3.7 Sketch and clearly label the transfer characteristic of the circuit in Figure below for -15 V <= VI <= +15 V. Assume that the diodes can be represented by the constant-voltage drop model with VD = 0.7 V. Also assume that the zener voltage is 6.8 V and that r₂ is negligibly small. O 1 ΚΩ w D4 DA D2 4 N 14 4 D₁ Vo D3