Recall that a rectifier is a device that allows current to pass through it in one direction only. A diode can serve as a rectifier because it permits generous current flow in only one direction-the direction of forward bias. Rectification k +,he same as clipping at the 0-V level: all of the waveform below (or above) the zero-axis is eliminated. Recall, however, that a diode rectifier has certain intervals of nonconduction and produces resulting “gaps” at the zero-crossing points of the output voltage, due to the fact that the input must overcome the diode drop (0.7 V for silicon) before conduction begins. See Figures 3-19 and 3-20. In powersuppry applications, where input voltages are quite large, these gaps are of no concern. However, in many other applications, especially in instrumentation, the 0.7-V drop can be a significant portion of the total input voltage swing and can seriously affect circuit performance. For example, most ac instruments rectify ac inputs so they can be measured by a device that responds to de levels. It is obvious that small ac signals could not be measured if it were always necessary for them to reach 0.7 V before rectification could begin. For these applications, precision rectifiers are necessary.
One way to obtain precision rectification using an operational amplifier and a diode. The circuit is essentially a non inverting voltage follower when the diode is forward biased. When Vjll is positive, the output of the amplifier, is positive, the diode is forward biased, and a low-resistance path is established between v” and V-, as necessary for a voltage follower. The load voltage, VI., then follows the positive variations of Vjll = V I. Note that even a very small positive value of Vjll will cause this result because of the large differential gain of the amplifier; that is, the large gain and the action of the feedback cause the usual result that v+ “” V-. Note also that the drop across the diode does not appear in VI
When the input goes negative, becomes negative, and the diode is reverse biased. This effectively opens the feedback loop, so VL no longer follows Vin’ The amplifier itself, now operating open-loop, is quickly driven to its maximum negative output, thus holding the diode well into reverse bias.
Another precision rectifier circuit is shown in Figure 15-20. In this circuit, the load voltage is an amplified and inverted version of the negative variations in the 0 when the input is positive. Also in contrast with the previous circuit, the amplifier in this rectifier is not driven t ore of Its output extremes. When Vi” is negative, the amplifier output, v,,, is positive, so diode D, is reverse biased and diode D2 is forward biased. O. is open and connects the amplifier output through R, to . Thus, the circuit behaves like an ordinary inverting amplifier with gain. The load voItageis an amplified and inverted (positive) version ‘of the negative variations in Vi”‘ When Vi” becomes positive, Vo is negative, DI is forward biased, and D2 is reverse biased. D, shorts the output Vo to o. which is held at virtual ground, so VI- is 0, It is an exercise at the end of the chapter to analyze this circuit when th~ diodes are turned around.