Use a self-powered op amp to create a low-leakage rectifier
An amplifier that works at 0.8V is key.
Martin Tomasz, Sageloop Designs, San Francisco, CA; Edited by Paul Rako and Fran Granville -- EDN, October 20, 2011
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:
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-torail
inputs and outputs and no phase
inversion when operating near the rails.
The amplifier operates at power-supply
voltages as low as 0.8V. You directly
connect the op amp’s noninverting
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 0.8V.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 subthreshold range.
The amplifier quickly
turns off due to the exponential relationship
of the gate-to-source voltage
to the drain-to-source current in the
subthreshold 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 1.5V. The active-diode circuit rectifies
the peak harvested voltage in conditions
of rapidly changing light and minimizes
reverse leakage to the cells.
Talkback
-
The biggest limitation I see is that this circuit cannot deliver voltages higher than the Absolute Maximum Rating of the opamp (namely 2.75V for the TS1001).
This is certainly not a problem in very low voltage applications (where the extremely low minimum working voltage of the TS1001 can be of great advantage), but it sets big restrictions on the applications that can exploit it.
Roberto Mariani - 2011-10-11 02:32:39 PST -
Martin,
The Spice simulation of this circuitry with an ideal Op Amp is working, but shows problems under load.
For Brad Peeters only. The thetang.com/FET Bridge does not work in reverse mode. The control voltage polarity is changed. Also the author did not recommend try it with a load capacitor.
Vladimir Doubovis - 2011-1-11 11:35:52 PDT -
FYI the op amp is Touchstone Semiconductor TS1001.
Martin Tomasz - 2011-27-10 14:50:04 PDT -
The circuit works like a very low forward-drop diode, with "reverse leakage" under 1uA (because the op amp draws under 1uA). The motivation was to replace a Schottky diode with something that has even lower forward drop and less reverse leakage current than a 1N5817 over temp (in the diagram, the op amp and FET combination equivalently replaces a diode).
These active diode circuits have been around, but you need a good sub-uA,sub-V, well-behaved (rail-rail, no phase reversal while voltage powering up) op amp to make a "low leakage Schottky" improvement. I found this circuit is good in general for reverse current protection in micropower circuits- requires no on/off control and draws nearly zero current. Note that real schottkys will always have a speed advantage over a circuit like this, but FET operation in subthreshold at low currents means even a slow op amp can move to turn off the FET surprisingly quickly, and the reverse current never exceeds the forward current and then drops rapidly, even if you short the input instantaneously- so it's reasonably good protection.
Martin Tomasz - 2011-27-10 14:39:56 PDT -
Ah, but you CAN use a MOSFET in reverse to make a low-voltage drop rectifier. Check out www.thetaeng.com/FETBridge.htm for another example.
Brad Peeters - 2011-26-10 22:15:19 PDT
