77-op-amp-absolute-value-amplifier-with-one-diode

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One realization of absolute value amplifier with op-amp and one diode is given in figure bellow.

op-amp-absolute-value-amplifier-one-diode

Simulate op-amp absolute value amplifier.

In order to analyze given circuit, analysis has to be separated.

  • VIN>0.

If input voltage is positive, inverting input of OP1 is on higher potential then non-inverting input which is on the ground level. It means that output of OP1 is negative, i.e. anode of the diode D is negative. Cathode of the diode D is on positive VIN level, so diode D is inverse biased. Equivalent circuit is given in figure bellow.

op_amp_bas_1_diode_sch01a

Now, non-inverting input of OP2 is connected via R3 to VIN. Inverting input is also connected to VIN via 2R1. Both inputs are on the same potential, OP2 operates in amplifying mode, and inverting input is shortly connected to the output. If VIN>0, VOUT=VIN.

 

  • VIN<0

If input voltage is negative, inverting input of OP1 is on lower potential then non-inverting input which is on the ground level. It means that output of OP1 is positive, i.e. anode of the diode D is positive. Cathode of the diode D is on negative VIN level, so diode D is forward biased. Equivalent circuit is given in figure bellow.

op_amp_bas_1_diode_sch02

Now, inverting input of OP1 is on the virtual ground, i.e. on potential of zero volts (which was not the case when input was positive). Current through R1 flows from point A (zero volts) to input terminal (negative voltage). Current through R2 is with the same magnitude and since R1=R2, potential of point B is positive and equal to –VIN. Non-inverting input is on the same positive potential of –VIN. Inverting input of OP2 is shortly connected to output, so output now is –VIN. If VIN>0, VOUT=-VIN.

op_amp_abs_eq01

op_amp_abs_eq02

Go to op-amp absolute value amplifier simulation.

 

External links:

Our video tutorial about op-amp absolute value amplifier

Op-amp absolute value amplifier on edn

Op-amp absolute value amplifier on westhost

Op-amp absolute value amplifier on electronics.stackexchange

 

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