(which in turn, is the current through the load). Thus, the load current is constant (neglecting the output resistance of the transistor due to the Early effect) and the circuit operates as a constant current source. As long as the temperature remains constant (or doesn't vary much), the load current will be independent of the supply voltage, R1 and the transistor's gain. R2 allows the load current to be set at any desirable value and is calculated by where is typically 0.65 V for a silicon device. ( is also the emitter current and is assumed to be the same as the collector or required load current, provided is sufficiently large). Resistance is calculated as where = 1.2 to 2 (so that is low enough to ensure adequate ), and is the lowest acceptable current gain for the particular transistor type being used. LED current source The Zener diode can be replaced by any other diode; e.g., a light-emitting diode LED1 as shown in Figure 5. The LED voltage drop () is now used to derive the constant voltage and also has the additional advantage of tracking (compensating) changes due to temperature. is calculated as and as , where ID is the LED current Transistor current source with diode compensation Temperature changes will change the output current delivered by the circuit of Figure 4 because is sensitive to temperature. Temperature dependence can be compensated using the circuit of Figure 6 that includes a standard diode, D, (of the same semiconductor material as the transistor) in series with the Zener diode as shown in the image on the left. The diode drop () tracks the changes due to temperature and thus significantly counteracts temperature dependence of the CCS. Resistance is now calculated as Since , (In practice, is never exactly equal to and hence it only suppresses the change in rather than nulling it out.) is calculated as (the compensating diode's forward voltage drop, , appears in the equation and is typically 0.65 V for silicon devices.) Note that this only works well if DZ1 is a reference diode or another stable voltage source. Together with 'normal' Zener diodes especially with lower Zener voltages (<5V) the diode might even worsen overall temperature dependency. Current mirror with emitter degeneration Series negative feedback is also used in the two-transistor current mirror with emitter degeneration. Negative feedback is a basic feature in some current