There are many variants on two common designs. The tube screamer uses diode clipping around an op amp feedback loop. The distortion + has diode clipping to ground after the main gain stage. We use the diode clipping to ground arrangement, but we have added adjustable asymmetry, and an expansion control which will control how much the character of the distortion changes as you push the guitar harder. This is great for crunch rhythm tones, and gives nice drive sounds when driven hard.