger and costlier than other piezoelectric crystals. Quartz is a compromise between piezoelectric property and strength and also readily available in nature. This makes quartz most preferred piezoelectric crystal. When piezoelectric crystals are used in oscillator circuits, they are suitably cut and then mounted between two metal plates. An oscillator circuit in which piezoelectric crystal is used is shown here.

(ii). Non-sinusoidal or Relaxation Oscillators

Relaxation oscillator or Non-sinusoidal Oscillators generate square, rectangular or saw tooth waveform. Such oscillators can provide output at frequencies ranging from zero to 20MHz. Circuits of most common relaxation oscillators are listed here.   

(a). 555 Timer

555 timer IC can be used to make a relaxation oscillator. An Astable multivibrator using 555 IC is shown in the figure below. Here the capacitor charges through the resistor divider circuit and discharges through the IC. Voltage across the capacitor triggers the timer circuit to work as an astable multivibrator.  

(b). UJT relaxation Oscillator  

Circuit shows a relaxation oscillator using a UJT transistor. Capacitor is charged through the resistor and the voltage across the capacitor increases exponentially until it attains the peak point voltage. At peak point voltage, UJT switches on and the capacitor rapidly discharges through R1. Capacitor voltage drops to a low value turning off the transistor. This process is continued to generate a saw tooth waveform at the output.  

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