Rectifier that tracks positive and negative peaks

　　Signal waveforms ranging from music to complex control systems may contain unequal positive and negative peak amplitudes. "Envelope following" circuits can track unequal peaks, and the ability to select expected peaks can enhance the circuit's performance (Reference 1). The circuit in Figure 1 applies a new approach to the classic absolute value circuit. Applying an input signal to R ₁ (full wave) produces an output equal to the absolute value of the input. Applying an input signal to R ₆ (positive) or R ₇ (negative) produces a positive or negative half-cycle output, respectively. Figure 2 depicts all three operating modes.

Understanding this circuit is simple if you consider that op amp IC _1A strives to keep its inverting input at virtual ground. For example, applying -1V to the negative input R ₇ will drive the anode of D ₁ to -333 mV. The output of IC _1A (pin 1) drives the cathode of D ₂ to a positive enough value to force the anode voltage of D _{2 to 333 mV.} Since the input to IC _1A is now 0V, D ₁ is effectively reverse biased and outside the circuit. The 333 mV from the D ₂ cathode is also applied to the non-inverting input of IC _1B (pin 5), and IC _1B must drive its output (pin 7) to 1V, thereby balancing its input voltage. The inverting input of IC _1B (pin 6) reaches 333 mV. Therefore the voltage drop across R ₄ is equal to 666 mV. One-third of the input current flows through R2 and R3 in series _, and _two -thirds flows in _R4 . To achieve unity gain, the value of R ₇ is equal to the value of R ₂ + R ₃ in parallel with R ₄ .

Applying a positive input to R ₇ causes the output of IC _1A to go negative by a voltage equal to the forward diode voltage drop, and therefore keeps the anode of D ₁ at ground. D ₂ is reverse biased and both inputs to IC _{1B are 0V, the output of the circuit is therefore 0V.} Applying an input voltage to R ₆ produces a similar operation. Positive inputs result in an equal positive output, while negative inputs result in a 0V output. You can ignore the effects of IC _1B 's high input impedance, they are negligible. To maintain unity gain, the value of R ₆ is twice that of R ₃ .

Resistors R ₁ , R ₂ , R ₃ , R ₄ and R ₅ are equal values ​​and have close tolerances. Please note: IC ₁ 's power connection requires a bypass capacitor (not shown). To minimize errors, a low impedance source or buffer amplifier should be used to drive the circuit. You can use a three-position rotary switch to select the input mode, or use a pass/pass/pass toggle switch from Digi-Key (such as the 7211 from C&K Components) or similar switch, wired together to form a three-way selector. (See the manufacturer's manual for connection diagrams.) You can also use separate connectors for the inputs, but only one input can be connected at a time.