Transimpedance amplifier is an active current to voltage converter since it uses an active component like Op-Amp to convert the input current to a proportional output voltage. At its simplest, it's an operational amplifier with a feedback resistor, and the output voltage follows Ohm's law: V_out = I × R_F, where I is the input current and R_F is the feedback. Transimpedance amplifier is simply a current to voltage amplifier. Transimpedance comes from the term 'transfer impedance'.