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ElectronicsI_L21 - Lecture 21 CE Resistance in Saturation...

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Lecture 21
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CE Resistance in Saturation The BJT operated in common emitter mode in saturation acts like a small valued resistor The value tends to be from a few ohms to a few tens of ohms. •There is also a voltage offset present •This can be developed into a model of the BJT in saturation •The collector terminal is a resistor and battery
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BJT in Saturation Because there is an offset voltage in the saturated BJT model, they are less attractive as a switch. The MOSFET in comparison cross the origin, meaning that the off state will result in zero volts out. We can use the Ebers-Moll model to describe the current through the BJT with a constant current applied to the base for saturation operation. ( ) + = R F V v R V v B F C T CE T CE e e I i β β α β 1
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Breakdown Because the BJT consists of p-n junctions, there is the possibility to have breakdown occur. Needless to say this leads to non-ideal behavior In a common base configuration, the collector- base junction breaks down at a voltage BV CBO with no emitter current (>50V).
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