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Circuit design reference diagram

Source: InternetPublisher:MartinFowler Updated: 2013/03/12

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, and the advantage is that a filter capacitor can be connected in parallel with the 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 resistor requires 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 consists of two channels, one of which is R5 and the other is an op amp. The A2 composite structure also has the disadvantages of a composite operational amplifier.

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, and the gain can be increased at the output. 2 non-inverting amplifier isolation. Another disadvantage is that the input impedance of the positive half cycle and the negative half cycle is not equal, requiring the internal resistance of the input signal 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 and 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 to be small, otherwise the output waveform Asymmetric.

Figure 10 is designed using the characteristics of the 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. Use Be careful about the nonlinearity of a single power supply op amp when the signal is very small. Moreover, the 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 the op amp A1 is positive, A1 The negative feedback is formed by a composite amplifier composed of diode D2 and operational amplifier A2. Due to the composite (product) effect of the two operational amplifiers, the gain of the loop may be too high and oscillation is likely to occur.

There are still some precision full-wave circuits that are not available Input, 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 that of the high-impedance type, and it is not included specifically. Others use the output of A1 to only connect a diode. It is not included because when the diode is cut off, A1 is in an open-loop state.

Conclusion:
Although there are ten types of precision full-wave circuits here, after careful analysis, it is found that there are not many excellent ones. To be precise, there are only three types, which are the previous three. Although
the classic circuit in Figure 1 has many matching resistors, it can be completely implemented 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 advantageous thing is that you can add capacitor filtering to R5.

The advantage of the circuit in Figure 2 is that it has fewer matching resistors, as long as a pair of matching resistors are enough.

The advantage of Figure 3 is that it has high input impedance. There

are several other types, some of which are in D2 During the half-cycle of conduction, the negative feedback of A1 is realized through the recombination of A2, which may cause self-excitation for some op amps. Some input impedances of the two half-waves are not equal, which requires higher signal sources.
Two single op amp types Although the purpose of rectification can be achieved, the input\output characteristics are very poor. The input\output needs to be isolated by a follower or a non-inverting amplifier.
Each circuit has its own design features. I hope we can learn useful lessons from the ingenious design of its circuits. For example, the design of a single power supply full-wave circuit and the design of a composite feedback circuit are very useful design ideas and methods. If you can analyze the circuit principles of each diagram and derive each formula, you will benefit.

Final conclusion For your reference when designing circuits.

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MartinFowler

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