Fold It

The problem

Build a folded-cascode stage. An NMOS driver (gate on the signal, source on ground) has its drain on a folding node. A PMOS current source from VDD (gate “vbp2”) also feeds that node. A PMOS cascode (gate “vbp1”) takes the node’s current to the output “out”; below “out” an NMOS cascode (gate “vbn1”) sits on an NMOS current source (gate “vbn2”) to ground.

The stage has so much gain that nothing could set its output voltage by hand, so it biases itself: a 1 GΩ resistor from the output to the gate lets the DC level settle by feedback, and a 10 µF capacitor couples the AC input to the gate while keeping the source’s DC level out of the way. The resistor is so large that the feedback has no effect on the signal.

The top current source supplies the driver’s current plus the cascode branch’s current, so it must carry about twice as much (use twice the multiplier). The signal current of the driver is “folded” through the PMOS cascode into the output, which can then swing from about 0.3 V to 1.5 V: much more than a telescopic stage.

The test bench

• Supply: VDD = 1.8 V.
• The test bench hangs a 200 fF load capacitor from “out” to ground and applies the AC stimulus to “in” only.
• Set the “in” source to 0 V: the 10 µF coupling capacitor blocks DC.

Numbers to hit

• ◆Gain at least 56 dB
• ◆Unity-gain frequency at least 100 MHz
• ◆Output DC level between 0.5 V and 1.1 V
• ◆Power at most 350 µW
• ◆Every transistor in saturation