Transconductance, also known as mutual conductance, is a property of certain electronic components. Conductance is the reciprocal of resistance; transconductance, meanwhile, is the ratio of the current change at the output port to the voltage change at the input port. It is written as gm. For direct current, transconductance is defined as follows:

g_m = {\Delta I_\mathrm{out} \over \Delta V_\mathrm{in}}

For small signal alternating current, the definition is simpler:

g_m = {i_\mathrm{out} \over v_\mathrm{in}}

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