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
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
RC networks, LC tanks, and crystals may appear in sine-wave oscillator circuits. An amplifier can be
made into a sine-wave oscillator by providing regenerative feedback through an RC network.