(a) Suppose that the npn and pnp transistors in the input stage of the NE5234 op amp are changed into n-channel and p-channel MOS transistors, respectively. If transistors conducting nonzero current operate in strong inversion, the transconductance of the revised circuit depends on the common-mode input voltage, V_IC. Explain why.

(b) Fig. 6.63 shows one circuit that overcomes this problem. Let R represent the gains of the M₈ €“ M₇ and the M₁₁ €“ M₁₂ current mirrors. When V_B1 ‰ª V_IC ‰ª V_DD ˆ’ V_B2, M₅ €“ M₈ as well as M₁₁ and M₁₂ are off. Therefore, the tail current for M₁ €“ M₂ is I_B1, and the tail current for M₃€“M₄ is I_B2. On the other hand, when 0 ‰¤ V_IC ‰ª V_B1, M₁ and M₂ are off, and the tail current in M₃ and M₄ becomes RI_B1 + I_B2. Similarly, when V_DD ‰¥ V_IC ‰« V_DD ˆ’ V_B2, M₃ and M₄ are off, and the tail current in M₁ and M₂ becomes I_B1 + RI_B2. Find the value of R for which the transconductance of the input stage when only one differential pair is active is the same as when both pairs are active. Assume that I_B1 = I_B2 and µ_n C_ox,n (W/L)_n = µ_p C_ox,p (W/L)_p for M₁ €“ M₄. Also, assume that M₉, M₁₀, M₁₃, and M₁₄ operate in saturation.

Fig. 6.63:

Transcribed Image Text:

Vpp R14 R13 2. V82 o Bias, M4A Маз M42 2 M41 M2 M4 M6 9+ Vodt - 10 M5 M4 Мз M10 - BiasCM Mg Mg 4 M7 Тв1 VB1 3. R10