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BJT structure heavily doped ~ 10^15 provides the carriers lightly doped ~ 10^8 lightly doped ~ 10^6 note: this is a current of electrons (npn case) and so the conventional current flows from collector to emitter. BJT characteristics BJT characteristics BJT modes of operation Mode EBJ CBJ Cutoff Reverse Reverse Forward active Forward Reverse Reverse active Reverse Forward Saturation Forward Forward BJT modes of operation Cutoff: In cutoff, both junctions reverse biased. There is very little current flow, which corresponds to a logical "off", or an open switch. Forward-active (or simply, active): The emitter-base junction is forward biased and the base-collector junction is reverse biased. Most bipolar transistors are designed to afford the greatest common-emitter current gain, βf in forward-active mode. If this is the case, the collector-emitter current is approximately proportional to the base current, but many times larger, for small base current variations. Reverse-active (or inverse-active or inverted): By reversing the biasing conditions of the forward-active region, a bipolar transistor goes into reverse-active mode. In this mode, the emitter and collector regions switch roles. Since most BJTs are designed to maximise current gain in forward-active mode, the βf in inverted mode is several times smaller. This transistor mode is seldom used. The reverse bias breakdown voltage to the base may be an order of magnitude lower in this region. Saturation: With both junctions forward-biased, a BJT is in saturation mode and facilitates current conduction from the emitter to the collector. This mode corresponds to a logical "on", or a closed switch. BJT structure (active) IE - E IC VCE + C current of electrons for npn transistor – conventional current flows from collector to emitter. + VBE VCB IB + B BJT equations (active) = Common-base current gain (0.9-0.999; typical 0.99) VBE iC iE iC I S e iE IS VT VBE e VT BJT equations (active) = Common-emitter current gain (10-1000; typical 50-200) VBE iC iB iB iC I S e IS VBE e VT VT BJT equations (active) = Common-base current gain (0.9-0.999; typical 0.99) = Common-emitter current gain (10-1000; typical 50-200) 1 1 BJT large signal models (forward active) F ~ forward BJT large signal models (reverse) R ~ reverse Common-base current gain (0.1-0.5) BJT transistor is not a symmetrical device BJT Ebers-Moll (EM) model BJT structure The npn transistor has beta=100 and exhibits an Ic=1mA at VBE=0.7V. Design the circuit so that a current of 2mA flows through collector and a voltage of +5V appears at the collector. BJT equations The voltage at the emitter was measured and found to be -0.7V. If beta=50, find IE, IB, IC and VC. BJT equations A given npn transistor has beta=100. Determine the region of operation if: a) IB=50uA and IC=3mA b) IB=50uA and VCE=5V c) VBE=-2V and VCE=-1V BJT equations RB=200kΩ, RC=1kΩ, VCC=15V, beta=100. Solve for IC and VCE BJT equations RB=200kΩ, RC=1kΩ, VCC=15V, beta=100. Solve for IC and VCE