Bipolar Cascode In order to relax the trade-off between the output impedance
and the voltage headroom, we can replace the degeneration resistor with a transis-
tor. Depicted in Fig. 9.2(a) for the bipolar version, the idea is to introduce a high
small-signal resistance (= rO2 ) in the emitter of Q 1 while consuming a headroom in-
dependent of the current. In this case, Q 2 requires a headroom of approximately
0.4 V to remain in soft saturation. This configuration is called the “cascode” stage.2 To
emphasize that Q 1 and Q 2 play distinctly different roles here, we call Q 1 the cascode
transistor and Q 2 the degeneration transistor. Note that IC 1 ≈ IC 2 if β11.