Oscillator: An oscillator is a device that generates periodic alternating output signals without any form of the input signal. It keeps producing an output signal as long as there is a supply of DC power in the circuit. They are also known as waveform generators.

Operation of an Oscillator

Basic Oscillator Circuit

Figure: Basic Oscillator Circuit

A basic oscillator circuit consists of an amplifier which forms a positive feedback and frequency selective network, the figure above shows this where there is no application of external input. Due to some random movements of electrons, some noise voltage VO appears at the output. A portion of this voltage will be fed back to the input. As a result, the output increases as AβVO.

If the AβVO term is equal to VO, then the output regenerates itself.
If AβVO = VO (i.e. Aβ=1), the amplifier will oscillate if the phase shift through the amplifier and the feedback circuit is 00 to 3600.

Oscillator Waveform

Figure: a

If Aβ>1 input increases and output signals will build up. The figure below shows the waveform for this condition.

Oscillator Waveform

Figure: Oscillation build-up

For Aβ<1, there is a decrease in the input signal and the output signal will die out. The figure below shows the waveform for this condition.

Oscillator Waveform

Figure: Damped Oscillation

Barkhausen Criteria of Oscillation

The certain condition that must be fulfilled for sustained oscillation is known as Barkhausen criteria and they are:

  1. The loop gain of the circuit must be equal to or greater than unity.
    i.e. Aβ≥1.
  2. The phase shift around the circuit must be 00 to 3600.

Types of Oscillator Circuits

There are distinctly two different types of oscillator:

1. Sinusoidal Oscillator

A sinusoidal oscillator is a type of wave generator in which a frequency selective network is placed in the feedback path of the amplifier ( such as, a transistor or an op-amp). They produce an output having a sine waveform. The circuit will oscillate at the frequency at which the total phase shift around the loop is zero, such that the magnitude of the loop gain of the circuit is equal to or greater than zero.

Some of the sinusoidal waveform generators are:

(I) LC Oscillator

These oscillators use a combination of an amplifier and an LC feedback network to produce oscillations. They find application in the generation of high-frequency signals (from 10 kHz to 100 kHz). However, these oscillators are not suitable for generating low-frequency signals as the components to generate low frequency will be too bulky and heavy.

Some of the examples are Colpitts, Hartley, Crystal, Clapp oscillator, etc.

(II) RC Oscillator

These oscillators use a combination of an amplifier and an RC feedback network to produce oscillations. They find application in the generation of low-frequency signals (from a few Hz to hundreds of kHz).

Some examples of RC oscillators are the Wein bridge and RC phase shift oscillator.

2.Non-sinusoidal Wave Generator

A non-sinusoidal oscillator produces an output with rectangular or square waveforms. Multivibrators refer to the generators having rectangular waveforms. There are three types of multivibrators:

  1. Monostable Multivibrator
  2. Astable Multivibrator
  3. Bistable Multivibrator (Schmitt Trigger)

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