This problem presents a very interesting addition to the folded-cascode op-amp circuit of Fig. 13.10, designed to deal ith the situation during amplifier slewing. In particular, the additional circuitry does two things: It prevents Q1 and Q11 from going nto the triode region, and it increases the current available to charge CL and thus increases the slew rate. The circuit is shown in Fig. P13.27 (with the current-mirror circuit omitted, for simplicity). Observe that three transistors are added: Q14, which is biased by a constant-current source (20 μA), establishes the dc currents in Q9 and Q10. Assume with respect to Q9 and Q10 that each has a W/L ratio 10 times that of Q14. The other two additional ransistors are Q12 and Q13, which are diode connected and are normally cut off.
(a) For Vid = 0, find the bias current in each of Q1, Q2, Q3, Q4, Q14, Q9, and Q10. Also, for the dc voltages shown, and assuming Vtn =|Vtp |= 0.45 V and that all conducting devices are operating at |VOV | = 0.15 V, show that Q12 and Q13 will be cut off.
(b) For an input differential signal that causes Q2 to turn off and Q1 to conduct the entire bias current (320 μA), Q12 turns on (while Q13 remains off). Noting that the drain current of Q12 adds to the 20 μA flowing through Q14, find the current that now flows through Q10 and onto CL.
By what factor does the slew rate increase relative to the value without Q12 present? Also give an approximate estimate of the voltage at the drain of Q1 during the slewing transient.
