While high quality voltage sources are very common, current sources as components have remained elusive. Two-terminal current sources generate a new set of problems, especially if high accuracy and stability over temperature are desired. The current source must operate over a wide voltage range, exhibit high DC and AC impedance connected in series with unknown reactance, and exhibit good regulation and temperature coefficient. For optimal two-terminal solutions, no power supply bypass capacitor should be used since it degrades AC impedance. Older solutions using depletion mode FETs have wide variability in current and temperature coefficients. A two-transistor, two-Zener diode, two-terminal current source is shown above. It does a reasonable job of providing a two-terminal current, but its accuracy is limited to a few percent. The circuit operates open loop and as such cannot provide accuracies of closed loop feedback circuits. Since the temperature coefficients of the Zener and diode do not match the transistor perfectly, the circuit has drift and inaccuracies due to the variability in Zeners and transistor VBEs (base-emitter voltages). Further, the circuit requires a minimum of about 3 Volts across it in order to operate properly.