PROBLEM 044 · Hard · The single-stage (five-transistor) op amp
Five Alive
Five-transistor OTA
The problem
Build the classic five-transistor operational transconductance amplifier. An NMOS differential pair shares a tail current source. A PMOS current mirror (one diode-connected transistor, one copy) serves as the load, and the output is taken from the drain of the mirror’s copy transistor, the net named “out”.
The mirror converts the pair’s differential current into a single-ended output current without halving it, and the high resistance at the output gives much more gain than resistor loads. One input inverts and the other does not: the input on the diode-connected side of the pair is the non-inverting one. Drive that with “inp”, and mark the other gate with a Net label named “inn”: the test bench closes a DC feedback loop through “inn” so the output finds its own level.
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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