Precision diode,Opamp half wave rectifier

Definition of Precision diode or super diode

Precision diode or super diode is a combination of opamp and diode which is superior rectifier.

Operation of Precision diode

For normal silicon diode to be on the minimum positive bias should be greater than the cut in voltage. Hence the diodes when used in rectification will have a dead time in which they will not conduct currents even when the voltages are positive (0< Vd< Vcut in voltage). An ideal diode should have zero cut in voltage. A simple half wave rectifier is shown in the figure

Precision diode

Precision diode

When Vout < V’out, diode will be on and the circuit reduces to voltage follower in which Vout = Vin. Because of high gain of op amps the input cut in voltage will be reduced to Vγ/Avo, where Avo is the open loop gain of opamp. Thus for even for very small input positive voltages the output will be high, the combination of op amp and diode acts as diode with zero cut in voltage hence it is named as precision diode. The output of precision diode(ideal diode) and practical diode rectifier with drop across diode(Vd) and non zero cut in voltage(Vγ) for an input sine wave is shown in the figure below

ideal diode v/s practical diode output

ideal diode v/s practical diode output

The combination of op amp and diode(precision diode) acts as diode with zero cut in voltage.For normal silicon diodes there exists a dead time when the voltages are within (0< Vd< Vcut in voltage).Because of high gain of op amps even for very small input voltages in rising mode the output will be high and the dead time can be made negligible. An precision diode has practical zero cut in voltage. 

Disadvantages of using precision diode circuit 

  • When the input voltage is negative the diode is off, a large differential voltage appear between inputs of opamp which is not desirable.
  • when the input voltage exceeds saturation voltage the output clamps at saturation voltage.

Opamp Half wave rectifier

The modified version of half wave rectifier using opamp is shown in the figure

Op amp half wave rectifier

Op amp half wave rectifier

Circuit Operation of Opamp Half wave rectifier

Consider a sine wave signal applied to half wave rectifier circuit.When Vi > 0 volts the half wave rectifier can be simplified and redrawn as follows

When vi>0 simplified HWR

redrawn HWR for Vi >0 V

When Vi>0 V

Then the diodes D1 and D2 are forward biased, The diode D1 shorts the output of the opamp to the inverting terminal. Using the concept of virtual ground the inverting terminal will be at ground potential (Non-inverting terminal will be at 0 v ). Hence the output of opamp will be zero volts. 

When vi>0 simplified HWR

When vi>0 simplified HWR

When Vi<0 V

The diodes D1, D2 will be reverse biased and are open. The circuit of HWR is redrawn and simplified as shown below.

The combination shown in the figure resembles a inverting amplifier with gain -Rf/R1. Hence the output of opamp

V0 = -Rf/R1 (Vi)

The output will be positive as Vi is negative.

HWR redrawn for Vi <0 V

HWR redrawn for Vi >0 V

 

Simplified HWR for Vi < 0

Simplified HWR for Vi < 0

 

 

 

 

 

 

The characteristics can be summarized as follows

                                        Vi > 0 v ; D1, D2 ON  ;Vo = 0 

                                Vi < 0 v ; D1, D2 OFF  ;Vo = –(Rf/R1)*Vi    

Input,output characteristics of HWR

Input,output characteristics of HWR

 

Recent Posts