The following is a guest blog from Analog Devices. The authors are Chau Tran and Fotjana Bida. Their biographies are at the end of this article.
A full-wave bridge rectifier converts an AC signal to a full-wave DC signal. Typically, a bridge formed by four diodes achieves full-wave rectification. Figure 1 shows four diodes arranged in series pairs, with two diodes conducting current during each half cycle. At any given time, two diodes are forward biased, while the other two are reverse biased, effectively eliminating them from the circuit.
The result is a DC output, where the current flowing through the load is the same during both half cycles. A smoothing capacitor can be added to the output if the rectifier is to be used as a DC power supply. The main advantage of this bridge circuit is that it does not require a special center-tapped transformer, thereby reducing its size and cost.
This classic circuit has many disadvantages, however. The current flowing through the load is unidirectional, so the DC voltage developed across the load should have an average value of
In reality, however, during each half cycle, the current flows through two diodes, so the output voltage amplitude is two diode drops less than the input amplitude.
For example, with a 5-V peak input, the peak output will be about 3.8 V. The ripple frequency will be twice the supply frequency; for example, with a 60-Hz supply, the ripple frequency will be 120 Hz. In addition, the circuit suffers from crossover distortion and temperature drift.
The circuit shown in Figure 2 improves the performance of the classic four-diode bridge by employing two low-cost, high-performance difference amplifiers and two low-cost diodes to eliminate the loss at the output. This approach achieves better precision, lower cost, and lower power consumption than conventional techniques.
In this circuit, VIN is a sine wave. During a positive half cycle, diode D1 conducts. Both amplifiers A1 and A2 act as inverters. The result is a positive voltage at VOUT with amplitude exactly the same as that of the input. During a negative half cycle, diode D2 conducts. Amplifier A1 now has a gain of –2/3, while A2 has a gain of +3/2. The net gain of –1 provides a positive voltage at VOUT with amplitude opposite that of the input. The combination gives a loss-free full-wave rectifier. Signals as large as ±10 V can be handled at frequencies as high as 10 kHz.
This design has several inherent performance advantages, including cost, crossover distortion, gain error, and noise. The gain accuracy of the rectified output is determined by the 10 kΩ resistors. Precisely matched, these laser-wafer-trimmed resistors guarantee gain error of less than 0.02%. The circuit's noise gain is only 2, resulting in lower noise, offset, and drift.
The circuit's performance is demonstrated in Figure 3 with a 20-V p-p input signal at 1 kHz. The fact that the output overlaps the positive cycle of the input means that the exceptional input and output characteristics of the difference amplifiers create a negligible loss.
Unlike the classic circuit, the characteristics of the two diodes in the new circuit have no effect on the output voltage. Therefore, the performance over temperature is better.
A precision full-wave rectifier built with two difference amplifiers and two diodes offers several advantages over traditional designs. Specially, the output voltage shows no loss as compared to the input voltage. The difference amplifier solution has no crossover recovery problem and is optimized for low drift over a wide temperature range.
Chau Tran works in the Linear Products Group at Analog Devices in Wilmington, Mass. He joined the company in 1984. In 1990, he graduated with an MSEE degree from Tufts University. He holds more than 10 patents and has authored more than 10 technical articles.
Fotjana Bida is a product development engineer for the Linear Products Group at Analog Devices in Wilmington, Mass. She joined the company in 2007 and has worked in design as well as in product development of precision signal-processing components. She holds a MSEE from Northeastern University.