Bent Wave

The problem

Build a common-source amplifier with a degeneration resistor (source to ground through a resistor) and a load resistor from VDD to the drain “out”, driven by an Input source “in”. The test bench applies a 100 mV peak sinewave worth of input around the DC level you chose and computes the harmonic distortion of the output from the stage’s DC transfer curve.

A square-law transistor makes a second harmonic of relative size A/(4·Vov) for an input of amplitude A. A resistor in the source feeds back the output current and reduces every harmonic by the factor 1 + gm·Rs (for the second) and by its square (for the third). Distortion falls fast as the degeneration grows: you are trading gain for linearity.

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 also sweeps the source “in” from 0.2 V below to 0.2 V above your chosen DC level and takes the stage’s transfer curve.
• The total harmonic distortion (THD) is computed by passing a 100 mV peak sinewave through that curve and taking the Fourier components (2nd to 6th harmonics) relative to the fundamental.

Numbers to hit

• ◆Gain at least 6 dB
• ◆THD at 100 mV peak input at most 1.5 %
• ◆Output DC level between 0.8 V and 1.5 V
• ◆Every transistor in saturation