Question 7.16: Determine the input-referred thermal noise voltage of the co...
Determine the input-referred thermal noise voltage of the complementary common-source stage shown in Fig. 7.43.
With the input signal set to zero, this circuit produces the same output noise voltage as the circuit in Fig. 7.42(a) does. But the complementary stage provides a higher voltage gain, \left(g_{m 1}+g_{m 2}\right)\left(r_{O 1} \| r_{O 2}\right) The input-referred noise voltage is thus given by
\overline{V_{n, i n}^2}=\frac{4 k T \gamma}{g_{m 1}+g_{m 2}} (7.83)
an expected result because M_1 and M_2 operate in “parallel,” and hence their transconductances add. Why does this topology exhibit a lower input noise than the circuit of Fig. 7.42(a)? In both cases, M_2 injects noise to the output node, but in the complementary stage, this device operates as a transconductor and amplifies the input.
