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 voltageVI.
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 |
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.