Early Effect - Large-signal Model

Large-signal Model

In the forward active region the Early effect modifies the collector current and the forward common-emitter current gain, as typically described by the following equations:

Where

• is the collector–emitter voltage
• is the thermal voltage ; see thermal voltage: role in semiconductor physics
• is the Early voltage (typically 15 V to 150 V; smaller for smaller devices)
• is forward common-emitter current gain at zero bias.

Some models base the collector current correction factor on the collector–base voltage V_CB (as described in base-width modulation) instead of the collector–emitter voltage V_CE. Using V_CB may be more physically plausible, in agreement with the physical origin of the effect, which is a widening of the collector–base depletion layer that depends on V_CB. Computer models such as those used in SPICE use the collector–base voltage V_CB.