What is Oscillator? – Types of Oscillators:

An oscillator is the basic element of all ac signal sources and generates sinusoidal signals of known frequency and amplitude. It is one of the basic and useful instruments used in electrical and electronic measurements. For example, an oscillator finds wide applications in electronic communication equipment. In AM (amplitude modulation) and FM (frequency modulation) superheterodyne receivers. “local” oscillators are used to assist in the reduction of the incoming radio frequency (RF) to a lower intermediate frequency (IF). Oscillator circuits are also employed in the “exciter” section of a transmitter to generate the RF carrier. Other applications include their use as “clocks” in digital systems such as microcomputers, in the sweep circuits found in TV sets and oscilloscopes.

Since sinusoidal waveforms are encountered so frequently in electronic measurement work, the oscillator (sinewave generator) represents the largest single category of signal generators. This device covers the frequency range from a few Hz to many GHz.

Although we speak of an oscillator as “generating” a sinusoidal signal, it is to be noted that it does not create energy, but merely acts as an energy converter. It simply converts unidirectional current drawn from a dc source of supply into alternating current of desired frequency. The function of an oscillator is reverse of that of a rectifier and, therefore, sometimes called inverter. However, we generally think of oscillator circuits as providing an ac voltage signal.

It is noteworthy here that although an alternator (ac generator) generates sinusoidal ac power of 50 Hz, it cannot be called an oscillator. An alternator is a mechanical device that has rotating parts, converts mechanical energy into ac energy but cannot produce ac energy of high frequency whereas an oscillator is a non-rotating electronic device that converts dc energy into ac energy of frequency ranging from a few Hz to many GHz.

Though alternator generates large amount of ac power but for several applications such as radio transmitters and receivers, radars etc., an oscillator is preferred owing to its numerous advantages over alternators for such applications. These advantages are:

• Portable and cheap in cost.
• An oscillator is a non-rotating device. Consequently, there is no wear and tear and hence longer life.
• Frequency of oscillation may be conveniently varied.
• Voltage or currents of any frequency (20 Hz to extremely high frequencies) adjustable over a wide range can he generated.
• Frequency once set remains constant for a considerable period of time.
• Voltages free from harmonics as well as rich with harmonics can be generated by sinusoidal oscillators and relaxation oscillators respectively.
• Operation of an oscillator is silent, as there is no moving part in it.
• High operation efficiency—due to absence of moving part, there is no wastage of energy owing to friction.

For having a sinusoidal oscillator, we require an amplifier with positive feedback. The idea is to use the feedback signal in place of an input signal. If the loop gain and phase are correct, there will be an output signal even though there is no external input signal. In other words, an oscillator is an amplifier that is modified by positive feedback to supply its own input signal.

The difference between an amplifier and an oscillator can be explained with the help of Fig. 21.1.

As we know that, an amplifier strengthens the input signal without any change in its waveform and frequency. The additional power required comes from the external dc source. Thus an amplifier is essentially an energy converter that draws energy from a dc supply and converts it into ac energy at signal frequency, the energy conversion process being controlled by the input signal. On the other hand an oscillator does not require any external signal either to start or maintain the process of energy conversion and the energy conversion process is controlled by the oscillator circuit itself, as illustrated in Fig. 21.1. In an oscillator, the output signal frequency depends on the passive components employed in the circuit and can be varied as per needs. Oscillator may provide fixed or variable frequency.

When employed as an instrument, an oscillator may be called with different names such as test oscillator (or signal generator), standard signal generator (or function generator) depending on the function performed by the device, type of signal produced by it, order of sophistication etc.

Oscillators may be broadly divided into two categories viz. harmonic oscillators and relaxation oscillators. Both types can include active devices such as BJTs, FETs and op-amps, and passive components such as resistors, inductors and capacitors. In harmonic oscillators, the energy always flows in one direction—from the active to passive components. However, in relaxation oscillators, the energy is exchanged between the active

and passive components. In harmonic oscillators, the frequency of oscillations is determined by the feedback path. However, in relaxation oscillators, the frequency is determined by time constants-specifically, the charge and discharge time constants during the exchange of energy. Harmonic oscillators can develop low-distortion sinusoidal output waveforms, but relaxation oscillators can only generate nonsinusoidal waveforms such as sawtooth, square or triangular. Harmonic oscillators will be discussed here.

Oscillators can also be categorized on the basis of design principle used, and the frequency range over which they are employed.

The oscillators, according to the design principle used, are feedback oscillators and negative resistance oscillators. Feedback oscillators are widely employed.

The oscillators, according to operating frequency, may be classified as follows :