Step 3. Design output-side resistor RL
Determine RL from the IC-VCE characteristics of the output transistor. In order to use for signal transmission, it is necessary to fully satisfy the "L" level of the device connected to the load side.
Here, we set VCE = 0.3 V as the target value.
When IF = 10 mA, VCE = 0.9 V at RL = 1 kΩ, which is insufficient. At RL = 2 kΩ, VCE = about 0.2 V, which satisfies the target value. Therefore, choose RL = 2 kΩ. In actual design, the impedance on the load side must also be considered.