Design Ideas: March 14, 1996
Two op amps make fast full-wave rectifier
Fernando Werlang,
CPqD-Telebras,
Campinas, Brazil
The circuit in Figure 1 is a precision full-wave rectifier with the following characteristics:
- Minimal delay time between the input and output (with only one op amp in the chain);
- Similar delay times for positive and negative inputs;
- No need to match diodes or adjust resistors.
When VIN>0V, the output of op amp IC1 is negative, so the reverse-biased diode D1 blocks the output path of IC1 to VOUT. The output of op amp IC2 is positive, so the forward-biased diode D2 sets the voltage VB at VIN(R6/(R6+R5)). The gain of this noninverting amplifier is (R6/(R6+R5))(1+R3/R4).
When VIN<0V, the output of op amp IC1 is positive, so VA=0V, diode D1 becomes forward-biased, and the gain of the inverting amplifier is R2/R1. The output of op amp IC2 is negative, so the reverse-biased diode D2 blocks the output path of IC2 to VOUT. Three cases are possible for the circuit in Figure 1:
- For unity gain, R3=R5=0 Ohm, R4=R6=infinity, and R1=R2. For the project that led to this design, R1=R2=4.75k±1%, and the input is a sine wave at 25 Hz and 4V rms.
- For gains greater than unity, R5=0 Ohm, R6=infinity, and the gain is R2/R1=(1+R3/R4). Satisfying this gain equation yields equal gains for positive and negative input signals.
- For gains lower than unity, R3=0 Ohm, R4=infinity, and the gain is R2/R1=R6/(R6+R5). Satisfying this gain equation yields equal gains for positive and negative input signals. (DI #1837)
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