PROBLEM 105 · Hard · A fault in: self-biased current reference
Dead on Arrival
A reference that may never start
The problem
The circuit on the canvas was built for the problem “Self-biased current reference”, but it does not meet the specification. In simulation the current looks right, but the circuit has two stable states, and nothing makes it choose the right one when the power comes on.
Find what is wrong and repair it: change a value, move a wire or add a part. Everything the original problem asked for still applies (the targets are listed below), and the live readings and the colours of the transistors will show you where to look. Check your circuit when you think it is fixed.
The test bench
- Supply: VDD = 1.8 V.
- The test bench reports the supply current (the sum of the two branches) at 1.8 V and sweeps the supply from 1.4 V to 2.2 V, so you can see how little the current moves.
Numbers to hit
- Supply current at 1.8 V between 25 µA and 70 µA
- Supply sensitivity at most 20 %/V
- Power at most 140 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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