PROBLEM 104 · Medium · A fault in: five-transistor ota
Gain Heist
An OTA with no gain
The problem
The circuit on the canvas was built for the problem “Five-transistor OTA”, but it does not meet the specification. The gain is far below what five transistors should give, and one of the mirror transistors is not behaving as a mirror.
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 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 the source named “inp”; give that source a DC level of your choice (the input common-mode).
Numbers to hit
- DC gain at least 35 dB
- Unity-gain bandwidth at least 20 MHz
- Phase margin at least 60 °
- Output DC level between 0.6 V and 1.3 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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