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Firstly, they can't be driven by extremely high impedance signals, because some small base current must flow to enable conduction. When a small current between the base and the collector is applied, however, the B-E junction depletion layer collapses; and because of how thin the base layer is, the depletion layer on the B-C junction is also compromised. In normal operation, collector is connected to a more positive region, and emitter is more negative; in this configuration, the B-C junction is reverse biased and does not conduct. BJTs consist of a nominally non-conductive junction, most commonly n-p-n - with the outer layers connected to the collector and emitter terminals, and a very thin p-type layer sandwiched in between connected to the base terminal (see image below, left). This arrangement of connections - shown below on the left - results in a normal p-n diode that allows conduction from source to drain - but no conduction the other way round; MOSFET transistors are operated with this junction reverse-biased - i.e., drain more positive than source in case of n-p-n devices.

When turning on a lightbulb or a LED, this is not a problem - but when driving inductive or capacitive loads (including the gates of MOSFET transistors), or encoding information as voltage levels, it may be more desirable to offer two-pole operation, where the output can be switched between two low-impedance rails. This divider is not a perfect voltage source, however: when you connect any resistive load (R3) between A and B - for example, what are electric cables the lightbulb shown on the schematic - it will introduce a new resistance parallel with R2. The action of a capacitor in parallel with a load appears to be reminiscent to the behavior of a series inductor. IR); the output voltage of the circuit will be identical to that of the supply - and will drop quickly, but thanks to parasitic capacitances not instantaneously, when the load is connected again. For starters, there is a voltage offset present between the input and the output; for inputs between 0 and 0.6V, the output will be simply clipped at 0V. Connecting the emitter to a negative voltage at least 0.6V higher than the lowest signal voltage is a potential solution to the clipping problem - but it may be impractical in some settings.

Similarly to MOSFETs, there is also a minimum base-emitter voltage required to overcome the potential of the BE junction (VBE), usually in the 0.6V - 0.8V range. The interesting property of a diode (or any other p-n junction) is that while in this mode, the device will always maintain a potential close to that threshold voltage across its terminals: this electric field is necessary to keep the junction conductive, and the diode will develop an apparent resistance needed to maintain it. Thyristors (also known as silicon controlled rectifiers, SCRs) are four-layer semiconductors that behave like latching transistors - i.e., keep conducting even after the base current stops; this effect can be also approximated by two discrete transistors (image). Their p-n-p counterparts (PNP) operate with the emitter being more positive, and conduct when a current flows from the emitter to the base. Other common types of field-effect transistors are p-channel MOSFETs, which use p-n-p junctions, and switch on with a gate voltage is lower than the source voltage (and have slightly inferior electrical characteristics); four-terminal MOSFETs with no internal connection between the source and the middle semiconductor layer (the fourth terminal connected to this area is referred to as "body"), useful in some switching applications; less common depletion mode MOSFETs, which have a reversed operation, and are normally conductive until a field is applied to disrupt the channel; and somewhat simpler, depletion-only JFETs, which do not feature a glass insulator, and exhibit higher transconductance, but have the undeniable benefit of not having a gate-to-source threshold voltage to speak of.

The use of fuses in low-voltage, low-power consumer electronics is often a matter of a judgment call; but if the power supply can source significant currents, enough to blow a hole in the circuit board, adding a fuse may be a good idea. Some people use separate symbols for Zener (image) and Schottky (image) diodes, but you should certainly not bank on this. With higher currents, the measured voltage across the will increase subtly due to the resistance of the semiconductor material itself - but in most cases, this effect is not very pronounced (some Schottky diodes are an exception). This latter calculation is somewhat cumbersome, but for simplicity, it's good to remember that any number of identical resistors in parallel will have an equivalent resistance equal to R/count, and the power rating will increase accordingly; and that if and if one of the resistors in parallel has a resistance several orders of magnitude lower than the rest, the resistances and power ratings of the remaining resistors may often be safely ignored altogether.