When OUT goes low, Q4 is off while Q3 saturates and transmits negative current I to C1. Keep V_IN < 1 V so Q2 won't saturate

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Other Materials from the Main Document

• Article «A two-way mirror - current mirror that is»
• Figure 1. D1, Q3, and Q4 comprise the two-way current mirror. It passively conducts Q2's collector current to C1 via Q3's base-collector when OUT versus C1 is negative, but becomes an active inverting unity-gain current source when OUT goes positive and reverses th
• Figure 2. When OUT goes low, Q4 is off while Q3 saturates and transmits negative current I to C1. Keep V_IN < 1 V so Q2 won't saturate
• Figure 3. With OUT high, the mirror goes active and inverts I into a positive current to charge C1