For the level-restoring circuit of Fig. 15.19, let k1n = 3k1p = 75 μA/V2, VDD = 3.3 V, |Vt0 |= 0.8 V, γ = 0.5 V1/2, 2φf = 0.6 V, (W/L)1 = (W/L)n = 1.2 μm/0.8 μm, (W/L)p = 3.6 μm/0.8 μm, and C = 20 fF. Also let Vb = VDD. Now for vA rising to VDD, and Q1 charging C and causing vO1 to rise, show that the value of vO1 that causes vO2 to drop by a threshold voltage below VDD (i.e., to 2.5 V) so that QR turns on, is approximately VDD/2 and thus occurs at t _tLH . What is the capacitor-charging current available at this time (i.e., just rior to QR turning on)? What is it at vO1 = 0? What is the average current vailable for charging C? Estimate the time tPLH . (Note that after QR turns on, vO1 rises to VDD.)
