Precision Full-wave Rectifier using Low-Powered Single-Supply Op-Amps (eg. LM324/358)

On a low-powered battery-operated project that I'm working on, I needed  to get a fully-rectified output of a sine-wave like audio level input signal. Op-Amps based circuits are usually the handiest ways of doing this since they enable us to avoid forward voltage drop problems which occur in diodes based rectifier circuits. At consumer audio signal levels (less than +/- 1V), these diode voltage drops can wreck havoc on the input signals.

However, most op-amp based rectifier circuits are designed for split-supply op-amps that typically run on +/-9V to +/-15V with a reference ground or use exotic & hard to find single-supply variants. These designs don't work for commonly & cheaply available low-power single-supply op-amps like LM324/358 which are best suited for battery operated devices. After some dabbling however, I came up with this circuit which seems to do the job :-)

The top-left op-amp sub-circuit   is an ideal diode from which we can tap the positive half of the sine-wave. Since the single supply op-amp can't swing its output below ground, the feedback circuit copies the positive half of the waveform but grounds the negative half.

The bottom left-hand sub-circuit is simply a unity gain inverting amplifier which inverts the negative parts of the input waveform to positive. However, since again this single supply op-amp can't swing it's output below ground, it is unable to invert the positive parts of the input waveform to negative and grounds it instead.

So now we have two halves of the final signal that we want - a half-rectified waveform from the "Ideal Diode" & inverted+half-rectified waveform from the inverting amplifier. We can simply combine these two using a standard summing amplifier design to get a fully-rectified output that's free from diode voltage drops. Also, increasing the value of the resistance marked as R above 100k can increase the gain of the output.