Understanding Active Probes

This presentation provides a technical introduction to single-ended active FET probes and explains why they are essential for certain oscilloscope applications. Active probes use a field-effect transistor (FET) in the probe tip, enabling very low input capacitance, typically single-digit picofarads compared to tens of picofarads for passive probes. This results in higher input impedance, significantly reduced circuit loading, and bandwidth well into the GHz range. The video also covers how active probes provide high offset range independent of the oscilloscope’s vertical scale settings, making them ideal for measuring small AC components on large DC offsets. Trade-offs including cost, voltage range, and physical robustness are discussed.

What you’ll learn:

• How active probes minimize circuit loading through low input capacitance, resulting in less signal distortion and reduced ringing compared to passive probes
• Why active probes offer significantly higher bandwidth (GHz range) and are recommended for high-frequency signals, fast rise times, and square or pulsed waveforms
• How the built-in offset capability of active probes enables improved vertical resolution when measuring small signal details on top of large DC offsets

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