Master the Basics: How to Detect the Signal Peak
A peak detector is a simple but powerful circuit that captures and holds the maximum voltage reached by an input signal. It's commonly used in audio systems, measurement devices, and signal monitoring circuits to track the highest point of a waveform.
What Does a Peak Detector Do?
A peak detector circuit stores the highest voltage a signal reaches over time. For example, in audio processing, peak detectors help systems monitor signal volume and adjust output accordingly.
Building a Basic Peak Detector
The simplest version of a peak detector consists of:
- A diode in series with
- A capacitor
Here’s how it works:
- When the input voltage increases, the diode conducts and charges the capacitor up to that voltage.
- When the input voltage drops, the diode blocks reverse current, and the capacitor holds the charge—representing the peak value.
Accounting for the Diode Drop
Real-world diodes have a forward voltage drop (typically around 0.6V for a 1N4148). This means the capacitor voltage will always be a little lower than the true input peak.
To estimate the actual peak voltage:
Measured Output + Diode Drop ≈ Actual Peak Voltage
For example, if the output voltage is 2.4V, the actual peak input was around 3.0V.
Demonstration Example
Using a 1N4148 diode and a 100 µF capacitor:
- When the input rises to 2V, the output holds at ~1.4V.
- When the input drops back to 1V, the output remains at 1.4V.
- If the input jumps to 3V, the output increases to ~2.4V and stays there.
Over time, the voltage across the capacitor will slowly leak and decrease unless reset intentionally.
Adding Auto-Reset with a Resistor
To automatically reset the peak detector output, add a resistor in parallel with the capacitor. This allows the stored charge to slowly discharge after a peak event.
The discharge time depends on the RC time constant, and a good rule of thumb is:
Discharge Time ≈ 5 × R × C
For a 100 µF capacitor and a 10 kΩ resistor:
5 × 10,000 × 0.0001 = 5 seconds
This means the output will reset approximately five seconds after the peak occurs.
Real-World Behavior
With a resistor added:
- A short voltage spike raises the output quickly.
- The voltage decays smoothly as the capacitor discharges through the resistor.
This setup is useful when you only care about peaks within a specific time window.
Key Takeways
A peak detector is an effective way to measure and hold the highest voltage seen in a signal. By using just a diode, capacitor, and optionally a resistor, you can build a compact circuit for signal tracking, audio processing, or any application that requires peak detection. Understanding how to size your components gives you full control over how long the peak is held and when it resets.
