PROBLEM 046 · Hard · The PMOS-input five-transistor OTA

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OTA with a PMOS input pair

The problem

Build a five-transistor operational transconductance amplifier: a differential pair of two PMOS (gates on “inp” and “inn”), a tail current source, and a NMOS current mirror load (diode-connected on the “inp” side). The output is the drain of the mirror’s copy, “out”. It is the mirror image of the NMOS version: the pair’s tail comes from VDD and the mirror load goes to ground.

Name the non-inverting input source “inp”. The inverting input is just a net label “inn” on the other gate: the test bench ties it to the output through a 1 GΩ resistor so that the amplifier sets its own bias.

The test bench

Supply: VDD = 1.8 V.
The test bench adds a 1p 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 33 dB
Unity-gain bandwidth at least 15 MHz
Phase margin at least 60 °
Output DC level between 0.3 V and 1.2 V
Power at most 150 µW
Every transistor in saturation

Design it yourself

Draw the circuit on a schematic canvas, size every transistor, and see it simulated in your browser with ngspice and the SkyWater SKY130 models. Every transistor lights up green, gold or red by operating region as you move the sliders.

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The problem

The test bench

Numbers to hit

Design it yourself

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