Precision full-wave signal rectifier needs no diodes

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.

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 V_IN > 0 V, then IC_1A's output, V_HALF, equals V_IN/2, and IC_1B operates as a subtracter, delivering an output voltage, V_OUT, equals V_IN. In effect, the circuit operates as a unity-gain follower. If V_IN, 0 V, then V_HALF = 0 V, and the circuit behaves as a unity-gain inverter and delivers an output of V_OUT = –V_IN. Figure 2 shows the circuit's input signal at V_IN; its intermediate voltage, V_HALF; and its output voltage, V_OUT.

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.