Gain Boost

The problem

Build a mirror whose output branch is M2 (a mirror transistor, gate on the reference’s gate net, source on ground), a cascode M3 on top of it (drain on “out”, source on M2’s drain), and an amplifier M4: an NMOS with its source on ground, its gate on M2’s drain, and its drain on M3’s gate. A 10 µA Current source from VDD pulls M4’s drain (and M3’s gate) up. The reference branch is a diode-connected NMOS fed by a 50 µA source.

M4 senses M2’s drain voltage and adjusts M3’s gate to hold it constant: the cascode’s gate is driven by feedback instead of a fixed bias. The output resistance is then about gm3·ro3·ro2 multiplied by M4’s gain: tens or hundreds of megohms, from five transistors.

The test bench

• Supply: VDD = 1.8 V.
• The reference current source you place should be 50 µA (into the diode-connected transistor). A second Current source of 10 µA feeds M4 (any value from 5 µA to 20 µA works).
• 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 48 µA and 52 µA
• ◆Output resistance (1.0 V to 1.6 V) at least 200 MΩ
• ◆Every transistor in saturation