Well, it depends greatly on what style you are playing and what sounds you are going for.
If you play in great concert halls with fantastic acoustics all the time, you might never need anything but your great acoustic instrument.
If you play at festivals, concert venues or small cafés in an amplified acoustic or electric setup then read on.
In this article I am going to show you a couple of different setups, from very basic to more advanced.
A lot of musicians (like me) have different rigs for different applications. Just like we are using different instruments for different pieces.
The most basic stage setups
This can work very well for small bands, if every musician play acoustic instruments and are trained at how to place themselves in front of the microphone.
A little more comprehensive is a multi microphone setup, either stereo or a microphone on a stand in front of every musician.
Read more about microphone placement elsewhere on the blog.
Still keeping it simple
Acoustic instrument with microphone attached to it, going through the venues available PA.
If you play acoustic music with an acoustic band or semi-acoustic band a miniature microphone can be attached to the instrument, making the player able to play louder and move more freely.
If you want to play even louder, a semi-acoustic setup with a more permanent attached onboard pickup is the way to go. (On stringed instruments they are often placed in or under the bridge.)
Most pickups need pre-amplification and you will have to add that to your rig.
Bigger PA's have monitors, but to be able to get consistent sound you could add a stage amplifier to your rig - either a combo or a small powered PA speaker.
Adding a combo will also make you able to play smaller gigs where no PA is available - instrument with pickup/microphone and using your amplifier for both monitor and "PA".
Becoming more advanced
Now you have an instrument, microphone/pickup, preamp and amplifier in your rig.
If you are still going for a fairly natural sound coming from your fiddle, but you want to be able to tweak or change your sound on stage.
Or you could add a delay to thicken up your sound.
Maybe add a reverb to soften your sound.
If you want to change you sound more radically you could add an octaver, which can add an octave above or below to you sounds or both. (Some octavers do not track the notes of bowed instruments well, so be care full to find a suitable one.)
If you play in a loud band with other electric instruments - guitar, bass and drums - you have to start considering going electric yourself.
The electric violin or viola will not sound like your acoustic. You will have to find a combination of instrument, preamp and amplifier to get a sound that suits your playing style.
The electric setup is often enhanced with a wider range of effects - like flanger, compression or distortion.
Some will add a multi-effects unit and some seek out a select hand-full of usable single pedals. Others will add a computer to their setup, which provides the effects and amplifier simulations.
The final possibility is to add midi to your setup. This makes you able to control your rig and if you have the right pickup and interface you can make your bowed instrument sound like other instruments - brass, guitars, synth etc.