PROBLEM 025 · Medium · Mirror with source resistors
Resistor Armor
Current mirror with source degeneration
The problem
Build a 1 : 1 mirror from two NMOS, with a resistor of the same value (about 3 kΩ) from each source to ground. The reference transistor is diode-connected and carries 50 µA from a Current source on VDD; the copy’s drain is “out”.
The resistors add negative feedback in both branches. A small difference between the two transistors now causes only a small difference between the currents, and the output resistance rises by about (1 + gm·R). The cost is headroom: the resistor drops 150 mV in each source, so the minimum output voltage rises by that much.
The test bench
- Supply: VDD = 1.8 V.
- Your reference is the Current source you place: set it to 50 µA.
- The test bench holds the net named “out” at 0.9 V with a voltage source, sweeps it from 0.1 V to 1.7 V, and measures the current your mirror exchanges with it.
Numbers to hit
- Output current at 0.9 V between 48 µA and 52 µA
- Output resistance (0.5 V to 1.5 V) at least 0.6 MΩ
- Current within 2% above at most 0.8 V
- 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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