PROBLEM 047 · Medium · Low-power design
Sip Power
Micropower OTA
The problem
Build a five-transistor operational transconductance amplifier: a differential pair of two NMOS (gates on “inp” and “inn”), a tail current source, and a PMOS current mirror load (diode-connected on the “inp” side). The output is the drain of the mirror’s copy, “out”. This one is for a battery-powered sensor: every microamp counts.
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 38 dB
- Unity-gain bandwidth at least 3 MHz
- Phase margin at least 60 °
- Output DC level between 0.6 V and 1.3 V
- Power at most 8 µW
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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- StargazerCascode both the pair and the load in a single stack: gain of 70 dB or more from one stage, at the price of headroom.