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09-30-2015

Precision full-wave signal rectifier needs no diodes

National Semiconductor » LMC6482

Jose Blanes

EDN

Rectifier circuits based on semiconductor diodes typically handle voltage levels that greatly exceed the diodes' forward-voltage drops, which generally don't affect the accuracy of the rectification process. However, the rectified signal's accuracy suffers when the diode's voltage drop exceeds the applied voltage. Precision rectifier circuits combine diodes and operational amplifiers to eliminate the effects of diode voltage drops and enable high-accuracy, small-signal rectification.

By taking advantage of modern operational amplifiers that can handle rail-to-rail inputs and outputs, the circuit in Figure 1 dispenses with diodes altogether, provides full-wave rectification, and operates from a single power supply.

Precision full-wave signal rectifier needs no diodes
Figure 1. This precision full-wave-rectifier circuit uses two op amps and no diodes.
When altering the basic design, note that resistors R3 and R4 are both twice
the value of R2 and that R1 and R5 are equal.

The circuit operates as follows: If VIN > 0 V, then IC1A's output, VHALF, equals VIN/2, and IC1B operates as a subtracter, delivering an output voltage, VOUT, equals VIN. In effect, the circuit operates as a unity-gain follower. If VIN, 0 V, then VHALF = 0 V, and the circuit behaves as a unity-gain inverter and delivers an output of VOUT = –VIN. Figure 2 shows the circuit's input signal at VIN; its intermediate voltage, VHALF; and its output voltage, VOUT.

Precision full-wave signal rectifier needs no diodes
Figure 2. From bottom to top, the waveforms show
VIN (CH1), VHALF (CH2), and VOUT (CH3).

The circuit uses a single National Semiconductor LMC6482 chip and operates in the linear regions of both operational amplifiers. Suggested applications include low-cost rectification for automatic gain control, signal demodulation, and process instrumentation. The circuit relies on only one device-dependent property: The amplifiers must not introduce phase inversion when the input voltage exceeds the negative power supply; the LMC6482 meets this requirement.

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