Rectifier tracks positive and negative peaks

Signals ranging from music to complex control-system waveforms may contain unequal positive and negative peak amplitudes. An "envelope-follower" circuit can track unequal peaks, but the ability to select a desired peak can enhance the circuit's performance (Reference 1). The circuit in Figure 1 applies a new twist to a classic absolute-value circuit. Applying an input signal to R₁ (full) produces an output equal to the input's absolute value. Applying an input signal to R₆ (positive) or R₇ (negative) produces outputs of positive or negative half-cycles, respectively. Figure 2 illustrates all three modes of operation.

Understanding the circuit is simple if you consider that op amp IC_1A strives to maintain its inverting input at virtual ground. For example, applying –1V to the negative input, R₇, drives the anode of D₁ to –333 mV. IC_1A's output, Pin 1, drives D₂'s cathode positive enough to force D₂'s anode voltage to 333 mV. Because IC_1A's inputs now rest at 0V, D₁ is effectively reverse-biased and out of the circuit. The 333 mV available at D₂'s cathode also applies to IC_1B's noninverting input, Pin 5, and IC_1B must balance its input voltages by driving its output, Pin 7, to 1V. IC_1B's inverting input, Pin 6, goes to 333 mV. The voltage drop across R₄ thus equals 666 mV. One-third of the input current flows through the series connection of R₂ and R₃, and two-thirds flows in R₄. To achieve unity gain, R₇'s value equals that of R₂+R₃ in parallel with R₄.

Applying a positive input to R₇ causes IC_1A's output to go negative by a voltage equal to one forward-diode drop and thus holds D₁'s anode at ground. D₂ is reverse-biased, and both of IC_1B's inputs rest at 0V. The circuit's output is thus 0V. Applying an input voltage at R₆ yields similar operation. A positive input causes an equal-value positive output, and a negative input produces a 0V output. You can ignore the effects of IC_1B's high input impedance, which are negligible. To maintain unity gain, the value of R₆ is twice that of R₃.

Resistors R₁, R₂, R₃, R₄, and R₅ are of equal value and close tolerance. Note that IC₁'s power-supply connections require bypass capacitors (not shown). To minimize errors, use a low-impedance source or buffer amplifier to drive the circuit. You can use a three-position rotary switch for input-mode selection, or an on/on/on toggle switch, such as C&K Components' 7211, available from Digi-Key Corp and other sources, or a similar switch, wired as a three-way selector. (See the manufacturer's data sheet for a connection diagram.) You can also use separate connectors for the inputs, but connect no more than one input at a time.