8.90 Consider the Wilson MOS mirror of Fig. 8.88(a) for the case of all transistors identical, with W/L = 10, μnCox = 400 µA/V2, Vtn = 0.5 V, and VA = 18 V. The mirror is fed with IREF = 180 µA.
(a) Obtain an estimate of VOV and VGS at which the three transistors are operating, by neglecting the Early effect.
(b) Noting that Q1 and Q2 are operating at different VDS, obtain an approximate value for the difference in their currents and hence determine IO.
(c) To eliminate the systematic error between IO and IREF caused by the difference in VDS between Q1 and Q2, a diode-connected transistor Q4 can be added to the circuit as shown in Fig. 8.38(c). What do you estimate IO to be now?
(d) What is the minimum allowable voltage at the output node of the mirror?
(e) Convince yourself that Q4 will have no effect on the output resistance of the mirror. Find Ro.
(f) What is the change in IO (both absolute value and percentage) that results from ΔVO = 1 V?