Category: microelectronics sedra and smith chapter 4
4.57 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?
*4.56 Design a diode voltage regulator to supply 1.5 V to a 1.5-k 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 k ? To 750 ? To 500 ? (Hint:Use the small-signal diodemodel to calculate all changes in output voltage.)
*4.55 Consider the voltage-regulator circuit shown in Fig P4.54 under the condition that a load current I L is drawn from the output terminal. (a) If the value of I L is sufficiently small that the corre- sponding change in regulator output voltageﬁV O is small enough to justify using the diode small-signal model, show that ﬁV O I L = − r d ∫R 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 I D , show
*4.53 In the circuit shown in Fig. P4.53, I is a dc current and v i is a sinusoidal signal with small amplitude (less than 10 mV) and a frequency of 100 kHz. Representing the diode by its small-signal resistance r d ,which is a function of I, sketch the small-signal equivalent circuit and use it to determine the sinusoidal output voltage V o , and thus find the phase shift between V i and V o . Find the value of I that will provide a phase shift of –45°, and find the range of phase shift achieved as I is varied over the range of 0.1 times to 10 times this value.
**4.52 In Problem 4.51 we investigated the operation of the circuit in Fig. P4.51 for small input signals. In this problem we wish to find the voltage-transfer characteristic (VTC) v O versus v I for −12V ≤ v I ≤ 12V for the case I = 1 mA and each of the diodes exhibits a voltage drop of 0.7 V at a current of 1 mA. Toward this end, use the diode exponential characteristic to construct a table that gives the values of: the current i O in the 10-k resistor, the current in each of the four diodes, the voltage drop across each of the four diodes, and the input voltage v I , for v O = 0, +1 V, +2 V, +5 V, +9 V, +9.9 V, +9.99 V, +10.5 V, +11 V, and +12 V. Use these data, with extrapolation to negative values of v I and v O , to sketch the required VTC. Also sketch the VTC that results if I is reduced to 0.5 mA. (Hint: From symmetry, observe that as v O increases and i O correspondingly increases, i D3 and i D2 increase by equal amounts and i D4 and i D1 decrease by (the same) equal amounts.)
**4.52 In Problem 4.51 we investigated the operation of the circuit in Fig. P4.51 for small input signals. In this ...