1N4148 Diode I-V Curve Tracer

Application

This circuit characterizes the 1N4148 by sweeping the voltage bias across the diode from reverse bias into forward bias. This style of setup is commonly called a diode I-V curve tracer, or a forward/reverse bias sweep.

Design Context

VBIAS is connected directly across the diode under test through the current probe. The DC sweep moves VBIAS from -1 V to +1 V in 10 mV steps, so the chart shows how diode current changes as the applied diode voltage crosses from reverse bias into forward conduction. The cathode is tied to the common return, so positive sweep values forward-bias the diode and negative values reverse-bias it.

Expected Behavior

For negative bias values, Id should remain very small because the diode is reverse biased below breakdown. Near zero bias, current should stay close to zero. As the sweep approaches the forward knee, the current should rise quickly, showing the exponential forward conduction region of the 1N4148 model.

Key Values

• Sweep source: VBIAS
• Sweep range: -1 V to +1 V
• Sweep increment: 0.01 V
• Measured voltage: Vd
• Measured current: Id

Suggested Experiments

• Narrow the sweep around 0.5 V to 0.8 V to inspect the forward knee in more detail.
• Reduce the sweep stop value if you want to focus on low-current small-signal operation.
• Compare the curve with a rectifier or Schottky diode to see how forward voltage and leakage differ.

Featured Component

The 1N4148 is the diode under test. It is a fast small-signal silicon switching diode, and this demo focuses on the model's basic current-versus-voltage behavior across reverse and forward bias.