Precision Rectifier using Operational Amplifier

Precision Rectifier:

The ordinary diodes cannot rectify voltages below the cut-in -voltage of the diode. A circuit which can act as an ideal diode or precision signal – processing rectifier circuit for rectifying voltages which are below the level of cut-in voltage of the diode can be designed by placing the diode in the feedback loop of an op-amp.

Precision diodes:

Figure shows the arrangement of a precision diode. It is a single diode arrangement and functions as a non-inverting precision half– wave rectifier circuit. If  V₁ in the circuit of figure is positive, the op-amp output V_OA also becomes positive. Then the closed loop condition is achieved for the op-amp and the output voltage V₀ = Vi . When Vi < 0, the voltage V_0A becomes negative and the diode is reverse biased. The loop is then broken and the output V₀ = 0.

Consider the open loop gain AOL of the op-amp is approximately 10⁴ and the cut-in voltage Vγ for silicon diode is ≈ 0.7V. When the input voltage Vi > Vγ / AOL, the output of the op-amp V_OA exceeds Vγ and the diode D conducts.

Then the circuit acts like a voltage follower for input voltage level  V_i > V_γ / AOL ,(i.e. when V_i > 0.7/10⁴ = 70μV), and the output voltage V₀ follows the input voltage during the positive half cycle for input voltages higher than 70μV as shown in figure.

When Vi is negative or less than V_γ / A_OL, the output of op-amp V_OA becomes negative, and the diode becomes reverse biased. The loop is then broken, and the op-amp swings down to negative saturation. However, the output terminal is now isolated from both the input signal and the output of the op-amp terminal thus V₀ =0.

No current is then delivered to the load RL except for the small bias current of the op-amp and the reverse saturation current of the diode.

This circuit is an example of a non-linear circuit, in which linear operation is achieved over the remaining region (V_i < 0). Since the output swings to negative saturation level when V_i < 0, the circuit is basically of saturating form. Thus the frequency response is also limited.

Applications: The precision diodes are used in

·           half wave rectifier,

·           Full-wave rectifier,

·           peak value detector,

·           Clipper and clamper circuits.

Disadvantage:

It can be observed that the precision diode as shown in figure operated in the first quadrant with Vi > 0 and V₀ > 0. The operation in third quadrant can be achieved by connecting the diode in reverse direction.

Half – wave Rectifier:

A non-saturating half wave precision rectifier circuit is shown in figure. When V_i > 0V, the voltage at the inverting input becomes positive, forcing the output V_OA to go negative. This results in forward biasing the diode D₁ and the op-amp output drops only by ≈ 0.7V below the inverting input voltage. Diode D₂ becomes reverse biased. The output voltage V₀ is zero when the input is positive.

When V_i > 0, the op-amp output V_OA becomes positive, forward biasing the diode D₂ and reverse biasing the diode D₁. The circuit then acts like an inverting amplifier circuit with a non-linear diode in the forward path. The gain of the circuit is unity when R_f = R_i.

The circuit operation can mathematically be expressed as     

V₀ = 0         when V_i > 0 and

V₀ = R_f/R_iV₁         forV_i < 0

The voltage V_oA at the op amp output is V_OA = - 0.7V  for  V_i>0   

V_OA= R_f/R_i V₁ + 0.7V    forV_i<0

Advantages:

·           it is a precision half wave rectifier and

·           it is a non saturating one.

The inverting characteristics of the output V0 can be circumvented by the use of an additional inversion for achieving a positive output.

Full wave Rectifier:

The first part of the Full wave circuit is a half wave rectifier circuit. The second part of the circuit is an inverting amplifier.

For positive input voltage V_i > 0V and assuming that R_F =R_i = R, the output voltage V_OA=V_i. The voltage V₀ appears as (-) input to the summing op-amp circuit formed by A₂, The is R/(R/2), as shown in figure.

The input Vi also appears as an input to the summing amplifier. Then, the net output is V₀=-V_i -2V₀= -V_i -2(-V_i) = Vi. Since V_i > 0V, V₀ will be positive, with its input output characteristics in first quadrant. For negative input Vi < 0V, the output V‘0 of the first part of rectifier circuit is zero. Thus, one input of the summing circuit has a value of zero. However, Vi is also applied as an input to the summer circuit formed by the op-amp A2.

The gain for this input id (-R/R) = -1, and hence the output is V₀ = -Vi. Since Vi is negative, V₀ will be inverted and will thus be positive. This corresponds to the second quadrant of the circuit.

To summarize the operation of the circuit,

V₀ = V_i when V_i < 0V and

V₀ = V_i for V_i > 0V, and hence

V₀ = |V_i |