Common-Source Amplifier

Use drain-current changes to produce an amplified, inverted voltage across a drain resistor.

V1 applies a 50 mV peak, 100 Hz signal on a 3 V gate bias. The source is grounded and RD = 2 kΩ connects the drain to 5 V. With VTO = 2 V and KP = 2 mA/V², the quiescent current is near 1 mA and drain voltage near 3 V.

$$g_m\approx K_P(V_{GS}-V_T)=2\,\mathrm{mS}$$ $$A_V\approx-g_mR_D=-4$$

The gate signal is green and drain output red. A small positive gate change increases current and lowers drain voltage. Output amplitude is about 0.2 V peak on its DC bias. The square-law model adds curvature for larger signals, and eventually the available drain-voltage swing causes clipping.

Read the results. The saved transient chart uses separate voltage and current panels; the table provides numerical samples. The first voltage probe is green and the second red. The first current probe is blue; a second current probe is purple.

Model limits. The generic level-1 MOSFET model is intended to explain bias and low-frequency behavior. It omits detailed subthreshold conduction, gate charge, body effects and thermal behavior; these are not part-specific design results.

Try it. Set V1 peak voltage to 0.1 V; output amplitude approximately doubles, with more visible curvature. Restore the original values after comparing the results.