The (complex) impedance presented to the input port of a device; the impedance a device is driven from. Generator impedance determines noise figure. It influences device gain and stability.

For maximum gain (minimum input reflection coefficient), _G = s₁₁'^*
[The load impedance is equal to the conjugate of the input reflection coefficient, modified by any loading from the load impedance.]

For noise, _G is set to present the correct correct impedance for minimum noise figure (_opt).

See also:
Gain, Impedance, Impedance Matching, Load Impedance, Reflection Coefficient, S-Parameters