I can vaguely remember a time when nobody wanted to amplify an acoustic guitar, and I don’t want to remember some of the results when they began to try. There is one basic problem ~ the human ear does not want to recognise an acoustic instrument when it is played at electric volumes. There is too much going on in an acoustic guitar, and all the detail gets lost. Any attempt to wind up the volume turns the guitar into a microphone and we get the opening to "I feel fine" ~ feedback.
Today, professional musicians must have absolutely reliable amplification. Everybody needs to be louder than everybody else. They take several approaches, each of which involves a compromise for the reasons given above.
The earliest, and still used by some, is a tiny microphone inside the soundhole. It works, and it sounds OK until you become famous and start playing to a sizeable audience.
The way through this is to add a second source, usually a piezo-electric pickup under the saddle in the bridge. Using the piezo to get the message across, and the microphone to add a little "air" to the sound has been the best approach for many. There is always the potential for feedback with a microphone, but with a separate mixer for the two signals, the sound can be tailored for the venue, and there are many systems and refinements that use that combination. It is also very popular for players with a percussive playing style.
For a long time, many players used "bugs" of one sort or another, little devices glued to the underside of the soundboard. They functioned, but not well enough for today’s requirements. There are new versions with several separate elements inside the
To avoid feedback (or almost avoid feedback), magnetic pickups attached to the soundhole take their signal source from the string rather than the vibrating soundboard. This is a very neat and successful solution, but it does lose a little of the acoustic property of the guitar, replacing it with a slightly softened, muted character, which a good sound engineer can trim to very good effect. It is a good solution, and new, even better systems based on this principle are becoming available. The pickups are detachable and easy to fit.
The most common pickup on acoustic guitars has for a long time now been a piezo element, under the saddle in a slot in the wooden bridge. It senses the string vibrations through the saddle and converts them into electricity, the same principle as the spark in a gas lighter. That output can be taken directly to an amplifier, but the impedance of these pickups is very high, so they do not handle all frequencies equally, and a small pre-amplifier with a battery inside the guitar makes a big difference. They are available with or without onboard control systems and external power sources. They can be coupled with other pickups, and are available in many varieties. They are not easy to fit properly, and there is a little "edgy" character to the sound, the opposite problem to magnetic pickups, but the better quality systems are very good, and feedback problems are low.
I do dislike cutting holes in the sides of an expensive handmade instrument, so I prefer to use the simplest of systems, but I am always happy to discuss all these options with a customer.
Although there is a wide variety and choice of pickups, and I frequently use many of them, my own suggestion for most players is to use one of the English "Headway" range. They manufacture a wide variety of very high quality systems for all types of instruments, and are constantly refining their products as a result of contact with professional players. Many makers of "high end" guitars consider them to be the best available.