Response to the following questions:
1. Consider the voltage-regulator circuit shown in Figure under the condition that a load current IL is drawn from the output terminal.
(a) If the value of IL is sufficiently small that the corresponding change in regulator output voltage_VO is small enough to justify using the diode small-signal model
This quantity is known as the load regulation and is usually expressed in mV/mA.
(b) If the value of R is selected such that at no load the voltage across the diode is 0.7 V and the diode current is ID, show that the expression derived in (a) becomes
Select the lowest possible value for ID that results in a load regulation whose magnitude is ≤ 5 mV/mA. If V+ is nominally 15 V, what value of R is required? Also, specify the diode required in terms of its IS .
(c) Generalize the expression derived in (b) for the case of m diodes connected in series and R adjusted to obtain VO = 0.7m V at no load.
2. Design a diode voltage regulator to supply 1.5 V to a 1.5-kΩ load. Use two diodes specified to have a 0.7-V drop at a current of 1 mA. The diodes are to be connected to a+5-V supply through a resistorR. Specify the value forR. What is the diode current with the load connected? What is the increase resulting in the output voltage when the load is disconnected? What change results if the load resistance is reduced to 1 kΩ? To 750Ω? To 500?4?
(Hint: Use the small-signal diode model to calculate all changes in output voltage.)
3. A voltage regulator consisting of two diodes in series fed with a constant-current source is used as a replacement for a single carbon-zinc cell (battery) of nominal voltage 1.5 V. The regulator load current varies from 2 mA to 7 mA. Constant-current supplies of 5 mA, 10 mA, and 15 mA are available. Which would you choose, and why? What change in output voltage would result when the load current varies over its full range?