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    Composite Amplifiers and How They Give the Best of Both Worlds

    March 9, 2018

     Computer planet with circuit grid

    The best of both worlds. This term is often thought to be unattainable or very difficult to achieve. I mean if you really think about the good things in life, it’s really one or the other. Personally, I’m a huge fan of ice cream and cake. For the longest time growing up, these two were, in fact, a one or the other type of a choice. Sure you can get ice cream and cake, which in fact was delicious, but having the two at the same time was thought of as stuff of legend.

    This all changed, when I was strolling through the store at the ripe age of 15 with a pocket full of change, ready to wet my whistle with some tasty sweets. I remember the moment fondly when I strolled passed the frozen section, stopped dead in my tracks, and for the first time witnessed this incredible invention of the ice cream cake. My mind was blown. How long had I been living in the dark? It didn’t matter. I had truly found the best of both of these delicious worlds.

    Sure, there have been some fantastic innovations in the recent past, but specifically, when dealing with op amps, we find ourselves limited to single dimensioned choices. Whether it be speed, power, precision, or any combination therein the best of both worlds is a tall order. Enter composite amplifiers. Composite amplifiers are multiple amps entwined within each other’s routing schemes in the attempt to give us the unicorn we are after: the best of both worlds.

    Series vs Composite Amplifier Design

    Let’s assume you have been tasked with the design of a high power output amplifier while at the same time keeping the quiet precision to a T. Not an easy task with single amplifiers these days. However, if we were wise about the selection of multiple amps, we may be able to (relatively) easy design a circuit with the best of both of these single amps.

    Elementary design practices may lead you to place these amps in series, however, this is not the case. Say you plan to place a high precision amp first and a high power output in series in series just after. You will certainly be able to achieve the power that is required, however, the precision of the first amp is, in effect, lost in the second stage. So how are we to mitigate this?

    The solution lies within composite design. Placing the second amp (our high powered amp) within the feedback loop of the first. Providing our high DC precision of the first stage as well as the high powered output stage, this design additionally gives us extra bandwidth when compared to the first series design.

    Taking it even further, including the second amplifier within the feedback loop of the first enables the second amp to leverage the distortion and noise cancellation properties of the first ultimately rendering the entire circuit as our unicorn, ice cream cake, best of both worlds design. Look at us go!

    Amplifier circuit
    Using a series amplifier design may leave you with a loss of precision. Choose a composite design to leverage precision and power.

    Factors to Consider

    Although this circuit will give us what we are ultimately after, careful consideration must be taken in the amplifier choice, as well as the overall design of the circuit (isn’t this the case for every circuit).

    Pay attention to the stability, common-mode violations of the input stage, slew rate, bandwidth performance, as well as noise peaking factors of the amplifiers as this will absolutely affect end-game performance.

    Heating will also be a consideration that will be needed in the design stage. Due to the higher output power of the second stage, isolating the thermal offshoots will help guarantee precision of the first stage isn’t compromised.

    Choosing a second-stage, higher-power amp that is faster than the first stage will increase the stability of the overall design. As an example, amplifiers similar to the ADA4870 are a good option as they tend to be unaffected by gain-bandwidth product variations. This, in turn, leaves the bandwidth of the second amp independent of the gain and can be adjusted by the feedback resistors’ value. This will be an important variable to adjust in optimizing the stability of the circuit.

    Engineer working
    Taking the time to tweak the design will ultimately benefit the stability of the circuit.

    Spend the Time, Do the Work

    With all these factors to consider (and then some), you’ll certainly need to do your due diligence and allow ample amount of time for design optimization during the early design stages. A great way to verify these design factors is to run the circuit through the gauntlet of SPICE simulation software (such as Altium design verification). 

    These simulation models open up the variability doors to fine tune the adjustment of open-loop gain and phase, output current limits, capacitive load drives, slew rates, supply current, and many other factors that are specific to your own composite amplifier design.

    Ultimately Attaining the Ice Cream Cake

    Although the old adage of having the best of both worlds is often difficult to attain in circuit design, and in particular amplifier design, using clever solutions such as the incorporation of a second amp no just in series but in a composite manner makes it possible to achieve this goal.

    Of course, you’ll need to take care of and consider many factors while embarking on the design, but with the correct amount of design tweaking, and utilizing great software features within Altium Designer’s platform, you’ll be empowered to look like a champion when tasked with these difficult design problems.

    If you want to know how Altium’s design features, such as design verification, can make your composite design a walk in the park, talk to an Altium expert today.

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