Question: In the following, a string of three diodes is used to provide a constant voltage of about 2.1 V. We want to calculate the percentage change in this regulated voltage caused by (a) a ± 10% change in the power-supply voltage and (b) connection of a 1-kΩ load resistance. Assume n = 2.
To be solved using small signal model: I solved the first part, the input voltage is an ac wave with amplitude 1V on a dc-offset of 10V. The output ac-voltage (peak to peak) across the string of the three diodes is 37.1 mV. The incremental resistance, rd = 6.3Ω (of each diode).
The current through the circuit, I = 7.9 mA.
The method to model diode circuits that I am following is: 1. Perform dc analysis, (not considering the ac components) 2. Eliminate all dc sources, using small signal approximation; replace the diodes with their incremental resistance.
Now in the second part, after connecting the load resistance, RL. 2.1 mA current flows into the load, therefore the current through the diodes is (7.9 - 2.1) mA i.e. 5.8 mA.
So the incremental resistance should also change, as rd = nVt/Id --- (1)
The solution given is -39.7 mV. How should I proceed in the second part? --- (2)
(1) and (2) are the questions.
Note: The question is from Microelectronics Circuits, by Sedra and Smith.
EDIT: Solution: Due to a load resistance of 1kΩ the current flowing through the diode would decrease by 2.1 mA. Thus the decrease in voltage of the diodes is, -2.1*rd = -2.1*18.9 = -39.7 V.
I don't understand why rd is not changing despite a change in the diode current.