Module 4
A Precision Full-Wave Rectifier
The precision rectifier, which is also known as a super diode, is a configuration
obtained with an operational amplifier in order to have a circuit behaving like an ideal
diode or rectifier.[1] It can be useful for high-precision signal processing.
The basic circuit implementing such a feature is shown on the right, where RL can be any
load. When the input voltage is negative, there is a negative voltage on the diode, too, so
it works like an open circuit, there is no current in the load and the output voltage is zero.
When the input is positive, it is amplified by the operational amplifier and it turns the
diode on. There is current in the load and, because of the feedback, the output voltage is
equal to the input.
In fact the threshold of the super diode is not actually zero, as it should be for an ideal
one, but it equals the threshold of the normal diode divided by the gain of the operational
amplifier, that is almost zero.
This basic configuration has a problem so it is not commonly used: when the input
becomes (even sightly) negative, the output of the operational amplifier can easily
become greater than the voltage supplied to the op-amp, thus causing saturation. Then, if
the input becomes positive again, the op-amp has to get out of the saturation to amplify
again. This change takes some time, and this greatly reduces the frequency response of
the circuit.
MPROVED CIRCUIT
An improved precision rectifier circuit.
An alternative version is given on the right.
In this case, when the input is greater than zero, D1 is OFF and D2 is ON, so the output is
zero, because one side R2 is connected to the virtual ground, and there is no current
through it. When the input is less than zero, D1 is ON and D2 is OFF, and the output is
like the input with an amplification of − R2 / R1. Its transfer characteristic is the
following:
This circuit has the benefit that the op-amp never goes in saturation, so the only thing
affecting its frequency response is the amplification and the gain-bandwidth product.
Similar circuitry can be used to create a precision full-wave rectifier circuit.
PEAK DETECTOR
With little modifications basic precision rectifier can be used also for detecting peak
levels of signal. In the following circuit a capacitor keeps the peak voltage level of signal
and switch can be used for resetting detected level.
THE HALF-WAVE RECTIFIER KEPT ONLY THOSE PARTS OF THE ORIGINAL INPUT SIGNAL THAT
WERE POSITIVE (OR NEGATIVE). IS THERE A WAY TO KEEP BOTH HALVES OF THE INPUT
SIGNAL, AND YET RENDER THEM BOTH WITH THE SAME OUTPUT POLARITY? THIS IS THE
BEHAVIOR OF A FULL-WAVE RECTIFIER
signal. The recovery time can be long for some devices. To speed up recovery time, you can limit the output
voltage level before saturation is reached. But how? A zener diode across the feedback resistor creates a fine
limiting function. Unfortunately, for high precision applications, the non-ideal characteristics of the zener
diode can wreck circuit
THE CIRCUIT SHOWN ABOVE PERFORMS FULL-WAVE RECTIFICATION ON
THE INPUT SIGNAL, AS SHOWN. IF YOU WISH THE FINAL OUTPUT TO BE
POSITIVE INSTEAD OF NEGATIVE, SIMPLY REVERSE THE TWO DIODES IN
THE HALF-WAVE RECTIFIER SECTION.
THE FULL-WAVE RECTIFIER DEPENDS ON THE FACT THAT BOTH THE HALF-
WAVE RECTIFIER AND THE SUMMING AMPLIFIER ARE PRECISION
CIRCUITS. IT OPERATES BY PRODUCING AN INVERTED HALF-WAVE-
RECTIFIED SIGNAL AND THEN ADDING THAT SIGNAL AT DOUBLE
AMPLITUDE TO THE ORIGINAL SIGNAL IN THE SUMMING AMPLIFIER. THE
RESULT IS A REVERSAL OF THE SELECTED POLARITY OF THE INPUT
SIGNAL.
THE RESISTOR VALUES SHOWN ARE REASONABLE; THE RESISTORS
THEMSELVES MUST BE OF HIGH PRECISION IN ORDER TO KEEP THE
RECTIFICATION PROCESS ACCURATE. IF FOR SOME REASON YOU MUST
BUILD SUCH A CIRCUIT WITH A DIFFERENT SET OF RESISTANCE VALUES,
YOU MUST MAINTAIN THE INDICATED 2:1 RESISTANCE RATIO, AND YOU
MUST STILL USE PRECISION RESISTORS IN ORDER TO OBTAIN ACCURATE
RESULTS.
OP AMP LIMITER
CIRCUIT
Suppose you've been asked to limit the maximum output voltage of an amplifier. Why?
Maybe the next stage will go up in smoke if its input swings beyond the maximum input
rating. Or, maybe an op amp output gets frequently slammed into a rail by a big input
accuracy. However, performance can be restored with a simple modification to the
topology.
LIMITING ACTION
During normal operation (no limiting), the op amp works like your basic inverting
amplifier. VS and R1 produce an input current I = VS / R1. (The negative input is at
virtual ground, or 0V.) This same I flows through feedback element R2 to develop the
output voltage, Vo = I ∙ R2. However, limiting occurs when the signal swings large
enough to force D1 and D2 to conduct. For positive signals, the clip level begins at
