Small-signal model for diode
Note: For the top question, I found that Vd0 is approximately 0.667 V and Rd was 0.667 ohms, please check if I arrive at the right answers.
As for the bottom, I am not sure where to start.
The graph below shows the V-I Characteristic for a particular diode. Determine a small signal (piece-wise linear) model for this diode if the bias current varies between 20 and 50mA. Sketch a schematic of this model and indicate values for parameters rd and VDo. Diode V-I Characteristic 100 90 80 70 60 40 30 20 10 0.00 0.10 0.20 0.30 0.40 0.50 0.60 0.70 0.80 0.90 1.00 Diode Voltage (V) The diode of problem (11) is being used in the following circuit to provide a reference voltage of approximately 0.7V to a particular sensor. The battery voltage has a nominal value of 12.6v, but can be expected to vary from 11.1 to 14.1 volts (a typical automotive application) 330 Ω 12.6 V D1 VREF V1 Using your small signal (piece-wise) linear model from problem (11) determine the expected variation in the output reference voltage as the battery voltage varies between 11.1 and 14.1V