Draw the transfer characteristic of the circuit shown in fig. 9.

Fig. 9
Solution:
When diode D1 is off, i1 = 0, D2 must be ON.

and vo = 10 - 5 x 0.25 = 7.5 V
vp = vo = 7.5 V
Therefore, D1 is reverse biased only if vi < 7.5 V
If D2 is off and D1 is ON, i2 = 0

and vp = 10 ( 0.04 vi - 0.1 ) + 2.5 = 0.4 vi + 1.5
For D2 to be reverse biased,

Between 7.5 V and 21.25 V both the diodes are ON.
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Fig. 10
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The transfer characteristic of the circuit is shown in fig. 10.