Degenerate

The problem

Build a common-source amplifier with a resistor between the source and ground (the degeneration resistor) and a load resistor from VDD to the drain. The gate is driven by an Input source named “in” and the output is the drain, “out”.

The source resistor feeds part of the output current back to the input. The gain becomes about Rd/(Rs + 1/gm): when Rs is much larger than 1/gm it is simply Rd/Rs, set by two resistors. That makes the gain nearly immune to the transistor’s own variation, at the cost of a lower gain. The test bench proves it: it also simulates the circuit with the transistor 20% weaker (multiplier × 0.8) and the gain must barely move.

The test bench

• Supply: VDD = 1.8 V.
• The test bench hangs a 200 fF load capacitor from “out” to ground and applies the AC stimulus to “in”.
• It repeats the measurement with every transistor’s multiplier scaled by 0.8 (a weaker transistor) and compares the two gains.

Numbers to hit

• ◆Gain between 9 dB and 14 dB
• ◆Gain change with a 20% weaker transistor at most 0.6 dB
• ◆Bandwidth (−3 dB) at least 60 MHz
• ◆Output DC level between 0.8 V and 1.5 V
• ◆Power at most 250 µW
• ◆Every transistor in saturation