Operational Amplifier Based Half Wave Rectifier:

Rectification is a process of converting an AC waveform into a single-direction pulsating DC. There is a filter block at the end of the rectification block. The purpose of this filter is to smooth the DC voltage (which is in the form of pulses). 

The process of rectification

Diode-based rectifier circuits are commonly employed in power supply designs. In these applications, the voltages being rectified have much greater values than the voltage drop of a diode. Passive rectifiers are suitable for large signals coming from transformer windings.

In some cases, like instrumentation applications, signals coming from sensors or transducers are of very small value in the order of millivolts. In these applications, it is impossible to employ diode rectifiers because of tiny signals (less than 0.7 V). So, here comes the concept of precision rectifiers.

What is an active rectifier?

There are many ways to implement active rectifiers. But I am going to discuss opamp-based precision rectifiers. I will discuss the half-wave precision rectifier in this post. Diodes along with Opamp form a special class of rectifiers known as precision rectifiers.

Precision Half Wave Rectifiers:

It consists of an Opamp and a diode in the feedback path. The input signal is at the Non-inverting terminal of the amplifier.

The circuit works as a voltage follower during the positive half cycle only. 

vO = vi   for vi >= 0

The relationship between the input and output of the given circuit.

During the positive half cycle of the input signal, the output is also positive. The diode starts to conduct and a perfectly positive half cycle appears at the output. During the negative half cycle, the diode remains off.

OpAmp based half wave rectifier

This circuit has some limitations. The biggest drawback of this circuit is the feedback loop. Have a look at the circuit. The feedback loop will open when the diode is off (in this case during the negative half cycle of the input signal, the diode turns off). Hence Opamp gets saturated. In each cycle of the input signal Opamp switches from the linear region and the saturation region.

Improved Circuit: Precision Half Wave Rectifier With Two Diodes:

To avoid the switching of Opamp from linear region to saturation region, again and again, this circuit is useful.

The improved and faster circuit for half wave rectification.

In this circuit, there are two diodes and two resistors along with the Opamp. The input signal is applied to the inverting terminal. In this circuit, there is negative feedback that never gets open and won’t let the Opamp to saturate. During the positive half cycle of the input signal, D2 is conducting. The output of the amplifier is negative (because of the inverting configuration), and the D2 is off during the positive half cycle. 

During the negative half cycle, the D2 I’d off. The output of the Opamp is positive. The anode of the diode D1 is at the positive potential and starts to conduct and establish negative feedback through R2. The current flows through R2 is equal to the current flows through R1. 

For R1 = R2, the transfer characteristics will be

vo = – vi   for vi <= 0

The relationship between input and output of the given circuit.

The prominent advantage of this circuit is that the feedback loop is closed throughout the circuit operation. D2 is included and hence the feedback loop remains closed.

Active Vs Passive  Rectifiers: | Difference Between Active and Passive Rectification:

So, let’s discuss the difference between active and passive half-wave rectification. As a beginner, you might not be aware of the active rectification and passive rectification.  

Passive rectifiers use simple silicon diodes to convert alternating current (AC) into direct current (DC). It is composed of a simple diode, followed by a load resistor.

A silicon diode is effectively converting AC to DC

A diode is a unidirectional device that allows the flow of current only in one direction hence effectively converting AC into DC. For practical diodes, there is a forward voltage drop of 0.7V (for a silicon diode). They are suitable for rectification of large voltage signals. 

What will happen if the input voltage is 1V, after rectification from a passive rectifier, the output voltage becomes 0.3V (there is a diode drop of 0.7V). 

In this case, active rectifiers are used. They will effectively convert small AC voltages into DC voltages. They are composed of super diodes ( A super diode consists of an Opamp followed by a diode. The basic active rectifier consists of a super diode that is an operational amplifier and a diode. The negative feedback and high gain are responsible for bypassing the diode’s threshold voltage.

Comparison between output of an active (precision) rectifiers Vs passive rectifiers

Leave a comment

Your email address will not be published. Required fields are marked *