There are many variations of the acoustic instrument due to the constant search for new sounds and performance. The amplification of the acoustic guitar is probably the part most interested in research. In fact, if in the recording studio the use of a mircophone gives good results without problems, that won't necissarily be true when playing live. Microphoning the guitar on stage involves a series of problems that shouldn't be underestimated, such as return from other instruments or feedback from the microphone itself. This is why the research of the use of dedicated transducers began already in the 70's.
Almost all manufacturers of acoustic guitars, both folk and classical, have models equipped with piezoelectric transducers. This is a small block of crystalline or ceramic material encapsulated in a metal or plastic cover. Usually this is placed under the bone of the bridge, where you find the point of maximum vibration of the top panel. A cable connects the transducer to a jack screwed to the resonance chamber, to which you can connect a send cable to the amplification. This is how it breifly works, as we will go more in-depth in the pages dedicated to the amplification of the acoustic guitar.
There are many variations with respect to materials and shapes of the acoustic guitar. Some of these have created origional models with surprising sound quality. I would like to name at least a few, to observe the main characteristics.

The Ovation guitars have surely been revolutionary from a technical point of view. In fact the designer, Charles Kaman, thought to apply some of the acoustic principles used in the construction of airplanes and helicopter to the acoustic guitar (the Ovation, in fact, was founded in the 1960's as a division of the Kaman aerospace organization).
The most noteable difference of an Ovation is the body: the back and one side panel are rounded and are composed of fiberglass (called Lyrachord). The inside of the body is therefore without reinforcers that could in some way block the sound. The structure of the radials below the top panel vary from model to model, giving different versions of the guitar, each with its own tune, adaptable to a precise musical type.
Like many other acoustic models, the Ovation is also amplified by an internal circuit that can be, depending on the case, more or less complex. Their sound, also in this respect, is unique and inimitabile. Of more recent conception is the series of Canadian guitars under the brand Godin. In fact, these have the shape and sound very similar to an electric instrument, as you can see in the figure. The Godin's also have a top wooden panel with a fiberglass body, but are conceptually different from a sonorous point of view because they are born to be amplified. Their strong point is in the sound that they generate thanks to an internal preamp, but also due to their particular internal structure, that has harmonic wooden strips, which are attributed to the acoustic quality of the sound in the instrument.
A common system to these and many other acoustic guitars is preamplification. While a good quality piezoelectric transducer transmits the signal relative to the harmonic vibrations of the strings without the help of audio amplification, the preamplifier controls the signal frequency and exit volume coming from the audio amplification apparatus. Many models were adopted by the manufacturers, such as the the electro-acoustic guitars by Takamine, Taylor, Yamaha and Ibanez. Almost all makers of acoustic guitars now have preamplified instruments, which are highly requested and maintain their principle role of being an acoustic guitar.

In the figure below we can see one of the Ovation models, the Adamas. The particularity of this instrument is in the top panel, which is made of 2 layers of carbon fiber sandwiched around a laminate of birch. The central soundhold has been substituted with a series of 22 smaller holes.