Mechanism of start of oscillation
The starting voltage is provided by noise,
which is produced due to random motion of electrons in resistors used in the
circuit. The noise voltage contains almost all the sinusoidal frequencies. This
low amplitude noise voltage gets amplified and appears at the output terminals.
The amplified noise drives the feedback network which is the phase shift
network. Because of this the feedback voltage is maximum at a particular
frequency, which in turn represents the frequency of oscillation.
LC Oscillator:
Oscillators are used in many electronic
circuits and systems providing the central “clock” signal that controls that
controls the sequential operation of the entire system. Oscillators convert a
DC input (the supply voltage) into an AC output (the waveform), which can have
a wide range of different wave shapes and frequencies that can be either
complicated in nature or simple sine waves depending upon the application.
Oscillators are also used in many pieces of
test equipment producing either sinusoidal sine wave, square, sawtooth or
triangular shaped waveforms or just a train of pulse of a variable or constant
width. LC Oscillators are commonly used in radio-frequency circuits because of
their good phase noise characteristics and their ease of implementation.
An Oscillator is basically anAmplifier with
“Positive Feedback”, or regenerative feedback (in-phase) and one of the many
problems in electronic circuit design is stooping amplifiers from oscillating
while trying to get oscillators to oscillate. Oscillators work because they
overcome the losses of their feedback resonator circuit either in the form of a
capacitor or both in the same circuit by applying DC energy at the required
frequency into this resonator circuit.
In other words, an oscillator is a an amplifier
which uses positive feedback that generates an output frequency without the use
of an input signal.
It is self sustaining. Then an oscillator has a
small signal feedback amplifier with an open-loop gain equal too or slightly
greater than one for oscillations to start but to continue oscillations the
average loop gain must return to unity. In addition to these reactive
components, an amplifying device such as an Operational Amplifier or Bipolar
Transistors required. Unlike an amplifier there is no external AC input
required to cause the Oscillator to work as the DC supply energy is converted
by the oscillator into AC energy at the required frequency.
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