PROBLEM 048 · Hard · Speed against power
Need for Speed
A fast 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 fast signal path: bandwidth first, power second.
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 250 MHz
- Phase margin at least 60 °
- Output DC level between 0.6 V and 1.3 V
- Power at most 600 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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