CMOS Analog Integrated Circuits

5.4    Folded-cascode amplifier

FIGURE 5.26(a) Circuit diagram and (b) small-signal equivalent model of a cascode amplifier.

An amplifier with a cascode configuration has the advantage of exhibiting a higher bandwidth due its improved isolation between the input and output nodes. Figure 5.26(a) shows the circuit diagram of a cascode amplifier, whose input stage is a common-source transistor driving a common-gate transistor loaded by a resistor. By replacing each transistor with its equivalent model, the small-signal circuit shown in Figure 5.26(b) can be derived. It is assumed that ${\upsilon }_{gs2}=-{\upsilon }_{s2}$ and ${\upsilon }_{bs2}=-{\upsilon }_{s2}$. Using Kirchhoff’s current law for the ...