Can you achieve high accuracy and high output power simply by connecting two op amps in series?

“Engineers often face various challenges and need to continuously develop new applications to meet a wide range of needs. Generally speaking, these needs are difficult to meet at the same time. For example, a high-speed, high-voltage operational amplifier (op amp), but also has high output power, and the same excellent DC accuracy, noise and distortion performance. There are very few operational amplifiers on the market that have all these characteristics. However, you can use two separate amplifiers to build this amplifier to form a composite amplifier. Combining two operational amplifiers together can integrate their respective advantages into one.In this way, compared with a single amplifier with the same gain

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Engineers often face various challenges and need to continuously develop new applications to meet a wide range of needs. Generally speaking, these needs are difficult to meet at the same time. For example, a high-speed, high-voltage operational amplifier (op amp), but also has high output power, and the same excellent DC accuracy, noise and distortion performance. There are very few operational amplifiers on the market that have all these characteristics. However, you can use two separate amplifiers to build this amplifier to form a composite amplifier. Combining two operational amplifiers together can integrate their respective advantages into one. In this way, compared with a single amplifier with the same gain, a combination of two operational amplifiers can achieve a higher bandwidth.

Composite amplifier

The composite amplifier is composed of two separate amplifiers, each with different characteristics. Figure 1 shows this structure. Amplifier 1 is a low-noise precision amplifier ADA4091-2. In this example, amplifier 2 is AD8397, which has high output power and can be used to drive other modules.

Figure 1. Schematic diagram of two operational amplifiers connected in series to form a composite amplifier

The configuration of the composite amplifier shown in Figure 1 is similar to that of the non-inverting amplifier, which has two external operating resistors R1 and R2. Think of two operational amplifiers connected in series as one amplifier. The total gain (G) is set by the resistance ratio, G = 1 + R1/R2. If the resistance ratio of R3 to R4 changes, it will affect the gain of amplifier 2 (G2), as well as the gain or output level of amplifier 1 (G1). However, R3 and R4 will not change the effective total gain. If G2 decreases, G1 will increase.

Bandwidth expansion

Another feature of the composite amplifier is that it has a higher bandwidth. Compared with a single amplifier, the bandwidth of a composite amplifier is higher. Therefore, if two identical amplifiers are used with a gain-bandwidth product (GBWP) of 100 MHz and a gain of G = 1, then the C3 dB bandwidth can be increased by about 27%. The higher the gain, the more obvious the effect, but the highest can only reach a certain limit. Once the limit is exceeded, it may become unstable. This kind of instability can also occur when the two gains are not evenly distributed. Generally speaking, the maximum bandwidth can be obtained when the gains of the two amplifiers are equally distributed. Using the above values ​​(GBWP = 100 MHz, G2 = 3.16, G = 10), when the total gain is 10, the C3 dB bandwidth of the combination of two amplifiers can reach 3 times that of a single amplifier.

This description is relatively simple. When the gain is evenly distributed, G2 will also obtain the same effective gain as amplifier 1. However, the open loop gain of each independent amplifier is higher. At low gains, for example, from 40 dB to 20 db, both amplifiers will operate in the low region of the open loop curve (see Figure 2). In this way, compared with a single amplifier with the same gain, a composite amplifier can obtain a higher bandwidth.

Figure 2. Bandwidth expansion through composite amplifiers

DC accuracy and noise

In a typical operational amplifier circuit, part of the output is fed to the inverting input. In this way, the output error can be corrected through the feedback path to improve accuracy. The combination shown in Figure 1 also provides a separate feedback path for amplifier 2, although it is also in the feedback path of amplifier 1. The output of the overall configuration will have a greater error due to amplifier 2, but this error will be corrected when it is fed back to amplifier 1. Therefore, the accuracy of the amplifier 1 can be maintained. The output offset is only proportional to the input offset error of the first amplifier, and has nothing to do with the offset voltage of the second amplifier.

The same is true for noise components. It is also corrected by feedback, where the AC signal is related to the bandwidth reservation of the two amplifier stages. As long as the first amplifier stage has sufficient bandwidth, it will correct the noise component of amplifier 2. So far, its output voltage noise density is dominant. However, if the bandwidth of amplifier 1 is exceeded, the noise component of the second amplifier begins to dominate. If the bandwidth of amplifier 1 is too high, or much higher than the bandwidth of amplifier 2, problems will arise. This may cause additional noise peaks in the output of the composite amplifier.

in conclusion

By connecting two amplifiers in series, the excellent characteristics of the two can be combined to obtain results that cannot be achieved with a single operational amplifier. For example, a high-precision amplifier with high output power and high bandwidth can be realized. The example circuit shown in Figure 1 uses the rail-to-rail amplifier AD8397 (C3 dB bandwidth = 69 MHz) and the precision amplifier ADA4091-2 (C3 dB bandwidth = 1.2 MHz). ) Bandwidth is more than 2 times higher (G = 10). In addition, the combination of AD8397 and various precision amplifiers can also reduce noise and improve THD characteristics. However, in the design, the amplifier configuration must also be modified to ensure the stability of the system. If all standards are considered, composite amplifiers may also be suitable for a wide range of demanding applications.