Folded Cascode

A folded-cascode stage takes a differential input pair and routes its signal current sideways into a cascode device of the opposite polarity instead of stacking a same-type cascode transistor directly on top — an NMOS input pair's current gets picked up by a PMOS cascode branch, folded through a current-source node rather than piled straight up the same rail — which is what lets the input pair's drains sit near one supply rail while the cascode devices operate near the other, recovering most of the headroom a straight telescopic cascode would have burned through stacking four same-polarity devices in a single column.

The "fold" is a literal change in current direction at a specific circuit node, not a metaphor. In a telescopic cascode, current flows straight up through the input device and then straight up again through the cascode device stacked directly above it — one unbroken column, one polarity, and every device in that column eats into the same shared headroom budget; in a folded-cascode stage, the input pair's drain current flows down into a current-source node biased to absorb exactly the DC bias current, and the AC signal component of that current gets redirected sideways and up through a cascode device of the opposite polarity sitting in its own separate branch — the current literally changes direction and crosses from an NMOS-dominated region of the circuit into a PMOS-dominated one at that fold node.

This is specifically what buys back headroom in low-voltage processes, and the mechanism is about separating columns, not eliminating devices. A folded-cascode stage still uses the same total device count as a telescopic one — input pair, cascode devices, current sources — but because the input pair's branch and the cascode branch are now electrically separate columns each needing only their own headroom rather than one shared column needing everyone's headroom stacked together, the input common-mode range can sit much closer to a supply rail without the whole stage running out of voltage margin, which is exactly the scarce resource in any modern CMOS process running on a 1.2 volt or lower supply.

It is the standard first stage of almost every single-stage CMOS operational amplifier for a reason beyond just headroom. Because the cascode branch is a separate node from the input pair's drains, folded-cascode op-amps can combine wide input common-mode range with a single high-impedance output node suitable for driving a second gain stage or an output buffer directly, and the output impedance still benefits from the same multiplicative gm-ro-ro cascode boost that a telescopic design gets — this combination of properties, not any one property alone, is why folded-cascode architecture appears as the default input stage in commercial general-purpose op-amp ICs across essentially every analog semiconductor vendor.

The tradeoff that doesn't disappear is extra current and extra noise, since folding needs its own dedicated bias branch. A telescopic cascode reuses the same current for the input pair and the cascode devices since they're stacked in series; a folded-cascode stage needs a separate current source to bias the fold node and sum correctly with the input pair's current, which means the cascode branch carries additional bias current beyond what the input pair alone draws — more total current means more power consumption, and the extra current-source devices contribute their own thermal noise directly into the signal path, so folded-cascode designs trade raw noise performance and power efficiency for the headroom and common-mode range gains that make them worth it in low-voltage designs.

Read a folded-cascode stage through a *column-separation* lens rather than a *cascode variant* lens: it keeps the exact same output-impedance multiplication a telescopic cascode delivers, but physically splits what would be one tall shared stack into two separate branches joined only at a current node, which is the specific move that frees the input pair to sit near a rail without dragging the cascode devices' headroom requirement along with it.