Ten circuit designs for precision full-wave rectification

The name of the precision full-wave rectifier circuit in the picture is purely my own, just for differentiation; unless otherwise specified, the gain is designed as 1.

Figure 1 is the most classic circuit. The advantage is that a filter capacitor can be connected in parallel to resistor R5. The resistor matching relationship is R1=R2, R4=R5=2R3; the gain can be adjusted by changing R5

The advantage of Figure 2 is that there are fewer matching resistors and only R1=R2 is required.

The advantage of Figure 3 is that the input impedance is high, and the matching resistors require R1=R2, R4=2R3

The matching resistors in Figure 4 are all equal, and the gain can also be changed by changing the resistor R1. The disadvantage is that in the negative half cycle of the input signal, the negative feedback of A1 is composed of two channels, one of which is R5, and the other is composed of the op amp A2. , also has the disadvantages of composite op amps.

Figure 5 and Figure 6 require R1=2R2=2R3 and the gain is 1/2. The disadvantage is: when the input signal is in the positive half cycle, the output impedance is relatively high. You can add a non-inverting amplifier with a gain of 2 to the output for isolation. Another disadvantage is that in the positive half cycle It is not equal to the input impedance of the negative half cycle, and the internal resistance of the input signal is required to be ignored.

Figure 7 Positive half cycle, D2 passes, gain = 1+(R2+R3)/R1; negative half cycle gain = -R3/R2; the absolute values ​​of the positive and negative half cycle gains are required to be equal. For example, if the gain is 2, you can choose R1=30K. R2=10K,R3=20K

The resistor matching relationship in Figure 8 is R1=R2

Figure 9 requires R1=R2, R4 can be used to adjust the gain, the gain is equal to 1+R4/R2; if R4=0, the gain is equal to 1; the disadvantage is that the input impedance of the positive and negative half-waves is not equal, and the internal resistance of the input signal is required. small, otherwise the output waveform will be asymmetrical.

Figure 10 is designed using the characteristics of a follower of a single power supply op amp. When the input signal of a single power supply follower is greater than 0, the output is a follower; when the input signal is less than 0, the output is 0. Be careful when using it. A single power supply op amp is nonlinear when the signal is very small. Moreover, a single power supply follower also has nonlinearity when a negative signal is input.

In the three circuits of Figures 7, 8, and 9, when the output of op amp A1 is positive, the negative feedback of A1 is formed by a composite amplifier composed of diode D2 and op amp A2. Due to the composite (product) effect of the two op amps, Maybe the gain of the loop is too high and oscillation is likely to occur.

There are still some precision full-wave circuits that have not been included. For example, there is a high-impedance type that connects the non-inverting input terminal of A2 to the inverting input terminal of A1. In fact, the principle is the same as this high-impedance type, so it is not included specifically. Others use A1. The output of only one diode is not included, because when the diode is turned off, A1 is in an open loop state.

in conclusion:

Although there are ten types of precision full-wave circuits here, after careful analysis, we found that there are not many excellent ones. To be precise, there are only three types, which are the first three types.

Although the classic circuit in Figure 1 has many matching resistors, it can be completely realized with six equal-value resistors R, of which resistor R3 can be connected in parallel with two R. The gain can be adjusted through R5, and the gain can be greater than 1 or less than 1. The most useful The advantage is that you can add capacitor filtering to R5.

The advantage of the circuit in Figure 2 is that there are few matching resistors, and only one pair of matching resistors is needed.

The advantage of Figure 3 is the high input impedance.

There are several other types, some of which realize the negative feedback of A1 through the recombination of A2 during the half-cycle of D2 conduction. For some op amps, self-excitation will occur. In some cases, the input impedance of the two half-waves is not equal, and the signal source requirements are relatively high. high.

Although the two single op amp types can achieve the purpose of rectification, their input and output characteristics are very poor. Followers or non-inverting amplifiers are required for both input and output isolation.

Each circuit has its own design features. I hope we can draw useful lessons from the ingenious design of its circuits. For example, the design of a single-power full-wave circuit and the design of a composite feedback circuit are very useful design ideas and methods. If we can It will be beneficial to analyze the circuit principles of each diagram and derive each formula.