In the design of a Widlar current source of Fig. 4.31 a to produce a specified output current, two resistors must be selected. Resistor R₁sets I_IN, and the emitter resistor R₂sets I_OUT. Assuming a supply voltage of V_CCand a desired output current I_OUT, determine the values of the two resistors so that the total resistance in the circuit is minimized. Your answer should be given as expressions for R₁and R₂in terms of V_CCand I_OUT. What values would these expressions give for Problem 4.20? Are these values practical?

Data from Prob. 4.20:

Design a Widlar current source using npn transistors that produces a 5 µA output current. Use Fig. 4.31a with identical transistors, V_CC = 30 V, and R₁ = 30 kΩ. Find the output resistance.

Fig. 4.31 a:

Transcribed Image Text:

Vcc IN | IOUT R1 Icı! Q2 Q1 R2