Signal conditioning High output drive capability with precision
A composite amplifier is an arrangement of two individual amplifiers configured in such a way as to realise the benefits of each individual amplifier while diminishing the drawbacks of each amplifier. In this article, Analog Devices’ Jino Loquinario discusses the major benefits and design considerations when implementing a composite amplifier configuration
would require us to think of a solution that is beyond the performance of current products that the market offers. For example, an application may require an amplifier that is high speed and high voltage with high output drive capability, but may also demand excellent dc precision, low noise, low distortion, etc. Amplifiers that meet the speed and output
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voltage/current requirements and amplifiers with outstanding dc precision are readily available in the market—many of them, in fact. However, all the requirements may not exist in a single amplifier. When faced with this problem, some would think it is impossible for us to meet the demands of such applications, and that we must settle for a mediocre solution and go with either a precision amplifier or a high speed amplifier, perhaps sacrificing some of the requirements. Fortunately, this is not entirely true. There is a solution for this in the form of a composite amplifier.
The ComposITe AmpLIfIer A composite amplifier is an arrangement of two individual amplifiers configured in such a way as to realise the benefits of each individual amplifier while diminishing the drawbacks of each amplifier.
t is normal, and almost expected, to be faced with applications for which a solution does not appear to exist. To meet their requirements
gain. To address this, it is helpful to view the composite amplifier as a single noninverting op amp contained within the large triangle, as in Figure 2. If we imagine the triangle is blacked out so that we could not see what is inside, then the gain of the noninverting op amp is 1 + R1/R2. Revealing the composite configuration inside the triangle doesn’t change anything—the gain of the whole thing is still controlled by the ratio of R1 and R2. In this configuration it is tempting to think that
changing the gain of AMP2 by means of R3 and R4 will affect the output level of AMP2, indicating a change in composite gain, but this is not the case. Increasing the gain around AMP2 via R3 and R4 will simply decrease the effective gain, and output level, of AMP1 such that the output of the composite (AMP2 output) remains unchanged. Alternatively, decreasing the gain around AMP2 will serve to increase the effective gain of AMP1. So, in general, the gain of the composite amplifier is only dependent on R1 and R2.
per cent higher –3 dB bandwidth, albeit with a small amount of peaking. However, at higher gains this benefit becomes much more noticeable. Figure 5 shows the composite amplifier in a gain
of 10. Note the composite gain is set to 10 via R1 and R2. The gain around AMP2 is set to
Composite Amplifiers
Figure 3. Composite amplifier at unity gain.
Figure 4. –3 dB BW improvement at unity gain.
Figure 2. Composite amplifier seen as a single amplifier.
This article will discuss the major benefits and
Figure 1. Simple composite amplifier configuration. Referring to Figure 1, AMP1 should have
excellent dc precision as well as the noise and distortion performance required by the application. AMP2 should provide the output drive requirements. In this arrangement, the amplifier (AMP2) with required output specifications is placed inside the feedback loop of an amplifier (AMP1) with the required input specifications. Some of the techniques and benefits of this arrangement will be discussed.
seTTIng The gAIn When initially encountering a composite amplifier, the first question that may arise is how to set the
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design considerations when implementing a composite amplifier configuration. The effects on bandwidth, dc precision, noise, and distortion will be highlighted.
BAnDwIDTh exTensIon One of the major benefits of implementing a composite amplifier is the extended bandwidth as compared to a single amplifier configured with the same gain. Referring to Figure 3 and Figure 4, let us say we
have two separate amplifiers each having a gain- bandwidth product (GBWP) of 100 MHz. Putting them together in a composite configuration will increase the effective GBWP of the combination. At unity gain, the composite amplifier offers a ~27
Figure 5. Composite amplifier configured for gain = 10.
Figure 6. –3 dB BW improvement for gain = 10. January 2021 Instrumentation Monthly
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