CMOS Analog IC Review Part I

3.6. Cascode Stage

The voltage gain can be written as -G_mR_out. In this circuit, G_m ≈ g_m1 and R_out ≈ (g_m2 + g_mb2)r_O2r_O1. The cascode configuration greatly increases the output impedance at the cost of headroom.

The DC value at the output of such configuration is poorly defined, hence the circuit must be biased in a negative-feedback loop.

The “poor man’s cascode” here places M₂ in triode region. If M₁ and M₂ have identical dimensions, the structure is equivalent to a single transistor having twice the length. However, if M₁ and M₂ have different V_TH, M₂ may operate in saturation.

Cascoding reduces the mismatch between ID1 and ID2 by a factor of (g_m3 + g_mb3)r_O3.