AN842 - Silicon Labs

Use a Self-Powered Op Amp to Create a Low-Leakage Rectifier
An Amplifier that Works at 0.8 V is Key
1. Introduction
Figure 1.
You can combine a carefully chosen op amp, a low-threshold P-channel MOSFET, and two feedback resistors to
make a rectifier circuit with less forward drop than a diode (Figure 1). The rectified output voltage powers the active
circuitry, so no additional power supply is necessary. The circuit’s quiescent current is lower than most Schottky
diodes’ reverse-leakage current. This circuit provides active rectification at voltage drops as low as 0.8V. At lower
voltages, the MOSFET’s body diode takes over as an ordinary diode.
The op-amp circuit turns on the MOSFET as a forward voltage develops between the input and the output voltages,
according to the following equation:
where VGATE is the MOSFET’s gate drive, VIN is the input voltage, and VOUT is the output voltage. You can
relate the input and the output voltages to the MOSFET’s drain-to-source and gate-to-source voltages, according
to the following equations:
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Copyright © 2014 by Silicon Laboratories
where VDS is the drain-to-source voltage and VGS is the gate-to-source voltage. Combine these equations to
relate the MOSFET’s gate drive to a function of the drain-to-source voltage:
Figure 2.
If you make R2 12 times larger in value than R1, a 40 mV voltage drop across the MOSFET’s drain-to-source
voltage is sufficient to turn on the MOSFET at low drain currents (Figure 2). You could choose a higher ratio to
further reduce the voltage drop within the limits of the op amp’s worst-case input-offset voltage of 6 mV. The op
amp is powered from output-reservoir capacitor C1. The amplifier has rail-to-rail inputs and outputs and no phase
inversion when operating near the rails. The amplifier operates at power-supply voltages as low as 0.8 V. You
directly connect the op amp’s non-inverting input to the VDD rail and the amp’s output to the gate of the MOSFET.
The circuit consumes slightly more than 1 µA when actively rectifying a 100 Hz sine wave, less current leakage
than that of most Schottky diodes. The BSH205 supports milliamp-level currents at a gate-to-source voltage of
0.8 V.
Rev. 1.0
Figure 3.
The op amp’s bandwidth limits the circuit to lower-frequency signals. At bandwidths higher than 500 Hz, the
amplifier’s gain begins to decline. As the signal frequency increases, the MOSFET remains off, and the body diode
of the MOSFET takes over the rectification function. An input with a fast fall time could potentially drag the output
with reverse current through the MOSFET. However, for small currents, the MOSFET operates in its sub-threshold
range. The amplifier quickly turns off due to the exponential relationship of the gate-to-source voltage to the drainto-source current in the sub-threshold range. The limiting factor is the amplifier’s slew rate of 1.5V/msec. As long
as you don’t load the circuit so heavily that you drive the MOSFET into its linear range, reverse currents won’t
exceed forward currents.
You can use the circuit in a micropower solar-harvesting application (Figure 3). Depending on the light, the BPW34
cells generate 10 to 30 µA at 0.8 to 1.5 V. The active-diode circuit rectifies the peak harvested voltage in conditions
of rapidly changing light and minimizes reverse leakage to the cells.
Please see the documentation for the TS1001 Op Amp. For additional information, contact Silicon Labs.
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