A fully differential op amp with CMFB is shown in Fig. 12.57. For M₁, M_1C, M₂, and M_2C, use W/L = (64 µm) / (0.8 µm). For M₃ €“ M₄, M₂₆ €“ M₂₇and M₁₁, W / L = (96 µm) / (1.4 µm). For M₂₁€“ M₂₄, W / L = (6 µm) / (0.8 µm). For M₁₄, M₂₅, and M₅₂, W / L = (16 µm) / (0.8 µm). For M₁₃, W / L = (1.4 µm) / (0.8 µm). Take V_CM = ˆ’0.65 V.

(a) Choose W values for M₁₂ and M₅₁ so that |I_D13|=20 µA. Use L = 0.8 µm for M₅₁ and L = 1.4 µm for M₁₂.

(b) Use SPICE to find the low-frequency op-amp gains v_od /v_id, v_oc/v_ic, v_od/v_ic, and v_oc/v_id with the CMFB active.

(c) Calculate the output slew rate dV_od/dt if a 4-pF capacitor is connected from each op-amp output to ground.

(d) What is the differential output voltage swing of this op amp? Assume V_ic = V_CM, and ignore body effect for this calculation.

(e) Repeat (b) when the input transistors are mismatched with W₁ = 63 µm and W₂ = 65 µm. (Note: With mismatch, the op-amp offset voltage is not zero.)

(f) Repeat (b) when the load transistors are mismatched with W₃ = 95 µm and W₄ = 97 µm.

Figure 12.57:

VDp = +1.65V %3D M27 M26 M4 M12 M11 Vo2 M24 M23 Vo1 M22 M21 M2C M1C V I 3 100 M13 M2o Vi2 M1 V;1 M25 M52 M51 M14 -Vss =-1.65V