Voltage gain of an amplifier is defined as ratio of output voltage to input voltage fed to amplifier. It is unit less and is denoted by Av.
Voltage gain Av = Vo/Vi
Current gain of an amplifier is defined as ratio of output current to input current fed to amplifier. It is unit less and is denoted by Av.
Voltage gain Ai = Io/Ii
Transconductance of an amplifier is defined as ratio of output current to input voltage fed to amplifier. It is specified in units of mhos (or) Siemens and is denoted by Rt.
Transconductance Rt = Vo/ Ii
Trans-resistance of an amplifier is defined as ratio of output voltage to input current fed to amplifier. It is specified in units of ohms and is denoted by Rt.
Trans-resistance Rt = Vo/ Ii