Opamps for Active Crossovers 379 With JFET inputs the problem is not the input bias currents of the input devices themselves, which are quite negligible, but the currents drawn by the non-linear junction capacitances inherent in field-effect devices. These capacitances are effectively connected to one of the supply rails. For P-channel JFETs, as used in the input stages of most JFET opamps, the important capacitances are between the input JFETs and the substrate, which is normally connected to the V- rail. See, for example, Jung [2]. According to the Burr-Brown data sheet for the OPA2134, "The P-channel JFETs in the input stage exhibit a varying input capacitance with applied CM voltage." It goes on to recommend that the input impedances should be matched if they are above 2 k, to cancel out the non-linearity. The amount of CM distortion generated by a given type of opamp is very important for our purposes here because active crossover circuitry very often uses voltage-followers, primarily in Sallen and Key filters, but also for general buffering purposes. This configuration is the worst case for CM distortion because the full output voltage appears as a CM signal on the inputs. With BJT opamps CM distortion can be rendered negligible by keeping the source impedances low, which is also a very good idea as it reduces noise and susceptibility to electrostatic interference. 13.3 Opamps Surveyed