Usually output of an amplifier is a voltage ,...but in case of Operational Transconductance Amplifier ,Iout (current ) is the output. This feature, makes it useful for Electronic control of amplifier gain .

Conductance is the reciprocal of resistance. If it is taken between one output parameter and one input parameter, then it is called transconductance i. e. the ratio of output current to the input voltage.It is given by gm= Iout/Vin

it is a transconductance unit which is "ampere/volt"

voltage amplifier feedback

current amplifier feedback

transconductance feedback

transresistance feedback

Roughly speaking, resistance.

Transconductance refers to the reciprocal of the amplifying device's internal resistance. The concept is particularly useful if the device is a voltage-controlled current source (tube or FET). In vacuum tube amplifiers, transconductance (Gm) is (u / Rp), where...

u is the amplification factor. u= (Gm x Rp).

Rp is the anode (drain) resistance. Rp is the internal resistance of the amplifying device.

Gfs is synonymous with Gm.

The reciprocal of Gm (or Gfs) is (Rp / u). Another term for this reciprocal is transresistance.