This is not a rectifier diode and no simple threshold. in the scheme, a single operational amplifier such as the LM358 and 5 resistors create a full-wave rectifier. The output voltage is 1/3 of the absolute value of the input signal

Diagram of operational without rectifier diode

In blu (LM358 with R1 and R2), the conventional configuration inverting.

The circuit behaves differently depending on the sign of the input voltage VI .

Choice of the operational amplifier

For the rectifier diode without an operational amplifier is required whose input voltage range extends to its negative supply (lower potential). The input voltage range of the LM358 is suitable for this application from -0,3 V a + VDC when it is powered by 0 V a + V CC. Furthermore, its saturated output voltage (Vsat) low must be close to the power supply negative, in this case the mass: 20mV max (for output current 1mA), which it is very low. It is a voltage Vce sat (saturation of the output bipolar transistor).

When the voltage is positive

When the input voltage is positive, the operational amplifier is saturated. Its output voltage (VO) It is therefore zero. VR It is the result VI attenuated by the divider bridge R3 and R4. The value of R4 should be twice that of R3.

VR = 1/3 VI

The case in which the voltage is negative

This time, the operational amplifier operates as inverter. Its gain is defined by the ratio -R2 / And R1 is -1. VO It is a weighted average of VR (which is -VI) e VI. Given that R4 = 2 R3, weighting is written:

VR= 2/3 VO + 1/3 VI

VR = -2/3 VO + 1/3 VI (V becauseO = -VI)

VR= – 1/3 VI

We can summarize the 2 almost (VI > 0 e VI <0) writing an absolute value:

VR = 1/3 | VI |

operating range

To prevent the amplifier to reverse the operation oF saturation, the input voltage will be limited to -10V. The upper limit for VI It depends on the maximum differential voltage that inputs bear.

The ideal range of LM358 voltage is

-10V < VI < + 10V.


The resistors can be chosen freely but with constraints: R2 = R1 R4 = 2 R3 is a full-wave rectifier that has the advantage of having the same reference potentials of the input voltage, unlike a bridge with 4 diodes.


3 replies
  1. theremino
    theremino says:

    The selenium rectifier reminded me of my first repair, It should be about 1967. Those were different times. Times when no one cared whether the boys 14 years goofed with his hands inside the valve radio. Only a few years later I had fun to go into reverse in cities with 500 (I was very good at driving in reverse) and the police, instead of arresting me would laugh .

    One day I was at home “Bobo” (my friend who was always skip the counter experiencing electromagnets of wire wound on large nails) and the radio his father had stopped working. After many trials we found that detaching and attaching the wire to the radio selenium rectifier he began to play for a while '. And we also got to understand that doing “touches and does not touch” sometimes the electrolyte is partially loaded and the radio was playing for a few seconds. So we went to retrieve another selenium rectifier (bigger) removed from another radio, we connected by twisting the wires and it worked!!!

  2. Amilcare
    Amilcare says:

    To tell the truth has small distortions due to the fact that the operational never saturate bringing its output to zero.
    The internal transistor output has a saturation currents of 20mV Item less than 2mA
    For this reason I chose all resistors 47K.
    The percentage of distortion can not determine a priori not knowing the extent of the incoming alternating signal.
    However in most cases the percentage of distribution will be very small.

  3. theremino
    theremino says:

    really interesting circuit, It will be useful for sure in some cases.
    There are other circuits that eliminate the voltage drop of the diode, but this also eliminates the diode and I think it has advantages. I have not done the simulations but should have minor variations due to temperature, switching faster and better balance between the two half-waves.


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