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An OSCILLATOR is a device that converts dc power to ac power at a predetermined frequency.
The requirements for an oscillator are AMPLIFICATION, REGENERATIVE FEEDBACK, and a
FREQUENCY-DETERMINING NETWORK.
An oscillator has two stability requirements, AMPLITUDE STABILITY and FREQUENCY
STABILITY.
FEEDBACK is the process of transferring energy from a high-level point in a system to a low-level
point. Feedback that aids the input signal is REGENERATIVE or POSITIVE. Feedback that opposes the
input signal is DEGENERATIVE or NEGATIVE.
The three basic circuit configurations used for oscillators are COMMON COLLECTOR,
COMMON BASE, and COMMON EMITTER.
In the COMMON-COLLECTOR configuration there is no PHASE SHIFT between input and
output. It is not necessary for the feedback network to provide a phase shift. Voltage gain is less than
unity (one) and power gain is low so it is very seldom used as an oscillator.
In the COMMON-BASE configuration, there is no PHASE SHIFT between input and output. It is
not necessary for the feedback network to provide a phase shift. Voltage and power gain are high enough
to give satisfactory operation in an oscillator circuit.
In the COMMON-EMITTER configuration, there is a 180-degree PHASE SHIFT between input
and output. The feedback network must provide another phase shift of 180 degrees. It has a high power
gain.
