Signal conditioning
Table 1. Bandwidth Extension on Different Amplifier Combinations for Gain of 10 and VOUT = 10 V p-p Figure 7. Gain splitting for maximum bandwidth.
redistribution of the gain between the two amplifiers. In this case, reducing the gain of AMP2 causes an increase in the effective gain of AMP1. The result is that AMP1 closed-loop BW is decreased as it operates higher on the open-loop curve and AMP2 closed-loop bandwidth is increased as it operates lower on the open-loop curve. If this slowing down of AMP1 and speeding up of AMP2 is adequately applied, the stability of the composite combination is restored. For this article, the AD8397 was picked as the
output stage (AMP2), interfaced with various precision amplifiers for AMP1 to demonstrate the benefits of a composite amplifier. The AD8397 is a high output current amplifier capable of delivering 310 mA.
Figure 8. Expected response of a single amplifier.
approximately 3.16, forcing the effective gain of AMP1 to be the same. Splitting the gain equally between the two amplifiers yields the greatest possible bandwidth. Figure 6 shows the frequency response for a
single amplifier at a gain of 10 compared to a composite amplifier configured with the same gain. In this case, the composite offers a ~300 per cent increase in –3 dB bandwidth. How is this possible? For a specific example, refer to Figure 7 and
Figure 8. We require a system gain of 40 dB and will use two identical amplifiers, each with an open- loop gain of 80 dB and a GBWP of 100 MHz. To realise the highest possible bandwidth for
the combination, we will split the required system gain equally between the two amplifiers, giving each of them a gain of 20 dB. So, setting the closed-loop gain of AMP2 to 20 dB forces the effective closed-loop gain of AMP1 to 20 dB as well. With this gain configuration, both amplifiers operate lower on the open-loop curve than either of them would at a gain of 40 dB. As a result, the composite will have higher bandwidth at the gain of 40 dB as compared to the single amplifier solution of the same gain. Although this may sound relatively simple and
easy to implement, proper care should be taken in designing the composite amplifier to have the highest possible bandwidth without sacrificing the stability of the combination. In real-world applications where amplifiers are nonideal, and probably nonidentical, a proper gain arrangement must be ensured to maintain stability. Also, note that the composite gain will roll off at –40 dB/decade, so one must be careful when distributing the gain between the two stages. In some cases, splitting the gain equally may not
be possible. To that point, equal distribution of the gain between the two amplifiers requires that the GBWP of AMP2 must always be greater than or equal to GBWP of AMP1, otherwise peaking— and possibly instability—will result. In a case where AMP1 GBWP must be greater than AMP2 GBWP, the instability can typically be corrected by
Instrumentation Monthly January 2021 Preserved dC PreCision Figure 11. Offset error contribution.
AMP1 while VOS2 represents a variable offset voltage for AMP2. Figure 12 shows that as VOS2 is swept from 0 mV to 100 mV, the output offset is
not affected by the magnitude of error (offset) contributed by AMP2. Instead, the output offset is proportional only to the error of AMP1 (50 µV multiplied by the composite gain of 100) and
remains at 5 mV regardless of the value of VOS2. Without the composite loop, we would expect the output error to increase as high as 500 mV.
Figure 9. Operational amplifier feedback loop.
In a typical operational amplifier circuit, a portion of the output is fed back to the inverting input. Errors that are present on the output which were generated in the loop are multiplied by the feedback factor (β) and subtracted out. This helps maintain the fidelity of the output with respect to the input multiplied by the closed-loop gain (A).
Figure 12. Composite output offset vs. VOS2. noise and distortion
Figure 10. Composite amplifier feedback loop. For the composite amplifier, amplifier A2 has its
own feedback loop, but A2 and its feedback loop are all inside the larger feedback loop of A1. The output now contains the larger errors due to A2 which are fed back to A1 and corrected. The larger correction signal results in the precision of A1 being preserved. The effect of this composite feedback loop can
be clearly seen in the circuit and results in Figure 11 and Figure 12. Figure 11 shows a composite amplifier comprised of two ideal op amps. The composite gain is 100 and the AMP2 gain is set to 5. VOS1 represents a 50 µV offset voltage for
The output noise and harmonic distortion of the composite amplifier are corrected in a similar fashion as the dc errors, but, in the case of ac parameters, the bandwidth of the two stages also comes into play. We will look at an example using output noise to illustrate this with the understanding that distortion cancellation occurs in much the same manner. Referring to the example circuit in Figure 13, for as
long as the first stage (AMP1) has enough bandwidth, it will correct for the larger noise of the second stage (AMP2). As AMP1 begins to run out of bandwidth, the noise from AMP2 will begin to dominate. However, if AMP1 has too much bandwidth, and peaking is present in the frequency response, a noise peak will be induced at the same frequency. For this example, resistors R5 and R6 in Figure
13 represent the inherent noise sources for AMP1 and AMP2 respectively. The top plot of Figure 14
Table 2. Output Offset Voltage for Gain of 100
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