E we have a PN junction if we apply a positive voltage from B E it will forward

E we have a pn junction if we apply a positive

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E we have a PN junction if we apply a positive voltage from B E, it will forward-bias this junction (it behaves like a diode). Thus, when conducting, the BJT admits a small current into the base connection through the forward-biased diode. This current is still very small relative to the collector/emitter current, but it is not zero like the gate current of the MOSFET. This means that the BJT is a pure current amplifier (current input, current output), unlike the MOSFET which was a transconductance amplifier (voltage input, current output). This also shows us the biggest reason why BJTs are still used they need only an input base-emitter voltage sufficient to forward-bias a diode to conduct/amplify (recall that this is in the range of 0.3V to 1.0V maximum). Compare this to the MOSFET, where we need a of at least , where was typically at least 1.0V or more. BJTs are also more mature in their development; they tend to be able to resist higher voltages, have lower parasitic capacitances (meaning they switch ON/OFF faster), and have better small-signal properties in some amplifier configurations. However, the MOSFET is catching up in many respects. For example, a BJT cannot compete with the MOSFET for high current transmission (as we will see). Hybrid designs such as the IGBT (Integrated-Gate BJT, essentially a MOSFET/BJT combo) exist which attempt to harness the positive aspects of both transistors. It is likely that you will encounter these devices when dealing with high power switching applications such as motor drivers and the like. BJT Symbol and Basic Operation Like the MOSFET, we have two primary variations of the BJT, depending on primary direction of current flow. However, the symbols are basically unified; there is only one symbol for each. We will begin our discussion of operation by again only considering the NPN transistor first. Later we will introduce the PNP, which we will see only causes minor variations. The following diagram shows the symbols for both BJT types, along with the internal construction of each transistor. In general, we will analyze the B E junction/diode using the diode constant voltage model; we do not require the additional complexity of the PWL or exponential model in the majority of cases.
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222 General BJT Equations The basic operation of the BJT is fixed due to its construction. At all times, the emitter current must equal the sum of the base current and collector current: This rule can never be violated, regardless of operating mode or PNP/NPN type. You can think of the BJT as three pipes a small pipe for the base, and a larger pipe for the collector, both of which connect into a single largest pipe, the emitter. BJT Operating Modes Like the MOSFET, the BJT has three distinct operating modes which are roughly the same in terms of application and behavior. The conditions for the BJT, though, are based on the state (forward/reverse bias) of the B E PN junction. To begin, we will examine the simplest possible mode like we did for the MOSFET.
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223 Cut-Off Mode The cut-off mode for the BJT is very similar to the MOSFET. It occurs whenever the PN junction for
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