cascode

A cascode is a two-transistor stack: you put a second transistor on top of your gain device so they share the same current, but split the work. The bottom one (the input device) does the amplifying. The top one (the cascode) just sits there holding its source at a steady voltage, acting like a buffer between your delicate gain device and whatever the output is doing. Think of it as a bodyguard standing in front of the amplifier: it absorbs the shoving from the output node so the input device barely feels the output voltage move at all. Because the input device's drain voltage now stays nearly constant, two good things happen — its tiny channel-length-modulation feedback (the Early effect, which softens an ideal current source into a finite output resistance ro) almost stops mattering, and the Miller multiplication of its gate-drain capacitance collapses, which helps your bandwidth.

The payoff is gain. A plain common-source stage gives intrinsic gain of about gm*ro — the device's transconductance times its own output resistance, and ro is annoyingly low in short-channel processes. Stacking a cascode multiplies the looking-in output resistance by roughly the cascode's own gm*ro, so the node now presents something like gm*ro*ro instead of just ro. Your DC gain jumps by that same factor — easily 20 to 40 dB more — without burning any extra current, since both transistors carry the same bias. That is why cascodes are everywhere in op-amp output stages, current mirrors, and anywhere you need a stiff, high-impedance node.

The bill comes due in headroom. You've now stacked two drain-source voltages between supply and ground, and each transistor needs at least its overdrive Vov (with margin) to stay in saturation, so you spend roughly two Vov of voltage room instead of one. On a modern low-voltage supply that's expensive, and it's why folded cascodes and gain-boosting exist — clever ways to keep the gain win while clawing back some of the headroom you gave up.