Fold & Hold

The problem

Build a folded-cascode OTA. An NMOS pair (gates “inp” and “inn”) shares a tail Current source. Each drain is a folding node, also fed by a PMOS current source from VDD (gates on an Input source named “vbp”; use about twice the pair’s multiplier). From each folding node a PMOS cascode (gate “vcp”) carries current to the next node, where an NMOS cascode (gate “vcn”) stands on an NMOS mirror transistor to ground. The two mirror gates join the left NMOS cascode’s drain (the “inp” side); the right NMOS cascode’s drain is “out”.

The signal currents of the pair are folded up through the PMOS cascodes into the mirror: the input pair and the output stack sit side by side rather than on top of each other, so the pair can have its input near ground and the output can swing widely. It needs more current than a telescopic OTA (the folding sources carry the pair’s current plus the cascodes’).

The test bench

• Supply: VDD = 1.8 V.
• The test bench adds a 1 pF load capacitor on “out”, and a 1 GΩ feedback resistor from “out” to “inn” (with a 1 F capacitor from “inn” to a quiet source) so the amplifier biases itself but is open-loop for AC.
• It applies the AC stimulus to “inp” only.

Numbers to hit

• ◆DC gain at least 60 dB
• ◆Unity-gain bandwidth at least 30 MHz
• ◆Phase margin at least 60 °
• ◆Output DC level between 0.4 V and 1.3 V
• ◆Power at most 320 µW
• ◆Every transistor in saturation