Figure shows an interesting and very useful application of feedback to improve the performance of the current mirror formed by Q1 and Q2. Rather than connecting the drain of Q1 to the gate, as is the case in simple currentmirrors, an amplifier of gain +μ is connected between the drain and the gate. Note that the feedback loop does not include transistor Q2. The feedback loop ensures that the value of the gate-to-source voltage ofQ1 is such that Io1 equals Is. This regulated Vgs is also applied to Q2. Thus, if W/L of Q2 is n times W/L of Q1, Io2 = nIo1 = nIs. This current tracking, however, is not regulated by the feedback loop.
(a) Show that the feedback is negative.
(b) If μ is very large and the input resistance of the amplifier μ is infinite, what dc voltage appears at the drain of Q1?
If Q1 is to operate at an overdrive voltage of 0.2 V, what is the minimum value that VBIAS must have?
(c) Replacing Q1 by its small-signal model, find an expression for the small-signal input resistance Rin assuming finite gain but infinite input resistance for the amplifier μ. Note that here it is much easier to do the analysis directly than to use the feedback-analysis approach. For large μ, what does Rin become?
(d) What is the output resistance Rout?
