Double Stack

The problem

Build a cascode amplifier: a common-source NMOS (gate on the signal) with a second NMOS on top of it (the cascode, gate on a fixed DC voltage “vbn”), and a PMOS current source from VDD as the load (gate on “vbp”). The output is the drain of the cascode, “out”.

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 cascode transistor shields the input transistor from the output voltage, raising the NMOS side’s output resistance to about gm·ro². The PMOS current source then dominates, so the gain is about gm·ro_p: set “vbn” and “vbp” so that all three transistors stay saturated at about 50 µA.

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 46 dB
• ◆Bandwidth (−3 dB) at least 0.5 MHz
• ◆Output DC level between 0.55 V and 1.1 V
• ◆Power at most 200 µW
• ◆Every transistor in saturation