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EXAMPLE 10.5 Objective: To determine the currents in a Widlar current source circuit. Assume the Widlar source is biased at V+ = +5 V and V = -5 V, and assume resistor values R₁ = 7 k2 and RE = 4 k. Also assume VBE1 = 0.7 V. Solution: The reference current is found to be IREF = V+ - VBE1-V- R₁ 5-0.7-(-5) 7 = 1.33 mA The load current is found from the relation IORE = VT In IREF Io Io (4) 0.026 In 1.33 Io A transcendental equation cannot be solved directly. A computer solution or a trial and error solution yields 1o = 25.7 μα Comment: In this case, the difference between the two base-emitter voltages is IORE 103 mV. Again, a relatively small difference in the two base-emitter volt- ages can produce a relatively large difference between the reference and load currents. I. Consider the Widlar current source in Figure 10.9 (Textbook), with parameters given in Example 10.5 (Textbook). Choose appropriate transistor parameters. Connect a 50 k resistor between V* and the collector of Q₂ as a load. Using a computer simulation, determine IREF, lo, VBE1, and VBE2. V+ || Icz=lo Ro IREF R₁ -OV C2 Icil + + VBEI VBE2 IE2 RE V- Figure 10.9 Widlar current source or 2₁