MOSFET Current Ramp

Charge a gate capacitor with constant current and use source degeneration to produce a nearly linear current ramp after threshold.

I1 = 100 µA charges C1 = 1 µF from zero, raising the gate at 100 V/s. M1 has threshold 1 V and KP = 20 mA/V². Its 10 kΩ source resistor supplies local negative feedback, making drain current approximately follow the gate ramp after threshold. The drain receives 10 V through 1 kΩ.

$$\frac{dV_G}{dt}=\frac{I_1}{C_1}=100\,\mathrm{V/s}$$ $$I_D\approx\frac{V_G-V_T-V_{\mathrm{ov}}}{R_S}$$

Gate voltage is green, source voltage red and drain current blue. Current remains small until the gate crosses threshold, then rises nearly linearly; the finite overdrive leaves curvature near turn-on. A 1 GΩ return prevents a floating gate at DC. Initial conditions are used so the capacitor begins uncharged.

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 I1 to 200 µA; the gate voltage rises twice as fast and the current ramp becomes steeper. Restore the original values after comparing the results.