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Current transfer ratio of transistor coupler: It is expressed by the amplification ratio of the output current with respect to the input current like the transistor hFE.
Current Transfer Ratio=CTR
=IC/IF =Output (collector) current/input current ×100 (%)
When IF = 5 mA is input, IC = 10 mA is obtained.
CTR = IC /IF = 10 mA/5 mA×100 (%) = 200%
Reference: hFE of bipolar transistor
hFE (DC current gain)
= Collector current (IC)/base current (IB)