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phase inverter[′fāz in‚vərd·ər]
an electrical device that converts the input voltage to two voltages differing in phase by 180°. The simplest type of phase inverter is an electrical transformer with a symmetrical secondary winding that has a center tap. An oscillatory circuit is frequently used as a phase inverter; such circuits have center taps in the inductive or capacitive branch, connected to the center point of the induction coil or the common point of two series-connected capacitors, respectively.
In radio engineering, many devices use electron-tube phase inverters and, more recently, transistorized phase inverters with divided load. In such devices the output signals at the plate or collector and at the cathode or emitter are 180° out of phase. Other types of phase inverters use electron tubes, such as twin triodes, in a circuit with a common cathode or common grid, or they may be Darlington-connected transistors. Phase inverters are also used in measuring instruments and computer-technology devices.
A circuit having the primary function of changing the phase of a signal by 180°. The phase inverter is most commonly employed as the input stage for a push-pull amplifier. Therefore, the phase inverter must supply two voltages of equal magnitude and 180° phase difference. A variety of circuits are available for the phase inversion. See Push-pull amplifier
Overall fidelity of a phase inverter and push-pull amplifier can be adversely affected by improper design of the phase inverter. The principal design requirement is that frequency response of one input channel to the push-pull amplifier be identical to the frequency response of the other channel.
The simplest form of phase-inverter circuit is a transformer with a center-tapped secondary. Careful design of the transformer assures that the secondary voltages are equal. The transformer forms a good inverter when the inverter must supply power to the input of the push-pull amplifier. The transformer inverter has several disadvantages. It usually costs more, occupies more space, and weighs more than a transistor circuit. Furthermore, some means must be found to compensate for the frequency response of the transformer, which may not be as uniform as that which can be obtained from solid-state circuits. See Transformer
An amplifier that provides two equal output signals 180° out of phase is called a paraphase amplifier. If coupling capacitors can be omitted, the simplest paraphase amplifier is shown in the illustration. Approximately the same current flows through RL and RE, and therefore if RL and RE are equal, the ac output voltages from the collector and from the emitter are equal in magnitude and 180° out of phase.