Which statement about peak detectors at very fast input signals is most accurate?

Which statement about peak detectors at very fast input signals is most accurate?

• A. Slew rate is the only factor that limits tracking of fast peaks.
• B. The diode drop ensures precise peak detection at all frequencies.
• C. The detector tracks peaks perfectly regardless of device limitations.
• D. The diode drop, leakage, and recovery issues with very fast or low-level signals limit tracking of fast peaks.

Answer: (D)

When a peak detector relies on a diode and a capacitor, real-world nonidealities matter most for very fast or small signals. The fixed forward voltage drop of the diode means the detected peak is always lower than the actual peak, and this error becomes especially noticeable for small amplitudes. The capacitor stores the peak value, but it isn’t perfect: leakage through the diode and any leakage or loss in the capacitor causes that stored value to droop between peaks. For fast input signals, the diode also has a finite recovery time and capacitance, so it can’t switch perfectly instantaneously as the waveform changes, which leads to distortion or missed/blurred peaks. These combined effects—diode drop, leakage, and recovery dynamics—limit how accurately the detector can follow fast peaks, particularly when the signal levels are low.

When a peak detector relies on a diode and a capacitor, real-world nonidealities matter most for very fast or small signals. The fixed forward voltage drop of the diode means the detected peak is always lower than the actual peak, and this error becomes especially noticeable for small amplitudes. The capacitor stores the peak value, but it isn’t perfect: leakage through the diode and any leakage or loss in the capacitor causes that stored value to droop between peaks. For fast input signals, the diode also has a finite recovery time and capacitance, so it can’t switch perfectly instantaneously as the waveform changes, which leads to distortion or missed/blurred peaks. These combined effects—diode drop, leakage, and recovery dynamics—limit how accurately the detector can follow fast peaks, particularly when the signal levels are low.