Determined Conditions OR The determined conditions are relatively straight

# Determined conditions or the determined conditions

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Determined Conditions ( ) OR The determined conditions are relatively straight-forward. It is recommended that you use the first set (with ), as they are the most common. In general, you will start by calculating the base current, find the induced collector current, and then the emitter current is the sum of the two. In cases where the emitter is not at ground potential, we may have to perform an iterative calculation to get a final answer for all currents. For DC, the NPN BJT can be represented using the following equivalent circuit. Notice that this is basically identical to the MOSFET model, except we now have a diode from base to emitter. Note that we still have: | |

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225 DC Equivalent Model We are now ready to examine the last BJT mode, saturation (which is most like the MOSFET triode mode, if you recall). Saturation Mode The saturation mode for the BJT represents the ‘switching’ mode for the transistor. It is the state with the lowest , meaning it allows the transistor to conduct the most current from collector to emitter with the lowest dissipation. When using the BJT as a power switch, we will try to operate it in either the cut-off mode or saturation mode. For operation as a small-signal or large-signal amplifier, we will use the active mode. Required Conditions ( ) ( ) ( ) In this mode, we still require to be sufficient to forward-bias the B E PN junction. We also still require that the B C junction not be forward-biased. Notice that its forward drop is slightly higher on the average than with the active mode (0.5V-0.6V vs. 0.4V-0.5V). In this mode, the transistor attempts to maintain a fixed voltage drop from collector to emitter. Thus, the collector current in this mode is not determined directly by the base current, but rather by the rest of the collector/emitter circuit, given a constant voltage from C E of ~0.1 to 0.3V. For the purposes of this course, this simplified model is sufficient for all problems. In the real world, however, the issue is slightly more complex. Using the simplified model, the boundary between modes has some ambiguity. This comes from the fact that under this model, the voltage drop from collector to emitter remains fixed regardless of how much current flows from collector to emitter. In reality, as more current flows, the voltage drop increases from C E. This effect will usually be balanced, as the increased voltage drop will lower the current from C E due to any resistance in series, much like with the diode. Thus, the total effect is indeterminate without further information. With the diode, this meant having access to an accurate curve. For the
226 BJT, the relation is much more direct. Say that we have a transistor that nominally has a current gain of when operating in active mode. Now saw that we determine the transistor to actually be in saturation, with a current of flowing, as determined by replacing the C E connection with a constant drop of ~0.1V 0.3V and solving the circuit. In this case, we can find an equivalent gain for the transistor given this collector current. We assume that base current remains relatively the same

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