Copy Paste Pro

The problem

Build a 1 : 1 cascode current mirror from four NMOS transistors. A 50 µA current source from VDD feeds two diode-connected transistors stacked in series, which generate two bias voltages. A matching stack on the output side copies the current to the net “out”.

The upper transistor in the output stack shields the lower one from the output voltage, so the mirror’s output resistance is multiplied by about gm·ro. The current stays almost perfectly flat as “out” moves, as long as the output stays high enough to keep all four transistors saturated: that minimum is the circuit’s compliance voltage.

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 1.2 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 1.2 V between 49 µA and 51 µA
• ◆Change between 1.0 V and 1.6 V at most 0.5 %
• ◆Current within 2% of its value at 1.2 V down to at most 1.1 V
• ◆Every transistor in saturation