Junction field effect transistors (JFETs) are used and are useful in many different circuit topologies. Because these devices function as voltage controlled current sources (VCCS) as opposed to current controlled current sources (CCCS), they offer designers certain flexibilities in designs not available when using bipolar transistors (which are CCCS devices). JFETs can also be used as voltage controlled variable resistors and voltage-controlled switches. JFETs have extremely high input impedances (on the order of giga-ohms) and extremely low input leakage currents (on the order of pico-amps). Selected parts are designed to have excellent performance in high and very high frequency (VHF) circuits.