This typical quickly responding constant-current sink uses a bipolar transistor but suffers from base-current-induced error. Its normal output current is I_OUT = (V_REF/R₁) - I_B

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• Article «Error compensation improves bipolar-current sinks»
• Figure 1. This typical quickly responding constant-current sink uses a bipolar transistor but suffers from base-current-induced error. Its normal output current is I_OUT = (V_REF/R₁) - I_B
• Figure 2. Adding base-current error compensation improves the circuit's performance. Using perfectly matched resistors simplifies the output-current equation to I_OUT = (V_REF/R₁)
• Figure 3. You can further simplify the current sink's design by adding only two resistors, R₅ and R₆, to the original in Figure 1. The output-current equation remains I_OUT = (V_REF/R₁), as in Figure 2
• Figure 4. Adding RSPEEDUP improves the performance of a two-transistor Darlington output stage