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THE BASIC OSCILLATOR
In simple terms, an oscillator is an amplifier where sufficient energy is coupled back from the
output to the input to become unstable and start oscillating. The output port then provides the
wanted output power into the load and the overall circuit configuration determines the frequency
stability and sensitivity to load changes.
The primary purpose of an oscillator is to generate a given waveform at a constant peak
amplitude and specific frequency and to maintain this waveform within certain limits of amplitude
and frequency.
Any circuit requires 2 conditions to oscillate. Tracing the path from the input, round the feedback
network, back to the input there must be an overall phase shift of 0 degrees at one particular
frequency. In other words, any signal traveling around this loop, must be in phase with the
original signal as it arrives back at the input and thus add to the input signal.
As the signal travels around the loop, there will be a loss in the system (heat dissipation in the
components, losses in the amplifier etc). Therefore there must be some form of gain in the loop,
such that the signal arriving back at the input (having traveled around the loop) is as large (or
larger) compared to the original signal. If these 2 conditions are met, the oscillations will be
sustained.
Since a practical oscillator must oscillate at a predetermined frequency, a frequency-determining
device (fdd), sometimes referred to as a frequency-determining network (fdn), is needed. This
device acts as a filter, allowing only the desired frequency to pass. Without a frequency-determining
device, the stage will oscillate in a random manner, and a constant frequency will not
be maintained.
Fig 1.
Before discussing oscillators further, let's review the requirements for an oscillator. First,
amplification is required to provide the necessary gain for the signal. Second, sufficient
regenerative feedback is required to sustain oscillations. Third, a frequency-determining device is
needed to maintain the desired output frequency.
AMPLIFICATION. An oscillator must provide amplification. Amplification of signal power
occurs from input to output. In an oscillator, a portion of the output is fed back to sustain the
input, as shown in figure 1. Enough power must be fed back to the input circuit for the oscillator
to drive itself as does a signal generator. To cause the oscillator to be self-driven, the feedback
signal must also be regenerative
REGENERATIVE FEEDBACK (positive). Regenerative signals must have enough power to
compensate for circuit losses and to maintain oscillations.
FREQUENCY-DETERMINING DEVICE (fdd). A fdd is need to maintain a constant
frequency.
The basic oscillator requirements, in addition to the application, determine the type of oscillator
to be used. Let's consider some factors that account for the complexity and unique
characteristics of oscillators.
Virtually every piece of equipment that uses an oscillator has two stability requirements,
AMPLITUDE STABILITY and FREQUENCY STABILITY. Amplitude stability refers to the
ability of the oscillator to maintain a constant amplitude in the output waveform. The more
constant the amplitude of the output waveform, the better the amplitude stability. Frequency
stability refers to the ability of the oscillator to maintain its operating frequency. The less the
oscillator varies from its operating frequency, the better the frequency stability.
A constant frequency and amplitude can be achieved by taking extreme care to prevent
variations in LOAD, BIAS, and COMPONENT CHARACTERISTICS. Load variations can
greatly affect the amplitude and frequency stability of the output of an oscillator. Therefore,
maintaining the load as constant as possible is necessary to ensure a stable output.
As a result of changing temperature and humidity conditions, the value or characteristics of
components such as capacitors, resistors, and transistors can change. The changes in these
components also cause changes in amplitude and frequency.
Output power is another consideration in the use of oscillators. Generally
speaking, higher power is obtained at some sacrifice to stability. When both requirements are to
be met, a low-power, stable oscillator can be followed by a higher-power BUFFER
AMPLIFIER. The buffer provides isolation between the oscillator and the load to prevent
changes in the load from affecting the oscillator.
If the oscillator stage must develop higher power, efficiency becomes important. Many oscillators
use class C bias to increase efficiency. Other types of oscillators may use class A bias when high
efficiency is not required but distortion must be kept at a minimum. Other classes of bias may
also be used with certain oscillators.
CLASSIFICATION OF OSCILLATORS (GENERATORS)
Oscillator can be classified into two broad categories according to their output wave shapes,
sinusoidal and non-sinusoidal.
Sinusoidal Oscillators
A sinusoidal oscillator produces a sine-wave output signal. Ideally, the output signal is of
constant amplitude with no variation in frequency. Actually, something less than this is usually
obtained. The degree to which the ideal is approached depends upon such factors as class of
amplifier operation, amplifier characteristics, frequency stability, and amplitude stability.
Sine-wave generators produce signals ranging from low audio frequencies to ultrahigh radio and
microwave frequencies. Many low-frequency generators use resistors and capacitors to form
their frequency-determining networks and are referred to as RC OSCILLATORS. They are
widely used in the audio-frequency range.
Another type of sine-wave generator uses inductors and capacitors for its frequency-determining network.
This type is known as the LC OSCILLATOR. LC oscillators, which use
tank circuits, are commonly used for the higher radio frequencies. They are not suitable for use
as extremely low-frequency
oscillators because the inductors and capacitors would be large in size, heavy, and costly to
manufacture.
A third type of sine-wave generator is the CRYSTAL-CONTROLLED OSCILLATOR. The
crystal-controlled oscillator provides excellent frequency stability and is used from the middle of
the audio range through the radio frequency range.
Non-sinusoidal Oscillators
Non-sinusoidal oscillators generate complex waveforms, such as square, rectangular, trigger,
sawtooth, or trapezoidal. Because their outputs are generally characterized by a sudden change,
or relaxation, they are often referred to as RELAXATION OSCILLATORS. The signal
frequency of these oscillators is usually governed by the charge or discharge time of a capacitor
in series with a resistor. Some types, however, contain inductors that affect the output frequency.
Thus, like sinusoidal oscillators, both RC and LC networks are used for determining the
frequency of oscillation. Within this category of non-sinusoidal oscillators are
MULTIVIBRATORS, BLOCKING OSCILLATORS, SAWTOOTH GENERATORS, and
TRAPEZOIDAL GENERATORS.
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