A signal’s peak is typically measured with a diode. But if the diode is used alone, the input voltage must be much larger than the diode’s turn-on voltage to obtain acceptable accuracy. Since turn-on voltages range from 200 mV in germanium diodes to 700 mV in silicon diodes, a simple diode peak detector requires an input voltage of 2 to 7 V, respectively, to achieve a 10% error.
You can improve resolution and accuracy in low-power and single-supply apps by adding an active feedback loop. The technique employs a few components and a high-speed dual op amp to compensate for the diode turn-on voltage (see the figure 1).
 |
|
| Figure 1. | Adding a switched-feedback function to the simple diode peak detector improves resolution and accuracy even with small input signals. |
The diode provides the rectification. Similarly, the 100-kΩ resistor and 10-nF capacitor in parallel supply the low-pass filtering to average the peak signal. One op amp buffers this output while the other provides a high-impedance input and feedback node.
The resistor network surrounding the transistor is a clamping circuit. When the circuit input is higher than the average, the forward op amp’s output is also higher than average. The diode then conducts, as in the simple case, and the transistor is held off.
But when the input drops below the average level, the diode doesn’t conduct, and the transistor allows a feedback path to be created around the forward op amp. This switched feedback forces the negative input of the forward op amp to track the output voltage. This tracking is key for recovery.
