Peculiar precision full-wave rectifier wants no matched resistors

A basic analog utility is the precision energetic full-wave rectifier. Many various implementations exist of this theme, every with its personal supposed benefits. Nevertheless, one circuit ingredient wanted by (virtually) all energetic full-wave rectifier designs is an inverter with matched resistors to set its acquire to an correct -1.0. In such topologies, symmetry of rectification depends upon and may be no higher than the accuracy of this resistor match. For an instance, see a well-known (veritable basic!) design in Determine 1 with op-amp U1b performing because the inverter and R1 and R2 as its matched gainset resistors. Until R1 = R2, rectifier output for adverse Vin excursions are (very) unlikely to equal output for optimistic Vin excursions. 

Determine 1 Typical precision rectifier design with R1 and R2 matched symmetry-resistors.

For optimistic Vin inputs, D1 turns off and D2 conducts, establishing non-inverting unity acquire for the circuit that’s unaffected by resistor values: Vout/Vin = +1.

For adverse inputs, D1 conducts, D2 turns off and U1b turns into an inverter with acquire Vout/Vin = –R2/R1 = -1 provided that R2 = R1. In any other case, not, creating poor rectification symmetry.

Determine 2 exhibits one other (much less typical) design. However unconventional or not, listed here are Q2 and Q3 performing because the inverter and matched gainset symmetry-resistors R1 and R2 performing simply as in Determine 1.

Determine 2 Unconventional rectifier with discrete circuit inverter nonetheless makes use of symmetry-setting resistors: R1 and R2.

However now, simply to interrupt the monotony, regard Determine 3. Notice the (stunning) absence of matched resistors. Right here’s how this nonconformist works.

Determine 3 Unconventional precision rectifier design with out matched symmetry-resistors.

Q1 and Q2 present easy cross-over compensation to cancel the Vbe drops of Q3 and This fall. Consequently, adverse Vin excursions are inverted by A1 and output by This fall to filter R3C3. In the meantime, optimistic Vin excursions flip Q3 on, inflicting C2 to combine their time and present product: cost. The collected cost is saved as voltage on C2 which is added to subsequent reverse polarity half-cycles with Q3 and This fall performing as a easy full-wave cost pump. The online outcome: 

Vout = Avg(Abs(Vin)) R3 / R2 / R1.

Correct rectification symmetry is subsequently inherent so long as transistor Vbe’s match fairly properly which, being the identical kind and working in comparable contexts, they may.

Stephen Woodward’s relationship with EDN’s DI column goes again fairly a great distance. Over 100 submissions have been accepted since his first contribution again in 1974.

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