dB和V/V

dB作为增益单位忽略负反馈功放的方向（反向: Inverting）特性，因此在有的器件手册中使用V/V（voltage-over-voltage）表征放大器增益特性（而不忽略方向特性）。

举例如下：

Gain: 2 V/V or 20*log(|2|) = 6 dB and 0° phase difference
Gain: 0.5 V/V or 20*log(|0.5|) = -6 dB and 0° phase difference
Gain: -0.5 V/V or 20*log(|-0.5|) = -6 dB and 180° phase difference
Gain: -2 V/V or 20*log(|-2|) = 6 dB and 180° phase difference

参考链接：

http://electronics.stackexchange.com/questions/39771/my-amplifier-produced-a-negative-gain-when-it-should-be-positive-why