Stargazer

The problem

Build a telescopic OTA. An NMOS pair (gates “inp” and “inn”) shares a tail Current source. On each drain stands an NMOS cascode (gates on an Input source named “vcn”), then a PMOS cascode (gates on an Input source named “vcp”), then a PMOS mirror transistor with its source on VDD. The mirror’s two gates join the drain of the left PMOS cascode (the “inp” side): that node is the mirror’s diode. The output “out” is the drain of the right PMOS cascode.

Every device in the signal path is cascoded, so the output resistance rises from ro to gm·ro² on both sides. The gain climbs to 70 to 90 dB. The six transistors and the tail stacked between the rails leave little headroom: the output can only move over a fraction of a volt and the input common-mode range is narrow, so the bias voltages “vcn” and “vcp” must be placed carefully.

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 0.5 V and 1.2 V
• ◆Power at most 150 µW
• ◆Every transistor in saturation