Absolute value circuit diagram
The figure below illustrates an absolute value circuit, commonly referred to as a circuit that converts alternating current (AC) into direct current (DC). The simplest form of this circuit, shown in Figure (a), consists of a negative ideal diode and an addition circuit. This configuration produces an output voltage (U) that mirrors both halves of the input voltage (Ui). By connecting a capacitor (C1) to the inverting input and output of A2, the output becomes a smooth DC voltage. For optimal performance, the resistances should be balanced such that R1=R2, R5=2R4, and Av=R6R5. Figure (b) demonstrates a circuit with high input impedance. Its operational mechanism works as follows: during the positive half-cycle of the input voltage, VD1 turns on, and A1 operates in follower mode. Conversely, when the input voltage enters the negative half-cycle, VD2 activates, as illustrated in Figure (c). This particular design features equal resistances across all components. In the positive half-cycle, U1 equals Ui, and VD2 remains inactive, thus producing an output U equal to Ui. During the negative half-cycle, VD1 is inactive, and U2 outputs -Ui multiplied by the ratio of (R3+R4)/R2. If all resistors are equal, U2 becomes -1/3Ui, resulting in a total output of U=-Ui(2/3+1/3)=-Ui. Figure (d) presents a fundamental amplifying absolute value circuit employing an ideal diode. Op amps A1 and A2 utilize high-speed models like the LM318 and HA2525. In Figure (e), we see an example of a high-speed amplifying absolute value circuit. Here, the constant current source formed by VT and VT2, along with the voltage drops on RB1 and RB2, biases A1, simplifying the circuit while maintaining a frequency response of several hundred kHz or more. Figure (f) showcases an absolute value circuit incorporating an analog switch and a zero-crossing comparator. When the inverting input of A1 connects to its non-inverting counterpart, the potential becomes equal. With the analog switch DG201 activated—indicating a positive half-cycle—the A2 outputs a high level, entering the follower mode. Conversely, during the negative half-cycle, A2 outputs a low level, transitioning into inverter mode. The response speed of A2 and DG201 determines the circuit's highest operating frequency, ranging from low frequencies up to 10 kHz. As depicted in Figure (g), this standard DC conversion circuit transforms an AC input of 1V into a stable 10V DC output. Utilizing an ideal diode, the circuit ensures linear input and output behavior with a wide signal range. The potentiometer RP adjusts the gain, while C1 serves as the smoothing capacitor. Figure (h) represents another standard DC conversion circuit but employs a distinct rectification approach, featuring two smoothing capacitors, C1 and C2.
In summary, these various configurations of absolute value circuits showcase diverse methods for converting AC to DC. Each design caters to specific requirements, whether it involves high input impedance, amplification, or high-speed operation. Understanding these circuits is essential for applications ranging from signal processing to power supply systems. Engineers often choose the appropriate configuration based on factors such as frequency response, efficiency, and component availability. Furthermore, the integration of modern components like ideal diodes and advanced op amps continues to enhance the precision and reliability of these circuits in practical applications.
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