When we talk about rectifiers, the first thing that comes to mind is power supplies, but rectifiers are also used in many other circuits. Converting AC to DC is necessary in many high-precision signal processing circuits, and most circuits that measure real-world quantities first have to rectify sensor voltages. But even though regular diodes and bridges are adequate for many rectifying jobs, sometimes a different approach is needed.
A common rectifying circuit you build for a power supply will work perfectly, but it will be completely useless for high-precision signal processing circuits. The reason is simply that in many applications the signal we would like to rectify will be less than the voltage needed to turn a diode on. Even small-signal germanium diodes require about 0.3 volts to turn on. That may not seem like much, but, if you are working with signals in the millivolt range, you’ll have to find a way to handle the problem. The problem can be solved by using a precision rectifier.
The precision rectifier, also known as a super diode, is a configuration obtained with one or more operational amplifiers in order to have a circuit behave like an ideal diode and rectifier. Circuit designers have two standard methods for designing a precision rectifier. They can amplify the AC signal and then rectify it, or they can do both at once with a single operational amplifier. The latter method is often considered a much better way to get the job done.
The one-step approach to building a precision rectifier requires some way of isolating the positive and negative halves of the incoming AC, but after that AC has been amplified to a usable level. The circuit shown in figure 1 is a straightforward way of combining both amplification and rectification. Any op-amp IC can be used in this circuit. High input impedance, low offset voltage, frequency limit, and slew rate are among the factors you should consider when choosing an op-amp. Examine the requirements of your application and choose an appropriate device.
Figure 1. Half-wave precision rectifier circuit
Theory of operation
The circuit's theory of operation is similar to that of a diodes-only rectifier. During the positive half of the AC cycle the output of the op-amp forward biases D2 and current flows only through that diode. So, the output is zero because one side of R2 is connected to the virtual ground, and there is no current through it.
During the negative half of the input swing, however, D1 is forward biased, so current will flow through it and through R2. Therefore, DC will only show up across R2 during the negative part of the incoming AC cycle. Because we’re rectifying the voltage in the feedback loop of the op-amp and not at its input, the circuit will be able to handle very small AC signals. The inherent high gain of the op-amp allows us to rectify signals that are substantially below the voltage needed to forward -bias even small-signal germanium diodes.
When the input is negative, it is amplified by the operational amplifier which switches the diode D1 on. Current flows through the load and, because of the feedback, the output voltage is equal to the input voltage if R1=R2. The actual threshold of the super diode is very close to zero, but is not zero. It equals the actual threshold of the diode, divided by the gain of the operational amplifier.
Figure 2. Transfer funcion of the half-wave precision rectifier
At this point, one might ask if it is possible to build a simpler precision rectifier which will use only one diode. The answer is that such a basic configuration exists and it looks like the circuit presented on figure 3. This basic configuration has a problem so it is not commonly used. When the input becomes (even slightly) negative, the operational amplifier runs open loop, as there is no feedback signal through the diode. With a typical high open loop gain operational amplifier, the output saturates. If the input then becomes positive again, the op-amp has to get out of the saturated state before positive amplification can take place again. This change generates some ringing and takes some time, greatly reducing the frequency response of the circuit.
Figure 3. Basic half-wave precision rectifier
This circuit of figure 1 has the benefit that the op-amp never goes into saturation, but its output must change by one diode voltage drop (about 0.7V) each time the input signal crosses zero. Hence, the slew rate of the operational amplifier and its frequency response (gain-bandwidth product) will limit high frequency performance, especially for low signal levels. However, an error of less than 1% at 100 kHz is possible when we use high speed op-amps.
The op-amp shown in Fig.1 is set up as an inverting amplifier, so the output waveform will be180° out of phase with the input. The gain is equal to R2/R1, so, it will be equal to 1(0db) if R1=R2. You could switch inputs on the op amp to turn it into a non-inverting amplifier, but the phase difference comes in handy if you want to build a precision full-wave rectifier. A simple summing amplifier can be used to turn our circuit into a precision full-wave rectifier, but a bit of thought has to go into picking the summing resistors. As shown in Fig. 4, we're adding the original AC signal and twice the output of a unity-gain (0db) half-wave rectifier.
Figure 4. Full-wave precision rectifier
If R1 and R3 (in Fig. 4) had the same resistance, the output of the half-wave rectifier and the negative half of the input AC would be equal in magnitude, but 180° out of phase. In other words, the net result would be a voltage of zero. We can solve that problem by mixing in twice the half-wave voltage. If you decide to build the full-wave rectifier, it’s a good idea to use an IC, which has two operational amplifiers in its package.