Diode Approximation : (Large signal operations):
1.
Ideal Diode:    When forward biased, resistance offered is zero,
                When reverse biased resistance offered is infinity. It acts as a perfect switch .Fig10
2.
Second Approximation:
when forward voltage is more than 0.7 V, for Si then it conducts and offers zero
   resistance. Fig 11.
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3.
3rd Approximation:
when forward voltage is more than 0.7 v, then it conduct and offer bulk
   resistance Rf (slope of the current) fig.12
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VD = 0.7 + ID Rf.
When reverse biased resistance offered is very high. (and not infinity). Then the diode equivalent circuit is as
shown in fig13.
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Problem:
Calculate the voltage output for following inputs

(a) V1=V2=0.

(b) V1=V, V2=0.

(c) V=V2=V knew voltage =Vr?  .

Forward resistance is Rf.
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Solution:

(a). Vo = 0.

(b). V1 = V, V2 = V.
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(c) V1 = V 2 = V.
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