Series-output feedback connections create controlled current sources. This means that the load current will be constant in spite of changes in load impedance. Two variants are possible. Parallel-input connections create a current-sensing circuit, thus P-S feedback forms a current-controlled current-source, or ideal current amplifier. The current gain is set by the two feedback resistors, $$R_i$$ and $$R_f$$. Series-input connections create a voltage-sensing circuit, thus S-S feedback forms a voltage-controlled current-source, or voltage-to-current transducer. The transconductance is set by the feedback resistor $$R_i$$.