Small Signal Operation of Real diode:

Consider the diode circuit shown in fig. 3. V = VD + Id RL VD = V- IdRL This equation involves two unknowns and cannot be solved. The straight line represented  by the above equation is known as the load line. The load line passes through two points,                          I = 0, VD = V and                  VD= 0, I = V / RL.

Fig. 3

The slope of this line is equal to 1/ R_L. The other equation in terms of these two variables V_D & I_d, is given by the static characteristic. The point of intersection of straight line and diode characteristic gives the operating point as shown in fig. 4.

Fig. 4	Fig. 5

  Let us consider a circuit shown in fig. 5 having dc voltage and sinusoidal ac voltage. Say V = 1V, R_L=10 ohm.

The resulting input voltage is the sum of dc voltage and sinusoidal ac voltage. Therefore, as the diode voltage varies, diode current also varies, sinusoidally. The intersection of load line and diode characteristic for different input voltages gives the output voltage as shown in fig. 6.

Fig. 6

In certain applications only ac equivalent circuit is required. Since only ac response of the circuit is considered DC Source is not shown in the equivalent circuit of fig. 7. The resistance r_f represents the dynamic resistance or ac resistance of the diode. It is obtained by taking the ratio of Δ V_D/ Δ I_D at operating point.

Dynamic Resistance Δ r_D = Δ V_D / Δ I_D

Fig. 7

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