Oscillators

Introduction

Oscillators (wave generators) play a prominent role in the field of electronics. They generate signals from a few hertz to several gigahertz (10⁹ hertz). Modern oscillators use many different circuits and generate such outputs as sinusoidal, square, rectangular, sawtooth, and trapezoidal waveshapes. These waveshapes serve many useful purposes in the electronic circuits. For example, they are used extensively throughout the television receiver to reproduce both picture and sound.

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. Oscillators are classified according to the waveshapes they produce and the requirements needed for them to produce oscillations.

Classification of Oscillators

Oscillators can be classified into two broad categories according to their output waveshapes, sinusoidal and nonsinusoidal.

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 different 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.

Nonsinusoidal Oscillators

Nonsinusoidal 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 nonsinusoidal oscillators are multivibrators, blocking oscillators, sawtooth generators, and trapezoidal generators.

The Feedback Oscillator

The most common form of oscillator is a feedback oscillator. A feedback oscillator can be thought of as an amplifier that provides itself (through feedback) with an input signal.

A feedback oscillator must provide amplification. This section will use transistor amplifiers to present the concepts and principles of oscillators. Amplification of signal power occurs from input to output. In an oscillator, a portion of the output is fed back (through the feedback network) to sustain the input, as shown in the figure below. Enough power must be fed back to the input circuit for the oscillator to drive itself. To cause the oscillator to be self-driven, the feedback signal must also be regenerative (positive). Regenerative signals must have enough power to compensate for circuit losses and to maintain oscillations.

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.

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.

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 you should know from your study of transistor biasing, bias variations affect the operating point of the transistor. These variations may alter the amplification capabilities of the oscillator circuits as well. A well-regulated power supply and a bias-stabilizing circuit are required to ensure a constant, uniform signal 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, high 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 high power, efficiency becomes important. Many oscillators use class C bias to increase efficiency. Other types of oscillators may use class A bias when a high efficiency is not required but distortion must be kept at a minimum. Other classes of bias may also be used with certain oscillators.