PROBLEM 012 · Medium · The cascode stage
Stack Attack
Cascode amplifier
The problem
Build a cascode amplifier: a common-source NMOS (gate on the input, source on ground) with a second NMOS stacked on its drain. The upper transistor’s gate is held at a fixed bias, its drain is the output, and a load resistor goes from VDD to the output.
The upper device shields the lower one from the output swing, so the stage’s output resistance is multiplied by about gm·ro. With a very large load resistor that output resistance still matters, and the gain you can get far exceeds what a single transistor could give. The price is headroom: two transistors and a bias have to fit under 1.8 V.
The test bench
- Supply: VDD = 1.8 V.
- The test bench hangs a 200 fF load capacitor on the net named “out”.
- It applies the AC stimulus to the source named “in” only; your gate-bias source for the upper transistor is held quiet.
Numbers to hit
- Voltage gain at least 24 dB
- Bandwidth (−3 dB) at least 2 MHz
- Output DC level between 0.6 V and 1.5 V
- Power at most 60 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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