For this problem, use the op amp in Fig. 12.23 and the CMFB scheme in Fig. 12.17. Use the complement of the amplifier in Fig. 12.16b as the CM-sense amplifier, modified to give a negative dc gain. Assume the source followers in Fig. 12.17 have a low-frequency gain of 0.95 and R_cs= 15 kΩ. Use the transistor and op-amp operating-point data given in the example in Section 12.6.1.

Fig. 12.23:

Fig. 12.16(b)

Figure 12.17:

(a) Design the CM-sense amplifier so that the total low-frequency gain a_cms0 = v_oc / v_cms = ˆ’0.71, which is the same value as in the example in Section 12.6.1.

(b) With this CMFB circuit, what are the swing limits for each op-amp output voltage (V_o1 and V_o2)? Assume that the biasing current source in each source follower in Fig. 12.17 is implemented with a NMOS transistor, and the current-source and source-follower transistors operate with V_GS = 0.8 V and V_ov = 0.2V. For simplicity, assume V_GS is constant and take γ = 0.

(c) What value of V_CM gives the largest symmetric output swing?

(d) Verify that this CMFB circuit works correctly by running a SPICE simulation. Use the value of V_CM from part (c).

Transcribed Image Text:

VDD Vв1 V в1 M5 V one M10 Vo1 o Vi2 M2 Vo2 M1 Vit Vв2 M6 Мз Mg -Vss VDD VB M25 oV. M21 M22 VCM M24 M23 Veme = Vems -Vss (b)