1+1>2! This way you can achieve both high precision and high power

However, you can build this amplifier out of two separate amplifiers, forming a composite amplifier. By combining two op amps, you can combine the advantages of each into one. This allows you to achieve a higher bandwidth than a single amplifier with the same gain.

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A composite amplifier is a combination of two separate amplifiers, each with different characteristics. This structure is shown in Figure 1. Amplifier 1 is a low noise precision amplifier, the ADA4091-2. In this case, amplifier 2 is the AD8397, which has high output power and can be used to drive other modules.

The configuration of the composite amplifier shown in Figure 1 is similar to that of a non-inverting amplifier, which has two external operating resistors, R1 and R2. Think of the two op amps connected in series as one amplifier. The overall gain (G) is set by the resistor ratios, G = 1 + R1/R2. If the resistor ratio of R3 to R4 is changed, it affects the gain of amplifier 2 (G2) and also affects the gain or output level of amplifier 1 (G1). However, R3 and R4 do not change the effective overall gain. If G2 is decreased, G1 will increase.

Another characteristic of composite amplifiers is their higher bandwidth. Composite amplifiers have a higher bandwidth than the individual amplifiers. So, if you use two identical amplifiers with a gain bandwidth product (GBWP) of 100 MHz and a gain of G = 1, you can improve the –3 dB bandwidth by about 27%. The effect is more pronounced with higher gains, but only up to a certain limit. Beyond that limit, you may become unstable. This instability can also occur when the two gains are not equally distributed. Generally speaking, the maximum bandwidth is achieved when the gains of the two amplifiers are equally distributed. Using the above values ​​(GBWP = 100 MHz, G2 = 3.16, G = 10), the –3 dB bandwidth of the two amplifiers combined is three times that of the individual amplifiers at a total gain of 10.

This is a relatively simple illustration. When the gain is evenly distributed, G2 will also achieve the same effective gain as amplifier 1. However, the open-loop gain of each individual amplifier is higher. At lower gains, for example, from 40 dB down to 20 dB, both amplifiers will operate in the lower region of the open-loop curve (see Figure 2). This allows the composite amplifier to achieve a higher bandwidth than a single amplifier with the same gain.

In a typical op amp circuit, part of the output is fed to the inverting input. This allows the output error to 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 overall configuration output will have a larger error due to amplifier 2, but this error will be corrected when fed back to amplifier 1. Therefore, the accuracy of amplifier 1 can be maintained. The output offset is proportional only 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 the noise component. It is also corrected by feedback, where the AC signal is related to the bandwidth reserve of the two amplifier stages. As long as the first amplifier stage has enough bandwidth, it corrects the noise component of amplifier 2. Up to this point, its output voltage noise density dominates. However, if the bandwidth of amplifier 1 is exceeded, the noise component of the second amplifier starts to dominate. This can cause problems if the bandwidth of amplifier 1 is too high, or much higher than the bandwidth of amplifier 2. This can cause additional noise peaks in the output of the composite amplifier.

By connecting two amplifiers in series, it is possible to combine the excellent characteristics of both to achieve results that cannot be achieved using a single op amp. For example, a high-precision amplifier with high output power and wide bandwidth can be realized. The example circuit shown in Figure 1 uses the rail-to-rail amplifier AD8397 (–3 dB bandwidth = 69 MHz) and the precision amplifier ADA4091-2 (–3 dB bandwidth = 1.2 MHz), and the bandwidth obtained by combining the two is more than twice that of a single amplifier (amplifier 1) (G = 10). In addition, the combination of the AD8397 and various precision amplifiers can also reduce noise and improve THD characteristics. However, in the design, it is also necessary to ensure the stability of the system by correcting the amplifier configuration. If all criteria are considered, composite amplifiers may also be suitable for a wide range of demanding applications.