Configuration of Oscillators

Any of the three basic amplifier configurations (common collector, common base, or common emitter) described in NEETS, Module 7, Introduction to Solid-State Devices and Power Supplies, Chapter 2, may be used for the oscillator circuit.

However, certain considerations in the application of the circuit, such as the operating frequency and output power required, usually determine which of the three configurations is to be used. The three basic configurations of oscillators are shown in the figure below, views (A), (B), and (C).

Basic configurations COMMON-COLLECTOR-CONFIGURATION.

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Basic configurations COMMON-BASE-CONFIGURATION.

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Basic configurations COMMON-EMITTER-CONFIGURATION.

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COMMON-COLLECTOR CONFIGURATION

Since there is no phase reversal between the input and output circuits of a common-collector configuration, the feedback network does not need to provide a phase shift. However, since the voltage gain is less than unity and the power gain is low, the common-collector configuration is very seldom used in oscillator circuits.

COMMON-BASE CONFIGURATION

The power gain and voltage gain of the common-base configuration are high enough to give satisfactory operation in an oscillator circuit. The wide range between the input resistance and the output resistance make impedance matching slightly harder to achieve in the common-base circuit than in the common-emitter circuit. An advantage of the common-base configuration is that it exhibits better high- frequency response than does the common-emitter configuration.

COMMON-EMITTER CONFIGURATION

The common-emitter configuration has high power gain and is used in low-frequency applications. For the energy which is fed back from the output to be in phase with the energy at the input, the feedback network of a common-emitter oscillator must provide a phase shift of approximately 180 degrees. An advantage of the common-emitter configuration is that the medium resistance range of the input and output simplifies the job of impedance matching.

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