The pickups used to amplify a banjo are completely different than for guitars and other instruments. Compared to a guitar, a banjo head has significantly higher and more percussive resonance and is also in a higher frequency range. Slapping a guitar pickup on a banjo will result in a very tinny, thin sound with a tendency to clip, distort and feedback significantly. About 80% of the banjo sound comes from the resonance of the banjo head. Pickups for banjos need to be able to handle the added percussion and resonance of the banjo head, without adding any coloration to the sound. We have many different banjo pickups that use different techniques to accomplish this.
Contact Sensors
The most common approach for amplifying a banjo is to attach a sensor directly to the banjo head. Since the banjo has naturally high resonance, using passive high output sensors will typically produce enough signal to drive an amplifier directly (although you may still need a preamp if you are plugging into a PA system or mixer board.)
Schatten Design, for example, makes the highly acclaimed BJ-02 series of banjo pickups that all use a single sensor that mounts on the banjo head directly under the bridge feet. This sensor has a high resonance threshold and is resistant to feedback, and has a very natural sound without being tinny or harsh. The BJ-02 pickup is available in a variety of configurations with different types of jacks.Other examples of contact pickups include the K&K Banjo Twin, the McIntyre BF-60 Banjo Feather, the D-TAR SoundSpot Banjo Pickup, and the Shadow SHB1, all of which use pickup sensors that are specially designed for a banjo.
Bridge Replacements
The LR Baggs Banjo Pickup is a good example of a bridge replacement that has an embedded piezo pickup. Note that this pickup does not include a jack, so you would need to purchase that separately (LR Baggs is working on a new model that includes a jack). Another quality example is the Shadow SH930 pickup, which is made in Germany. Of course, replacing your bridge may also mean fitting the new pickup to match your original pickup, and dealing with the lead wire that extends from the pickup to the jack (typically by running it under the tailpiece) so this type of installation is not as straightforward as with a contact sensor.
Magnetic Pickups
If the banjo has steel strings, using a magnetic pickup is possible. However, there is not enough room to mount a magnetic pickup directly between the strings and the head, so the common approach is to attach a small metal plate to the banjo head and then use that plate to interact with the pickup's magnetic fields. The Fishman Rare Earth Banjo Pickup and the EMG ACB Banjo Pickup use this technique, which gives them excellent string clarity and feedback resistance. These magnetic pickups also include preamps, so the output is suitable for use with PA systems and mixer boards, making these a great choice for stage use.
Combination Systems
If you are serious about your banjo sound, you might consider a system that combines multiple sound sensors. The EMG ACB system combines the magnetic pickup with other sensors to detect the natural vibrations of the banjo head. Another example is the Schatten Design BJ-02M model that includes their very popular BJ-02 sensor along with a high quality condenser microphone and the Schatten Mini-Pre 2 belt-clip preamp/mixer for the best possible realism, an excellent choice for studio and recording use.