Half Wave Rectifier Using Opamp

OBJECTIVE:

To study the half wave rectifier using opamp or a precision rectifier.

THOERY:

Rectification Overview

The purpose of the rectifier section is to convert the incoming ac from a transformer or other ac power source to some form of pulsating dc. That is, it takes current that flows alternately in both directions as shown in the first figure to the right, and modifies it so that the output current flows only in one direction, as shown in the second and third figures below.



The circuit required to do this may be nothing more than a single diode, or it may be considerably more complex. However, all rectifier circuits may be classified into one of two categories, as follows:

Half Wave Rectifiers

An easy way to convert ac to pulsating dc is to simply allow half of the ac cycle to pass, while blocking current to prevent it from flowing during the other half cycle. The figure to the right shows the resulting output. Such circuits are known as half-wave rectifiers because they only work on half of the incoming ac wave.

Full Wave Rectifiers

The more common approach is to manipulate the incoming ac wave so that both halves are used to cause output current to flow in the same direction. The resulting waveform is shown to the right. Because these circuits operate on the entire incoming ac wave, they are known as full-wave rectifiers.

Half Wave Rectifier using op-amp

A half wave rectifier using OP amp is also known as a Precision rectifier or super diode, is a configuration obtained with an operational amplifier in order to have a circuit behaving like an ideal diode and rectifier.

Figure 1 shows the circuit of a Precision rectifier .On positive voltage swings the diode conducts and a voltage is developed across the resistor. On negative swings, the diode turns off and the output voltage is zero (no current through the resistor).

The basic idea behind the superdiode is to use the high-gain of an op-amp to mask the finite turn-on voltage (and other nonlinearities) of the diode. This is done by placing it in the negative feedback path as shown in fig 1. Any positive voltage at the op-amp “+” terminal is now sufficient to turn on the diode, and the negative feedback regulates the current through the load resistor to maintain an output voltage equal to the input voltage for these positive input voltages. For positive signals the circuit is a unity-gain buffer(fig 2). For negative signals, the output goes negative, and the diode turns off (fig 3). Because of the high open-loop gain of the op-amp, the circuit operates as a perfect switch for even very small voltages; hence it is called “superdiode”.

Fig 1. Circuit diagram of a precision rectifier and its input output relation.

When Diode is conducting: The feedback loop is closed, and the circuit looks like the buffer with small drop across diode. And Vo=Vin


Fig 2. The circuit behaves as a buffer for positive input voltages.

When Diode is not conducting: : i.e. Vi<0 And Vo = 0V.


Fig 3. The diode acts as an open circuit for negative value of voltages.


Fig 4. Input(a) and output (b) of a rectifier circuit.