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A Negative feedback occurs when the fed-back output signal has a relative phase of 180° with respect to the input signal (upside down). This situation is sometimes referred to as being ''out of phase'', but that term also is used to indicate other phase separations, as in "90° out of phase". Negative feedback can be used to correct output errors or to desensitize a system to unwanted fluctuations.<ref name=Bhattacharya>

A Negative feedback occurs when the fed-back output signal has a relative phase of 180° with respect to the input signal (upside down). This situation is sometimes referred to as being ''out of phase'', but that term also is used to indicate other phase separations, as in "90° out of phase". Negative feedback can be used to correct output errors or to desensitize a system to unwanted fluctuations.<ref name=Bhattacharya>

当反馈的输出信号与输入信号的相对相位为180°（上下颠倒）时，就会出现负反馈。这种情况有时被称为''失相''，但该术语也用于表示其他相位分离，如 "90°失相"。负反馈可用于纠正输出误差或使系统对不需要的波动失去敏感度。

当反馈的输出信号与输入信号的相对相位为180°（上下颠倒）时，就会出现负反馈。这种情况有时被称为''失相''，但该术语也用于表示其他相位分离，如 "90°失相"。负反馈可用于纠正输出误差或使系统对不需要的波动失去敏感度。

</ref> In feedback amplifiers, this correction is generally for waveform [[distortion]] reduction{{citation needed|date=October 2014}} or to establish a specified [[Gain (electronics)|gain]] level. A general expression for the gain of a negative feedback amplifier is the [[asymptotic gain model]].

</ref> In feedback amplifiers, this correction is generally for waveform [[distortion]] reduction{{citation needed|date=October 2014}} or to establish a specified [[Gain (electronics)|gain]] level. A general expression for the gain of a negative feedback amplifier is the [[asymptotic gain model]].

====Positive feedback====

====Positive feedback====