Precision Rectifier

What is Precision Rectifier :

It is a very small electronic circuit that is usually implemented by using an operational amplifier. A precision rectifier is used to precisely convert small AC signals into DC signals and without suffering from the kinds of voltage drop problems present in diode rectifiers.

In technical terms, this circuit "rectifies" the signal by passing through only those parts of the AC waveform positive or negative and is effective for giving a clean smooth DC output even from low-level voltage inputs. For those applications of signal processing based on detection and measuring proper low-level signals in application like audio, instrumentation circuits, or sensor circuits, Precision rectifiers are most famous.

Types of Precision Rectifier :

There are two main types of precision rectifiers using operational amplifiers:

• Half-Wave Precision Rectifier

This circuit only lets one half of the input AC waveform pass through which is either positive or negative to be rectified for a corresponding DC output.It uses a single op-amp and diode in order to provide a precision rectification without having to allow diode voltage drop; therefore it can be effectively applied for small signal rectification.

• Full-Wave Precision Rectifier

This type of rectifier rectifies both positive and negative halves of an input AC and gives a waveform output that is fullwave rectified.A common circuit of a fullwave precision rectifier uses the combination of two op amps and several diodes by inverting and adding together to give a continuous output waveform. Their frequency is doubled relative to the input AC signal that it is rectifying.These circuits find applications in signal processing, instrumentation, and audio systems wherein the detection of low-voltage signals is of crucial importance

Circuit Diagram of Half-Wave Precision Rectifier :

Working of Half-Wave Precision Rectifier :

when the input signal is positive, the op-amp output will go sufficiently positive to forwardbias the diode. As a result, it follows that the positive half cycle of the input will also appear at the output as, in general, the typical 0.7 V drop across the diode could be compensated for by the op-amp allowing the input to be well reproduced at the output with reasonable closeness. 

Now, when the input is negative, the diode is reversebiased preventing the flow of current inside the feedback loop. As a result, the output is kept at zero, which in effect clips off the negative half of the AC waveform. The final output is a DC signal that closely resembles the positive half of the applied AC input with high fidelity, even at very low voltage levels.

Waveform of Output :

Appications of Half-Wave Precision Rectifier :

• Peak Detection: Used in circuits to determine the peak values of AC signals, especially in RF signal processing and audio equipment.

• Signal Demodulation: Applied in demodulating AM signals in simpler radio receivers, where only one half-cycle of the waveform is needed to extract information.

• Audio Envelope Detection: Useful in audio circuits for detecting signal envelopes for compression, limiting, and other effects processing.

• Small Signal Detection: It is used in instrumentation where there is an accurate detection of low AC signals because it eliminates diode threshold losses that are commonly found in ordinary rectifiers.

• Overvoltage Protection: In some protection circuits, a half-wave precision rectifier can be used to detect voltage levels and initiate mechanisms for protection when a signal exceeds a certain threshold level.

Limitations of Half-Wave Precision Rectifier :

• The circuit has some serious limitations. The main one is speed. It will not work well with high frequency signals.

 • For a low frequency positive input signal, 100% negative feedback is applied when the diode conducts. The forward voltage is effectively removed by the feedback, and the inverting input follows the positive half of the input signal almost perfectly.

 • When the input signal becomes negative, the op amp has no feedback at all, so the output pin of the op amp swings negative as far as it can.

 • When the input signal becomes positive again, the op amp's output voltage will take a finite time to swing back to zero, then to forward bias the diode and produce an output. This time is determined by the op amp's slew rate, and even a very fast op amp will be limited to low frequencies.

Now when it comes to the analysis of the circuit, a half-wave rectifier circuit is good enough, but when it comes to a practical circuit, the half-wave rectifier just does not make practical sense.Because of that reason, a full-wave rectifier circuit was introduced, to achieve a full-wave precision rectifier

Circuit Diagram of Full-Wave Precision Rectifier :

Working of Full-Wave Precision Rectifier :

Stage 1: Inverting Rectifier. The first op-amp inverts only the positive half of the AC input. For a positive input, the output goes negative, forward-biasing the diode and producing a negative output. For a negative input, the diode is reverse-biased, and the output is zero.

Stage 2: The summing amplifier adds the input AC with the negative inverted output of the first stage. When the input AC is in the positive half-cycle, it passes directly to produce a positive output. The output of the first stage is negatively applied by the summing amplifier during the negative half-cycle of the input AC to give an output.

The rectification of the both halves of AC input is provided to get the output continuous and positive at double frequency of input. Both op-amps permit perfect rectification without threshold loss through diodes. So it is quite suitable for low-signal applications.

Waveform of Output :

Applications of Full-Wave Precision Rectifier :

• Signal Demodulation: Used in AM radio receivers to extract the original audio signal from modulated carrier waves.

• Instrumentation: This is very important in oscilloscopes, multimeters, and other measuring devices because it will preserve the integrity of signals without loss due to diode drops in measurement accuracy.

• Audio Processing: This is used in audio equipment for rectification of signals in envelope detection to obtain accurate sound reproduction and processing.

• Biomedical Signal Processing: It is used in ECG, EEG, and other bio-sign monitoring systems for detecting the small physiological signals by converting alternating bio-signals into the usable DC form.

• Power Measurement: True RMS (Root Mean Square) values of AC signals are found in wattmeters and other power analysis instruments. This will yield a much higher accuracy for low level power calculations.

Conclusion :

Precision rectifiers have an advantage of converting very low-level AC signals into a very high-accuracy level DC signal with no voltage drops unlike diodes. For detection purposes, the full wave is the best whereas peak detection and processing take place at the half-wave, with the fullwave taking an application in instrumentation used for biomedical monitoring. Areas such as detection of weak signal where measurement is significant hold significant importance.

References :

Circuit Digest. (n.d.). Half-Wave and Full-Wave Precision Rectifier Circuit using Op-Amp. Circuit Digest. Retrieved from https://circuitdigest.com/electronic-circuits/half-wave-and-full-wave-precision-rectifier-circuit-using-op-amp

ElecCircuit. (n.d.). Full-Wave Rectifier with an Op-Amp IC. ElecCircuit. Retrieved from https://www.eleccircuit.com/full-wave-rectifier-with-an-op-amp-ic/

Habash, N. (n.d.). ELG4135: Digital Circuits Laboratory. University of Ottawa. Retrieved from https://www.site.uottawa.ca/~rhabash/ELG4135L8.pdf